Gastrointestinal Physiology

Quick Answer

The gastrointestinal tract performs four critical functions: motility (propulsion and mixing), secretion (digestive enzymes, acid, bile), digestion (macromolecule breakdown), and absorption (nutrient and water uptake). GI motility is coordinated by the enteric nervous system (ENS) - the "second brain"

• containing 100 million neurons, operating via the myenteric (Auerbach's) and submucosal (Meissner's) plexuses. The migrating motor complex (MMC) cycles every 90-120 minutes during fasting, performing "housekeeping" to clear debris. Gastric acid secretion (2-3 L/day, pH 1-2) is regulated by the classic triad: gastrin (G cells), histamine (ECL cells via H2 receptors), and acetylcholine (vagus). Pancreatic secretion (1-2 L/day) provides bicarbonate (ductal cells) for acid neutralisation and digestive enzymes (acinar cells). The gut barrier is a single epithelial layer (surface area 32 m²) sealed by tight junctions (claudins, occludins) - failure leads to bacterial translocation and systemic inflammation. Splanchnic circulation receives 25% of cardiac output, with blood flow regulated by autoregulation and the hepatic artery buffer response (HABR). In critical illness, splanchnic hypoperfusion causes ischaemia-reperfusion injury, barrier dysfunction, and propagation of SIRS/sepsis. The gut-brain axis operates bidirectionally via vagal afferents, hormones, and the microbiome (10^14 bacteria, >1000 species).

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CICM Exam Focus

What Examiners Expect

First Part Written SAQ Topics:

• GI motility control: ENS organisation, MMC phases, gastric emptying regulation
• Gastric acid secretion: Parietal cell mechanisms, regulation (cephalic, gastric, intestinal phases), pharmacological targets
• Pancreatic secretion: Acinar vs ductal cell functions, hormonal regulation (secretin, CCK)
• Carbohydrate, protein, and fat absorption pathways
• Gut barrier function: Tight junctions, mucosal immunity, bacterial translocation
• Splanchnic circulation: Blood flow distribution, autoregulation, shock physiology
• GI hormones: Gastrin, secretin, CCK, motilin, GLP-1, ghrelin

First Part Viva Topics:

• Applied GI physiology in critical illness (ileus, stress ulceration)
• Prokinetic agents and mechanisms of action
• Gut barrier failure and bacterial translocation pathophysiology
• Splanchnic ischaemia and non-occlusive mesenteric ischaemia (NOMI)
• Enteral feeding and its effects on GI physiology
• Acid-base consequences of GI secretory losses

Common SAQ Stems:

1. "Describe the regulation of gastric acid secretion and the mechanisms of action of drugs that reduce gastric acid"
2. "Outline the control of gastrointestinal motility and explain the pathophysiology of ileus in critical illness"
3. "Describe the structure and function of the gut barrier and explain how it may fail in critical illness"
4. "Explain the regulation of splanchnic blood flow and its relevance to shock states"
5. "Describe the digestion and absorption of carbohydrates, proteins, and fats"

High-Yield Calculations:

• Gastric acid secretion: 2-3 L/day, H+ concentration 150 mmol/L, pH 1-2
• Splanchnic blood flow: 1500-2000 mL/min (25% of cardiac output)
• Gut surface area: 32 m² (200× skin surface area)
• MMC cycle duration: 90-120 minutes
• Gastric emptying half-life: Liquids 15-20 min, solids 60-120 min

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Key Points

1. Enteric nervous system (ENS): Contains 100 million neurons ("second brain"); myenteric plexus controls motility, submucosal plexus controls secretion; operates autonomously but modulated by parasympathetic (stimulatory) and sympathetic (inhibitory) inputs [1,2]

2. Migrating motor complex (MMC): Four-phase fasting motility pattern cycling every 90-120 minutes; Phase III (intense contractions) clears debris and bacteria; abolished by feeding; motilin is the key hormonal regulator [3,4]

3. Gastric acid secretion: Parietal cells secrete H+ via H+/K+-ATPase (proton pump); regulated by gastrin (G cells), histamine (ECL cells, H2 receptors), and acetylcholine (vagus, M3 receptors); inhibited by somatostatin, prostaglandins, and secretin [5,6]

4. Pancreatic secretion: Acinar cells secrete digestive enzymes (amylase, lipase, proteases as zymogens); ductal cells secrete bicarbonate (CFTR channel); stimulated by secretin (bicarbonate) and CCK (enzymes) [7,8]

5. Fat absorption: Requires bile salt emulsification, pancreatic lipase digestion, mixed micelle formation, and chylomicron synthesis in enterocytes; lymphatic drainage (not portal) for long-chain fatty acids [9,10]

6. Gut barrier function: Single epithelial layer sealed by tight junctions (claudins, occludins, ZO-1); mucus layer, secretory IgA, antimicrobial peptides provide additional defence; failure leads to bacterial translocation [11,12]

7. Splanchnic circulation: Receives 25% of cardiac output (1500-2000 mL/min); vulnerable to redistribution in shock; autoregulation maintained down to MAP 70 mmHg; non-occlusive mesenteric ischaemia (NOMI) in low-flow states [13,14]

8. Stress ulceration: Occurs in 75-100% of ICU patients within 24 hours; clinically significant bleeding in 1.5-6%; risk factors include mechanical ventilation >48h and coagulopathy; prophylaxis with PPIs or H2RAs [15,16]

9. Bacterial translocation: Gut bacteria and endotoxin cross compromised barrier; contributes to SIRS, sepsis, and MODS in critical illness; associated with splanchnic hypoperfusion, antibiotic dysbiosis, and mucosal atrophy [17,18]

10. Gut-brain axis: Bidirectional communication via vagal afferents, enteric hormones, and microbiome metabolites (short-chain fatty acids); influences immune function, metabolism, and behaviour; disrupted in critical illness [19,20]

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Gastrointestinal Tract Anatomy Overview

Structural Organisation

The gastrointestinal tract extends approximately 9 metres from mouth to anus, with the small intestine contributing 6-7 metres. The wall consists of four concentric layers from lumen outward. [21]

Mucosal Layer (Innermost):

• Epithelium: Single layer of columnar cells; surface area amplified 600-fold by circular folds (plicae circulares), villi, and microvilli
• Lamina propria: Loose connective tissue containing capillaries, lymphatics, immune cells (GALT)
• Muscularis mucosae: Thin smooth muscle layer controlling villus movement

Submucosal Layer:

• Dense connective tissue with blood vessels, lymphatics
• Submucosal plexus (Meissner's): Regulates secretion and blood flow
• Brunner's glands in duodenum (alkaline mucus secretion)

Muscularis Externa:

• Inner circular muscle: Controls lumen diameter, segmentation
• Outer longitudinal muscle: Controls gut length, peristalsis
• Myenteric plexus (Auerbach's): Between muscle layers; controls motility

Serosa/Adventitia (Outermost):

• Visceral peritoneum where intraperitoneal
• Adventitia (fibrous tissue) in retroperitoneal segments

Surface Area Amplification

The small intestine maximises absorptive surface area through hierarchical amplification: [22]

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Enteric Nervous System

Organisation and Function

The enteric nervous system (ENS) is the intrinsic nervous system of the GI tract, often termed the "second brain" due to its complexity and relative autonomy. It contains approximately 100 million neurons - comparable to the spinal cord - and can regulate GI function independently of CNS input. [1,2,23]

Myenteric Plexus (Auerbach's):

• Location: Between circular and longitudinal muscle layers
• Function: Primary control of GI motility
• Neuron types: Motor neurons (excitatory and inhibitory), interneurons, sensory neurons
• Extends entire length of GI tract
• Controls peristalsis, segmentation, and sphincter function

Submucosal Plexus (Meissner's):

• Location: Within submucosa
• Function: Control of secretion, blood flow, and absorption
• Less prominent than myenteric plexus
• Primarily in small and large intestine
• Regulates epithelial cell function

Neurotransmitters

Excitatory Neurotransmitters:

• Acetylcholine (ACh): Primary excitatory transmitter; increases smooth muscle contraction, increases secretion
• Substance P: Co-released with ACh; enhances contraction
• Serotonin (5-HT): Released from enterochromaffin cells; stimulates peristaltic reflex

Inhibitory Neurotransmitters:

• Vasoactive intestinal peptide (VIP): Relaxes smooth muscle; stimulates secretion
• Nitric oxide (NO): Primary inhibitory transmitter; essential for receptive relaxation and sphincter opening
• ATP: Purinergic inhibition of smooth muscle

Extrinsic Neural Control

Parasympathetic Innervation:

• Vagus nerve: Oesophagus to proximal colon
• Pelvic splanchnic nerves (S2-4): Distal colon, rectum
• Effect: Generally stimulatory - increases motility and secretion
• Preganglionic fibres synapse with ENS neurons
• Maintains "rest and digest" functions

Sympathetic Innervation:

• T5-L2 spinal segments via coeliac, superior and inferior mesenteric ganglia
• Effect: Generally inhibitory - decreases motility and secretion
• Directly innervates blood vessels (vasoconstriction)
• Contracts sphincters
• Overrides ENS during stress/shock ("fight or flight") [24]

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Gastrointestinal Motility

Types of GI Motility

Peristalsis:

• Coordinated wave of contraction preceded by relaxation
• Propels contents aborally (toward anus)
• Regulated by myenteric plexus
• Contraction behind bolus (ACh, substance P), relaxation ahead (VIP, NO)
• Rate varies: Oesophagus 2-4 cm/sec, small intestine 1-2 cm/min

Segmentation:

• Mixing contractions without net propulsion
• Predominant in small intestine during digestion
• Alternating contraction and relaxation of circular muscle
• Exposes chyme to mucosal surface for absorption

Tonic Contraction:

• Sustained partial contraction
• Characteristic of sphincters (LOS, pylorus, ICV, IAS)
• Maintains compartmentalisation between GI segments

Migrating Motor Complex (MMC)

The MMC is the characteristic fasting motility pattern that maintains GI "housekeeping" between meals, preventing bacterial overgrowth and clearing residual debris. [3,4,25]

Phases of the MMC:

• clears debris, bacteria | | Phase IV | 0-5 min | Transition to Phase I or feeding pattern | Transition |

Characteristics:

• Cycle duration: 90-120 minutes
• Originates in stomach or duodenum
• Propagates to terminal ileum
• Velocity: 5-10 cm/min
• Abolished by feeding (conversion to fed pattern within minutes)
• Resumes 2-4 hours after meal completion

Regulation:

• Motilin: Primary hormone; released from duodenal M cells during Phase III; stimulates gastric and duodenal contraction
• Erythromycin: Motilin receptor agonist; initiates Phase III-like activity
• Vagal innervation: Required for normal MMC; vagotomy disrupts pattern
• Somatostatin: Inhibits MMC
• Ghrelin: May contribute to MMC initiation

Clinical Significance in ICU:

• MMC absence → bacterial overgrowth, malabsorption
• Critical illness → MMC disruption → ileus, feeding intolerance
• Erythromycin as prokinetic exploits motilin receptor agonism
• Fasting during enteral feeding interruption allows MMC recovery [26]

Gastric Emptying

Gastric emptying is the coordinated process of transferring gastric contents to the duodenum at a rate matching duodenal digestive and absorptive capacity. [27,28]

Gastric Regions:

• Fundus: Reservoir function; receives food via receptive relaxation (VIP, NO); storage and controlled release
• Body: Mixing region; grinding contractions reduce particle size
• Antrum: Propulsion; powerful peristaltic contractions (3/min) force contents toward pylorus

Factors Affecting Gastric Emptying:

Emptying Rates:

• Water: T½ ~15-20 minutes
• Solid meal: T½ ~60-120 minutes
• Fat-rich meal: Significantly prolonged

Neural Control:

• Vagus nerve: Stimulates fundal relaxation and antral contraction
• Enterogastric reflex: Duodenal nutrients → inhibit gastric emptying
• Intrinsic ENS: Coordinates antral-pyloric function

Hormonal Control:

• Gastrin: Increases antral contractions (but overall effect variable)
• CCK: Contracts pylorus, relaxes fundus → slows emptying
• Secretin: Inhibits gastric motility and secretion
• GIP: Inhibits gastric motility
• Motilin: Stimulates gastric emptying (Phase III MMC)

Clinical Significance in ICU:

• Gastroparesis: 50-80% of mechanically ventilated patients
• Causes: Opioids, catecholamines, hyperglycaemia, sepsis, electrolyte disturbance
• Consequences: Aspiration risk, feeding intolerance, malnutrition
• Prokinetics: Metoclopramide (D2 antagonist, 5-HT4 agonist), erythromycin (motilin agonist) [29,30]

Small Intestinal Motility

Fed State (Digestive Pattern):

• Segmentation predominates: Mixing contractions for digestion/absorption
• Peristalsis: Slow aboral propulsion (1-2 cm/min)
• Transit time: 2-4 hours (duodenum to ileocaecal valve)

Fasting State:

• MMC pattern (described above)
• Clears residual contents
• Prevents bacterial overgrowth

Regulation:

• ENS coordinates segmentation and peristalsis
• Parasympathetic: Increases motility
• Sympathetic: Decreases motility
• CCK, motilin: Increase motility
• Secretin, glucagon: Decrease motility

Colonic Motility

Types of Colonic Contractions:

• Haustral contractions: Segmentation; mix contents, slow transit; 2-3/hour
• Peristaltic contractions: Propel contents aborally; infrequent
• Mass movements: High-amplitude propagated contractions; 1-3/day; move contents from transverse to sigmoid colon; associated with defecation urge

Transit Time:

• 24-48 hours (range 18-72 hours)
• Highly variable between individuals
• Prolonged in critical illness

Defecation:

• Internal anal sphincter (IAS): Smooth muscle; tonically contracted; involuntary
• External anal sphincter (EAS): Striated muscle; voluntary control
• Rectosphincteric reflex: Rectal distension → IAS relaxation
• Defecation: Voluntary EAS relaxation + Valsalva + colonic contraction [31]

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Gastric Acid Secretion

Parietal Cell Physiology

The parietal (oxyntic) cell is a highly specialised gastric gland cell responsible for hydrochloric acid secretion. It can generate a H+ concentration gradient of 3 million:1 (lumen pH 1 vs blood pH 7.4). [5,6,32]

Structural Adaptations:

• Abundant mitochondria (30-40% cell volume) for ATP generation
• Tubulovesicular system: Intracellular reservoir of H+/K+-ATPase
• Canalicular membrane: Extensive surface area for secretion
• Stimulation → tubulovesicle fusion with canalicular membrane → increased pump expression

Mechanism of HCl Secretion:

1. H+/K+-ATPase (Proton Pump):

   • Apical membrane; exchanges H+ for K+
   • ATP-dependent primary active transport
   • Generates 150 mmol/L H+ in gastric lumen
   • Target of proton pump inhibitors (PPIs)

2. Cl- Secretion:

   • Chloride channels on apical membrane
   • Cl- enters cell via basolateral Cl-/HCO3- exchanger

3. K+ Recycling:

   • Apical K+ channels allow K+ efflux
   • Maintains substrate for H+/K+-ATPase

4. HCO3- Generation (Alkaline Tide):

   • Intracellular carbonic anhydrase: H2O + CO2 → H+ + HCO3-
   • H+ secreted into lumen
   • HCO3- exits via basolateral Cl-/HCO3- exchanger
   • Results in post-prandial "alkaline tide" in venous blood

Quantitative Aspects:

• Gastric juice volume: 2-3 L/day
• H+ concentration: 150 mmol/L
• pH: 1-2 (basal); 1-1.5 (maximal stimulation)
• Basal acid output (BAO): 2-5 mmol/hour
• Maximal acid output (MAO): 20-40 mmol/hour

Regulation of Gastric Acid Secretion

Gastric acid secretion is regulated by three primary stimulants (gastrin, histamine, acetylcholine) and several inhibitors, operating through three phases. [5,6,33]

Stimulants (The Classic Triad):

1. Gastrin:

• Source: G cells in gastric antrum
• Stimulus: Amino acids, peptides, gastric distension, vagal stimulation (GRP)
• Receptor: CCK-B receptor on parietal cells and ECL cells
• Mechanism: Directly stimulates parietal cells; primarily stimulates histamine release from ECL cells
• Inhibition: Low gastric pH (<3) inhibits gastrin release (negative feedback)

2. Histamine:

• Source: Enterochromaffin-like (ECL) cells in gastric fundus
• Stimulus: Gastrin, acetylcholine
• Receptor: H2 receptor on parietal cells
• Mechanism: Increases cAMP → activates H+/K+-ATPase
• Key amplifier: Potentiates effects of gastrin and ACh
• Pharmacological target: H2 receptor antagonists (ranitidine, famotidine)

3. Acetylcholine:

• Source: Vagal efferents and ENS neurons
• Stimulus: Sight, smell, taste of food (cephalic phase); gastric distension
• Receptor: M3 muscarinic receptor on parietal cells
• Mechanism: Increases intracellular Ca2+ → activates H+/K+-ATPase
• Also stimulates ECL histamine release and G cell gastrin release
• Pharmacological target: Anticholinergics (limited use due to side effects)

Phases of Gastric Acid Secretion:

Inhibitors of Acid Secretion:

• Somatostatin: Released from D cells; inhibits G cells (gastrin release), ECL cells (histamine), and parietal cells (direct)
• Prostaglandins (PGE2): Inhibit parietal cell secretion; stimulate mucus/bicarbonate secretion; "cytoprotective"
• Secretin: Released from S cells in response to duodenal acid; inhibits gastric acid, stimulates pancreatic bicarbonate
• GIP: Inhibits acid secretion; stimulates insulin release
• Acid in duodenum: Triggers enterogastric reflex → inhibits gastric emptying and secretion

Pharmacological Targets

Proton Pump Inhibitors (PPIs):

• Mechanism: Irreversible inhibition of H+/K+-ATPase
• Examples: Omeprazole, esomeprazole, pantoprazole, lansoprazole
• Efficacy: Reduces acid secretion by 90-99%
• Duration: Effect persists until new pumps synthesised (24-48 hours)
• Clinical use: Peptic ulcer disease, GORD, stress ulcer prophylaxis, Zollinger-Ellison syndrome
• ICU considerations: Preferred for stress ulcer prophylaxis (superior efficacy vs H2RAs); IV formulations available [34,35]

H2 Receptor Antagonists (H2RAs):

• Mechanism: Competitive antagonism of histamine at H2 receptors
• Examples: Ranitidine, famotidine, cimetidine
• Efficacy: Reduces acid secretion by 50-70%
• Duration: 6-12 hours
• Limitations: Tachyphylaxis (tolerance) with continuous use
• ICU considerations: Alternative to PPIs; possibly lower infection risk (C. difficile, pneumonia) [36]

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Pancreatic Secretion

Anatomy and Organisation

The exocrine pancreas produces 1-2 L of pancreatic juice daily, containing digestive enzymes and bicarbonate. The functional unit is the pancreatic acinus with its associated duct system. [7,8,37]

Acinar Cells:

• Pyramidal epithelial cells arranged around central lumen
• Abundant rough ER and zymogen granules (apex)
• Synthesise and secrete digestive enzymes
• Store enzymes as inactive zymogens (prevent autodigestion)

Ductal Cells:

• Line pancreatic ductules and main duct
• Secrete bicarbonate-rich fluid
• CFTR chloride channel essential for Cl-/HCO3- exchange
• Modify primary secretion by ion exchange

Pancreatic Secretion Composition

Enzyme (Acinar Cell) Secretion:

Bicarbonate (Ductal Cell) Secretion:

• Concentration: Up to 150 mmol/L at maximal stimulation (isotonic to plasma)
• Mechanism: Cl-/HCO3- exchange (pendrin/SLC26A6); CFTR provides Cl- for exchange
• CO2 + H2O → H+ + HCO3- (carbonic anhydrase); H+ exits basolaterally via Na+/H+ exchanger
• Function: Neutralises gastric acid in duodenum; optimises pH for enzyme activity (pH 7-8)

Regulation of Pancreatic Secretion

Hormonal Control:

Secretin:

• Source: S cells in duodenal mucosa
• Stimulus: Duodenal pH <4.5, fatty acids
• Target: Pancreatic ductal cells
• Effect: Stimulates bicarbonate and water secretion
• "Nature's antacid"
• first hormone discovered (Bayliss and Starling, 1902)

Cholecystokinin (CCK):

• Source: I cells in duodenal and jejunal mucosa
• Stimulus: Fatty acids (>12 carbons), amino acids, peptides
• Target: Pancreatic acinar cells
• Effect: Stimulates enzyme secretion
• Also: Contracts gallbladder, relaxes sphincter of Oddi, stimulates pancreatic growth

Neural Control:

• Vagus nerve: Cephalic phase stimulation; potentiates hormone effects; primarily ACh-mediated enzyme secretion
• Enteropancreatic reflexes: Duodenal nutrients → ENS → pancreatic secretion

Phases of Pancreatic Secretion:

Protection Against Autodigestion

The pancreas has evolved multiple mechanisms to prevent autodigestion by its powerful proteolytic enzymes. [38]

Protective Mechanisms:

1. Zymogen secretion: Proteases stored/secreted as inactive precursors
2. Enterokinase activation: Trypsinogen activated only in duodenum by brush border enterokinase
3. Trypsin inhibitor: Pancreatic secretory trypsin inhibitor (PSTI/SPINK1) inactivates prematurely activated trypsin
4. Compartmentalisation: Enzymes in zymogen granules, separated from cell contents
5. Low intracellular calcium: Prevents premature enzyme activation
6. Autophagy: Degradation of abnormally activated enzymes

Clinical Significance - Acute Pancreatitis:

• Premature intrapancreatic trypsin activation → autodigestion
• Causes: Gallstones (obstruction), alcohol, hypertriglyceridaemia, drugs
• Trypsin activates other zymogens → cascade of tissue injury
• Phospholipase A2 → fat necrosis
• Elastase → vascular injury, haemorrhage
• Systemic release → SIRS, MODS [39]

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Bile Secretion and Biliary Physiology

Bile Production and Composition

Bile is produced continuously by hepatocytes (500-1000 mL/day) and modified by the biliary epithelium. It is essential for fat digestion and serves as an excretory pathway for cholesterol, bilirubin, and xenobiotics. [9,10,40]

Composition of Hepatic Bile:

• Water: 95%
• Bile acids/salts: 0.7% (12-18 mmol/L)
• Phospholipids: 0.5% (4-8 mmol/L, primarily phosphatidylcholine)
• Cholesterol: 0.3% (3-5 mmol/L)
• Bilirubin glucuronides: 0.05%
• Proteins, electrolytes (Na+, K+, Ca2+, Cl-, HCO3-)

Gallbladder Modification:

• Capacity: 30-60 mL
• Concentrates bile 5-20 fold by water and electrolyte absorption
• Stores bile between meals (fasting)
• Contracts in response to CCK (postprandially)

Bile Acids

Primary Bile Acids (Synthesised from Cholesterol):

• Cholic acid (3α,7α,12α-trihydroxy)
• Chenodeoxycholic acid (CDCA, 3α,7α-dihydroxy)
• Synthesis: 500-600 mg/day; CYP7A1 rate-limiting enzyme

Secondary Bile Acids (Bacterial Modification in Colon):

• Deoxycholic acid (from cholic acid)
• Lithocholic acid (from CDCA)

Conjugation:

• Glycine conjugates (75%): Glycocholic, glycochenodeoxycholic acid
• Taurine conjugates (25%): Taurocholic, taurochenodeoxycholic acid
• Conjugation lowers pKa → ionised at intestinal pH → prevents passive absorption → allows active reabsorption in terminal ileum

Bile Salt Function:

• Emulsification: Break fat globules into small droplets (increase surface area)
• Micelle formation: Solubilise fatty acids, monoglycerides, cholesterol, fat-soluble vitamins
• Enable lipase action: Colipase-lipase complex requires bile salt micelles
• Cholesterol solubilisation: Prevent gallstone formation

Enterohepatic Circulation

The enterohepatic circulation recycles 95% of bile acids, maintaining an efficient bile acid pool with minimal new synthesis. [40]

Cycle:

1. Bile acids secreted into bile (BSEP transporter, canalicular membrane)
2. Stored in gallbladder (fasting) or directly enter duodenum (fed)
3. Emulsify dietary lipids, form mixed micelles
4. Remain in gut lumen during lipid absorption (bile acids not absorbed with micelles)
5. Active reabsorption in terminal ileum (ASBT/SLC10A2 transporter, 95%)
6. Portal venous return to liver
7. Hepatocyte uptake (NTCP transporter, basolateral membrane)
8. Re-secretion into bile

Quantitative Aspects:

• Bile acid pool: 2-4 g
• Pool cycles: 6-12 times/day
• Daily bile acid secretion: 12-36 g (recycled)
• Faecal loss: 5% daily (500-600 mg)
• New synthesis: Matches faecal loss

Clinical Significance:

• Terminal ileum resection → bile acid malabsorption → steatorrhoea, vitamin malabsorption
• Cholestyramine (bile acid sequestrant) → interrupts enterohepatic circulation → reduces cholesterol
• Ileal disease → reduced bile acid pool → fat malabsorption

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Nutrient Absorption

Carbohydrate Digestion and Absorption

Dietary carbohydrates (250-350 g/day) are absorbed primarily as monosaccharides after enzymatic digestion. [41,42]

Dietary Carbohydrates:

• Starch (50-60%): Amylose (linear α-1,4 glucose) and amylopectin (branched α-1,4 + α-1,6)
• Sucrose (20-30%): Glucose + fructose
• Lactose (10%): Glucose + galactose
• Fibre: Non-digestible polysaccharides (cellulose, hemicellulose)

Digestion:

Absorption:

• Glucose and galactose: SGLT1 (Na+-glucose co-transporter) on apical membrane; secondary active transport coupled to Na+ gradient (Na+/K+-ATPase basolateral); GLUT2 on basolateral membrane → portal blood
• Fructose: GLUT5 facilitated diffusion (apical); GLUT2 (basolateral)

Clinical Significance:

• Lactose intolerance: Lactase deficiency → undigested lactose → osmotic diarrhoea, bacterial fermentation → bloating, gas
• SGLT2 inhibitors (empagliflozin, dapagliflozin): Inhibit renal glucose reabsorption (different transporter) - commonly used in diabetes and heart failure
• Critical illness: Brush border enzyme dysfunction → carbohydrate malabsorption → diarrhoea with enteral feeding

Protein Digestion and Absorption

Dietary protein (70-100 g/day) is absorbed as amino acids, dipeptides, and tripeptides after extensive enzymatic digestion. [43,44]

Digestion:

Absorption:

• Amino acids: Multiple Na+-dependent and Na+-independent transporters on apical membrane; classified by amino acid type (neutral, acidic, basic, imino)
• Dipeptides and tripeptides: H+-peptide co-transporter (PepT1); highly efficient; peptides hydrolysed intracellularly
• PepT1 transport: Major route of protein absorption (50-80% as di/tripeptides)
• Basolateral transport: Various amino acid transporters → portal blood

Clinical Significance:

• Parenteral amino acid solutions: Bypass GI digestion; direct IV amino acid delivery in gut failure
• Enteral protein: Stimulates GI function, maintains gut barrier; prefer polymeric formulas (require digestion)
• Elemental formulas: Pre-digested amino acids; for severe malabsorption (short bowel, pancreatitis)

Fat Digestion and Absorption

Dietary fat (50-100 g/day, primarily triglycerides) requires a complex sequence of emulsification, enzymatic digestion, micelle formation, and lymphatic absorption. [9,10,45]

Emulsification:

• Mechanical churning in stomach
• Bile salts: Amphipathic molecules; hydrophobic face adsorbs to fat droplet; hydrophilic face faces aqueous phase
• Creates small droplets (1 μm diameter) → increased surface area for lipase

Digestion:

Micelle Formation:

• Mixed micelles: Bile salts, phospholipids, cholesterol, fatty acids, monoglycerides
• Diameter: 4-5 nm
• Solubilise lipid digestion products in aqueous intestinal lumen
• Transport lipids to brush border membrane

Absorption:

• Micelles diffuse through unstirred water layer to brush border
• Lipid monomers dissociate from micelles
• Passive diffusion and facilitated transport (fatty acid transporter CD36, NPC1L1)
• Bile salts remain in lumen → terminal ileum reabsorption

Enterocyte Lipid Processing:

1. Fatty acid reesterification: Long-chain fatty acids + monoglycerides → triglycerides (smooth ER)
2. Chylomicron assembly: Triglycerides + cholesterol + phospholipids + apoB-48 → chylomicrons (80-90% triglyceride)
3. Lymphatic drainage: Chylomicrons too large for capillaries; enter lacteals → mesenteric lymphatics → thoracic duct → systemic circulation
4. Short/medium-chain fatty acids: Directly to portal blood (not chylomicron pathway)

Clinical Significance:

• Steatorrhoea: Fat malabsorption → pale, foul-smelling stools, fat-soluble vitamin deficiency
• Causes: Pancreatic insufficiency, bile salt deficiency, mucosal disease
• MCT oils: Medium-chain triglycerides absorbed directly to portal blood; useful in fat malabsorption
• Orlistat: Pancreatic lipase inhibitor; reduces fat absorption (weight loss drug)

Water and Electrolyte Absorption

The GI tract handles massive fluid volumes daily, with net absorption of 8-9 L/day, leaving only 100-200 mL in stool. [46,47]

Daily GI Fluid Balance:

Small Intestine Absorption:

• Primary absorption site (7-8 L/day)
• Mechanisms: Na+-coupled nutrient absorption (SGLT1, amino acid transporters), Na+/H+ exchange, Cl-/HCO3- exchange
• Follows osmotic gradients created by solute absorption

Large Intestine Absorption:

• Absorbs 1-1.5 L/day (can increase to 4-5 L with adaptation)
• Mechanisms: Electroneutral Na+/Cl- absorption; electrogenic Na+ absorption (ENaC, aldosterone-sensitive)
• Critical for final concentration of stool

Electrolyte Absorption:

Clinical Significance - GI Fluid Losses:

• Vomiting: Loss of H+, Cl-, K+ → hypochloraemic hypokalaemic metabolic alkalosis
• Diarrhoea: Loss of HCO3-, K+ → hyperchloraemic hypokalaemic metabolic acidosis
• High-output stoma: Massive Na+, water loss → dehydration, hyponatraemia
• Nasogastric suction: Similar to vomiting; requires replacement of H+, Cl-, K+ [48]

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Gut Barrier Function

Structure of the Gut Barrier

The gut barrier is a complex multi-layered defence system that permits nutrient absorption while excluding pathogens, toxins, and antigens. Barrier failure is a critical pathophysiological mechanism in ICU patients. [11,12,49]

Components of the Gut Barrier:

1. Mucus Layer:

• Inner layer: Dense, firmly attached, largely bacteria-free
• Outer layer: Loose, colonised by commensal bacteria
• Composition: MUC2 mucin (goblet cells), water, electrolytes, IgA, antimicrobial peptides
• Function: Physical barrier, lubricates epithelium, retains antimicrobials
• Thickness: 50-800 μm (varies by region; thickest in colon)

2. Epithelial Layer:

• Single layer of columnar epithelial cells
• Cell types: Enterocytes (absorption), goblet cells (mucus), Paneth cells (antimicrobial peptides), enteroendocrine cells (hormones), M cells (antigen sampling)
• Rapid turnover: 3-5 days (one of fastest renewing tissues)
• Stem cells: Base of crypts; continuous regeneration

3. Tight Junctions:

• Seal paracellular space between epithelial cells
• Proteins: Claudins (>24 subtypes), occludins, zonula occludens (ZO-1, ZO-2, ZO-3), junctional adhesion molecules (JAMs)
• Claudins: Determine permeability characteristics; some sealing (claudin-1, 3, 4, 5), some pore-forming (claudin-2, 10, 15)
• Dynamic regulation: Permeability can be modulated by signalling pathways
• Cytoskeletal connection: Actin-myosin ring; contraction increases permeability

4. Immune Components:

• Secretory IgA (sIgA): Dimeric IgA transported across epithelium; binds pathogens/antigens in lumen; prevents adhesion
• Antimicrobial peptides (AMPs): Defensins (α and β), cathelicidins, RegIIIγ; kill bacteria
• GALT: Peyer's patches, mesenteric lymph nodes, isolated lymphoid follicles; antigen sampling and immune response
• Intraepithelial lymphocytes (IELs): CD8+ T cells; surveillance and rapid response

Intestinal Permeability Regulation

Tight junction permeability is dynamically regulated by multiple pathways. [50,51]

Factors Increasing Permeability:

• Zonulin: Released in response to gliadin, bacteria; activates myosin light chain kinase (MLCK)
• Pro-inflammatory cytokines: TNF-α, IFN-γ, IL-1β, IL-6
• Pathogenic bacteria: Toxins disrupt tight junctions (e.g., C. difficile toxin A/B)
• Hypoxia/ischaemia: Epithelial ATP depletion → tight junction dysfunction
• Oxidative stress: Reactive oxygen species damage tight junction proteins

Factors Decreasing Permeability:

• Short-chain fatty acids (SCFAs): Butyrate, propionate, acetate; strengthen barrier
• Glutamine: Essential for enterocyte metabolism and tight junction integrity
• Epidermal growth factor (EGF): Promotes epithelial repair
• Glucocorticoids: Reduce inflammation, enhance tight junction function

Bacterial Translocation

Bacterial translocation is the passage of viable bacteria or bacterial products (endotoxin/LPS) from the intestinal lumen to normally sterile sites (mesenteric lymph nodes, portal blood, systemic circulation). [17,18,52]

Mechanisms:

1. Transcellular passage: Bacteria taken up by M cells or damaged enterocytes
2. Paracellular passage: Bacteria traverse disrupted tight junctions
3. Direct invasion: Pathogenic bacteria with invasive capabilities

Predisposing Factors (Critical Illness):

Consequences:

• Activation of systemic inflammatory response
• Portal endotoxaemia → hepatic Kupffer cell activation → cytokine release
• Bacteraemia → sepsis
• Gut as "motor" of MODS: Propagates and amplifies systemic inflammation
• Associated with poor outcomes in ICU patients [53]

Prevention Strategies:

• Early enteral nutrition: Maintains epithelial integrity, stimulates mucus/IgA
• Selective digestive decontamination (SDD): Controversial; reduces VAP but concerns re: resistance
• Probiotics: Limited evidence; possible benefit in specific populations
• Glutamine supplementation: Mixed evidence in critically ill
• Avoid unnecessary bowel rest: If gut works, use it

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Splanchnic Circulation

Anatomy and Blood Flow Distribution

The splanchnic circulation serves the gastrointestinal tract, liver, spleen, and pancreas. It receives approximately 25% of cardiac output at rest, making it the largest vascular bed in the body. [13,14,54]

Arterial Supply:

Blood Flow Distribution:

Portal Circulation:

• Portal vein: Receives venous drainage from stomach, intestines, spleen, pancreas
• Portal blood flow: 1000-1200 mL/min (75% of hepatic blood flow)
• Low pressure system: 7-10 mmHg
• Delivers nutrients and absorbed substances directly to liver for first-pass metabolism

Regulation of Splanchnic Blood Flow

Intrinsic Regulation:

Autoregulation:

• Maintains blood flow constant despite changes in perfusion pressure
• Less robust than cerebral or renal autoregulation
• Effective range: MAP 70-140 mmHg (narrower than other beds)
• Mechanisms: Myogenic response, metabolic vasodilation

Metabolic Hyperaemia (Postprandial):

• Intestinal blood flow increases 30-130% after meals
• Duration: 3-6 hours
• Mediators: Absorbed nutrients, CCK, VIP, bradykinin, adenosine
• Mucosal blood flow increases more than muscularis

Reactive Hyperaemia:

• Robust vasodilation after transient ischaemia
• Mediated by accumulated metabolites (adenosine, NO, prostaglandins)
• Important in reperfusion after shock resuscitation

Extrinsic Regulation:

Sympathetic Nervous System:

• Splanchnic vasculature richly innervated by sympathetic fibres
• α1-adrenoceptor mediated vasoconstriction
• Can reduce splanchnic blood flow by 40-50% in stress/shock
• "Blood reservoir" function: Splanchnic veins constrict, mobilising 1000+ mL blood to central circulation
• Vasoconstriction preferentially affects mucosa (more vulnerable to ischaemia) [55]

Humoral Factors:

Splanchnic Circulation in Shock

Redistribution Phenomenon:

• In hypovolaemic and cardiogenic shock, blood is redistributed away from splanchnic bed
• Splanchnic vasoconstriction is disproportionately greater than other beds
• Maintains perfusion to heart and brain at expense of gut
• Gut hypoperfusion occurs before systemic hypotension becomes apparent

Non-Occlusive Mesenteric Ischaemia (NOMI):

• Ischaemia without arterial/venous occlusion
• Caused by prolonged splanchnic vasoconstriction in low-flow states
• Risk factors: Cardiac failure, shock, vasopressors, cardiac surgery
• Affects watershed areas (splenic flexure, rectosigmoid)
• May present with abdominal pain, bloody diarrhoea, elevated lactate
• High mortality (50-90%) [56]

Ischaemia-Reperfusion Injury:

• Reperfusion paradoxically worsens injury
• Mechanisms: ROS generation (xanthine oxidase), neutrophil activation, complement activation
• Intestinal oedema, increased permeability, bacterial translocation
• Systemic release of cytokines → SIRS propagation

Effects of Vasopressors:

• Norepinephrine: α-mediated splanchnic vasoconstriction; effects on splanchnic flow debated (may improve if MAP increased from low baseline)
• Vasopressin: V1-mediated vasoconstriction; may reduce splanchnic flow; used in vasodilatory shock
• Dopamine: Low-dose "renal/splanchnic" dopamine not supported by evidence; higher doses → vasoconstriction
• Dobutamine: β1-mediated inotrope; may improve splanchnic flow by improving cardiac output

Splanchnic Monitoring in ICU:

• Gastric tonometry: Historical; measured pHi (mucosal pH) as marker of ischaemia
• Current practice: Indirect markers (lactate, pH, base deficit, clinical assessment)
• Near-infrared spectroscopy (NIRS): Experimental for splanchnic oxygenation
• Direct monitoring not routinely performed [57]

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Gastrointestinal Hormones

Major GI Hormones

Gastrointestinal hormones are peptides secreted by enteroendocrine cells that regulate motility, secretion, and absorption. They act via endocrine (bloodstream), paracrine (local), and neurocrine (neurotransmitter) pathways. [58,59]

Gastrin:

• Source: G cells (gastric antrum, duodenum)
• Stimulus: Amino acids, peptides, gastric distension, vagal stimulation (GRP)
• Actions: Stimulates gastric acid secretion (via ECL histamine release), stimulates gastric motility, trophic to gastric mucosa
• Inhibition: Gastric pH <3, somatostatin
• Clinical: Elevated in Zollinger-Ellison syndrome, PPI therapy, atrophic gastritis

Cholecystokinin (CCK):

• Source: I cells (duodenum, jejunum)
• Stimulus: Fatty acids (>12 carbons), amino acids, peptides
• Actions: Contracts gallbladder, relaxes sphincter of Oddi, stimulates pancreatic enzyme secretion, inhibits gastric emptying, satiety signal
• Receptors: CCK-A (peripheral), CCK-B (CNS and parietal cells, identical to gastrin receptor)
• Clinical: Mediates fat-induced satiety; CCK analogues studied for appetite suppression

Secretin:

• Source: S cells (duodenum)
• Stimulus: Duodenal pH <4.5, fatty acids
• Actions: Stimulates pancreatic bicarbonate secretion, inhibits gastric acid secretion, inhibits gastric emptying
• First hormone discovered (Bayliss and Starling, 1902)
• Clinical: Secretin stimulation test for diagnosis of gastrinoma

Gastric Inhibitory Polypeptide (GIP, Glucose-dependent Insulinotropic Polypeptide):

• Source: K cells (duodenum, jejunum)
• Stimulus: Glucose, fat, amino acids
• Actions: Stimulates insulin release (incretin effect), inhibits gastric acid secretion, inhibits gastric motility
• Clinical: Part of incretin system; GIP receptor agonists combined with GLP-1 agonists (tirzepatide) for diabetes/obesity

Glucagon-like Peptide-1 (GLP-1):

• Source: L cells (ileum, colon)
• Stimulus: Glucose, fatty acids
• Actions: Stimulates insulin release (incretin effect), inhibits glucagon secretion, delays gastric emptying, promotes satiety
• Clinical: GLP-1 receptor agonists (semaglutide, liraglutide) for diabetes and obesity; major drug class [60]

Motilin:

• Source: M cells (duodenum, jejunum)
• Stimulus: Fasting, duodenal alkalinisation
• Actions: Stimulates Phase III of MMC, accelerates gastric emptying
• Clinical: Erythromycin is a motilin receptor agonist; used as prokinetic

Ghrelin:

• Source: X/A cells (gastric fundus)
• Stimulus: Fasting (levels peak before meals)
• Actions: Stimulates appetite ("hunger hormone"), stimulates GH release, accelerates gastric emptying
• Antagonism: Ghrelin levels suppressed by feeding
• Clinical: Ghrelin mimetics studied for cachexia, gastroparesis

Somatostatin:

• Source: D cells (stomach, intestine, pancreas)
• Stimulus: Gastric acid, fat, glucose
• Actions: Inhibits gastrin, histamine, acid secretion, pancreatic secretion, intestinal motility; reduces splanchnic blood flow
• Clinical: Octreotide (somatostatin analogue) for variceal bleeding, carcinoid syndrome, acromegaly

The Incretin Effect

The incretin effect describes the observation that oral glucose produces a greater insulin response than intravenous glucose at the same blood glucose concentration. This is mediated by GLP-1 and GIP released from gut in response to nutrients. [60,61]

Clinical Significance:

• Incretins account for 50-70% of postprandial insulin secretion
• Incretin effect diminished in type 2 diabetes
• GLP-1 receptor agonists (semaglutide, liraglutide, dulaglutide) restore incretin effect
• DPP-4 inhibitors (sitagliptin, linagliptin) prevent incretin degradation
• Major therapeutic class for diabetes and obesity

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Gut-Brain Axis

Bidirectional Communication

The gut-brain axis describes the extensive bidirectional communication network between the gastrointestinal tract and the central nervous system. This involves neural, hormonal, and immune pathways. [19,20,62]

Components:

1. Vagal Pathway (Primary Neural Route):

• Afferent fibres (80%): Convey information from gut to brainstem (nucleus tractus solitarius)
• Sense stretch, nutrients, pH, osmolality, inflammatory mediators
• Efferent fibres (20%): Parasympathetic control of gut function
• Anti-inflammatory pathway: Vagal efferents → ACh → α7 nicotinic receptors on macrophages → reduced TNF-α release (cholinergic anti-inflammatory pathway)

2. Hormonal Pathway:

• Gut hormones (CCK, GLP-1, PYY, ghrelin) act on hypothalamus to regulate appetite
• Gut hormones influence mood, cognition, and stress responses
• Cortisol (HPA axis) affects gut function, permeability, microbiome

3. Immune Pathway:

• Cytokines produced in gut affect brain function
• Peripheral inflammation → sickness behaviour, cognitive impairment
• GALT is largest immune organ; communicates with systemic immunity

4. Microbiome-Brain Pathway:

• Gut bacteria produce neuroactive metabolites
• Short-chain fatty acids (SCFAs): Influence blood-brain barrier integrity, microglial function
• Neurotransmitters: Bacteria produce GABA, serotonin, dopamine precursors
• Tryptophan metabolism: Microbiome influences serotonin and kynurenine pathways

Clinical Implications in ICU

Gut-Brain Axis Disruption in Critical Illness:

• Altered consciousness → reduced vagal tone → ileus, barrier dysfunction
• Sedation → disrupts gut-brain signalling
• Antibiotics → dysbiosis → reduced SCFA production → barrier failure
• Systemic inflammation → sickness behaviour, delirium
• Stress response → HPA axis activation → gut dysfunction

Delirium and the Gut:

• Emerging evidence links gut dysfunction to ICU delirium
• Dysbiosis, endotoxaemia may contribute to neuroinflammation
• Probiotics studied for delirium prevention (limited evidence) [63]

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Gut Microbiome

Composition and Function

The human gut contains approximately 10^14 bacteria (10× human cells), comprising over 1000 species and encoding 150× more genes than the human genome. This complex ecosystem performs essential functions for host health. [64,65]

Composition:

• Dominant phyla: Firmicutes (30-50%), Bacteroidetes (25-40%), Actinobacteria, Proteobacteria, Verrucomicrobia
• Highly individual: "Fingerprint" of bacterial species unique to each person
• Influenced by: Diet, antibiotics, age, geography, genetics, disease states

Functions:

Short-Chain Fatty Acids (SCFAs)

SCFAs are the primary metabolic products of bacterial fermentation of dietary fibre. They have profound effects on gut and systemic health. [66]

Major SCFAs:

• Butyrate: Primary fuel for colonocytes (70% of energy); produced by Firmicutes
• Propionate: Absorbed to portal blood; substrate for hepatic gluconeogenesis
• Acetate: Absorbed systemically; substrate for lipogenesis and energy

Effects of Butyrate:

• Colonocyte fuel source (preferred over glucose)
• Strengthens tight junctions
• Anti-inflammatory (inhibits NF-κB)
• Promotes regulatory T cells (Tregs)
• Maintains colonisation resistance

Clinical Significance:

• Reduced SCFA production in dysbiosis → impaired barrier function
• Fibre-free enteral nutrition → reduced fermentation → reduced SCFAs
• C. difficile infection associated with loss of butyrate-producing bacteria

Dysbiosis in Critical Illness

Critical illness profoundly disrupts the gut microbiome, creating a state of dysbiosis that contributes to adverse outcomes. [67,68]

Causes of ICU Dysbiosis:

Consequences:

• Loss of colonisation resistance → pathogen overgrowth (C. difficile, VRE, ESBL organisms)
• Reduced SCFA production → barrier failure
• Increased permeability → bacterial translocation
• Altered immune function → susceptibility to sepsis

Potential Interventions:

• Probiotics: Limited evidence in ICU; some benefit for VAP prevention (certain strains)
• Prebiotics: Fibre supplementation to support beneficial bacteria
• Synbiotics: Combination of probiotics and prebiotics
• Faecal microbiota transplantation (FMT): Effective for recurrent C. difficile; studied for other indications
• Selective digestive decontamination (SDD): Controversial; reduces VAP but antibiotic resistance concerns [69]

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ICU Relevance: GI Dysfunction in Critical Illness

Ileus and GI Dysmotility

Gastrointestinal dysmotility is nearly universal in critically ill patients and significantly impacts nutrition delivery, infection risk, and outcomes. [70,71]

Definitions:

• Gastroparesis: Delayed gastric emptying without mechanical obstruction
• Ileus: Transient impairment of GI motility without mechanical cause
• Ogilvie syndrome: Acute colonic pseudo-obstruction; massively dilated colon without mechanical obstruction

Prevalence in ICU:

• Gastroparesis: 50-80% of mechanically ventilated patients
• Ileus: Present to some degree in nearly all ICU patients
• Clinically significant: 30-50% develop feeding intolerance

Causes:

Pathophysiology:

• Sympathetic activation → inhibits ENS and smooth muscle
• Inflammation → NO and VIP release → smooth muscle relaxation
• Opioids → μ-receptor activation → reduced ACh release
• Hyperglycaemia → delayed gastric emptying (glucose >10 mmol/L impairs motility)
• Catecholamines → direct smooth muscle inhibition

Consequences:

• Aspiration risk (gastric residuals, regurgitation)
• Enteral feeding intolerance → malnutrition
• Bacterial overgrowth → translocation, sepsis
• Abdominal distension → respiratory impairment
• Nausea, vomiting → patient discomfort

Management:

Prokinetic Agents:

General Measures:

• Correct electrolytes (K+, Mg2+, Ca2+, PO4)
• Optimise glycaemic control (target glucose <10 mmol/L)
• Minimise opioids (multimodal analgesia)
• Early mobilisation
• Bowel regimen (stool softeners, stimulant laxatives if no contraindication)
• Post-pyloric feeding if gastroparesis refractory [72]

Stress Ulceration

Stress-related mucosal disease (SRMD) is universal in critically ill patients, though clinically significant bleeding is less common with modern ICU care. [15,16,73]

Epidemiology:

• Endoscopic lesions: 75-100% of ICU patients within 24 hours
• Clinically significant bleeding: 1.5-6% (reduced with prophylaxis)
• Mortality with significant bleeding: Increased 2-4 fold

Risk Factors (ASHP Criteria):

• Major risk factors (prophylaxis recommended):
  • Mechanical ventilation >48 hours
  • Coagulopathy (platelets <50, INR >1.5, PTT >2× normal)
• Additional risk factors:
  • Sepsis, shock, burns >35% TBSA
  • Multiple trauma, head injury (Cushing ulcer)
  • Spinal cord injury
  • Hepatic failure, renal failure
  • History of GI bleeding
  • High-dose corticosteroids

Pathophysiology:

• Splanchnic hypoperfusion → mucosal ischaemia
• Reduced mucus and bicarbonate secretion
• Increased back-diffusion of H+ into mucosa
• Reperfusion injury with ROS generation
• Disrupted prostaglandin-mediated cytoprotection

Prophylaxis:

PPI vs H2RA Controversy:

• PPIs: More effective acid suppression; may have higher risk of C. difficile, pneumonia (debated)
• SUP-ICU trial (2018): Pantoprazole vs placebo; no difference in mortality, small reduction in clinically significant bleeding (PMID: 29668344)
• Current practice: Prophylaxis for high-risk patients; consider stopping when risk factors resolve [74,75]

Bacterial Translocation and Gut-Origin Sepsis

Bacterial translocation from the gut is increasingly recognised as a driver of systemic inflammation and sepsis in critically ill patients. [17,18,53]

Evidence:

• Endotoxaemia detected in 30-80% of critically ill patients
• Portal bacteraemia demonstrated in animal models and human studies
• Mesenteric lymph (not portal blood) may be primary route to systemic circulation
• "Gut hypothesis" of MODS: Gut as motor of systemic inflammation

Factors Promoting Translocation:

• Splanchnic hypoperfusion (shock, vasopressors)
• Barrier dysfunction (tight junction failure, epithelial apoptosis)
• Dysbiosis (antibiotics, acid suppression)
• Bowel rest (mucosal atrophy, reduced sIgA)
• Immunosuppression (reduced antimicrobial peptides)

Consequences:

• Systemic endotoxaemia → cytokine storm → SIRS
• Mesenteric lymph delivers gut-derived inflammatory mediators to pulmonary circulation → ARDS
• Portal bacteraemia → hepatic Kupffer cell activation
• Distant organ injury propagated by gut-derived inflammation

Prevention:

• Early enteral nutrition: Trophic effects, barrier maintenance
• Avoid unnecessary bowel rest
• Minimise unnecessary antibiotics
• Selective digestive decontamination: Reduces gram-negative colonisation (controversial)
• Optimise splanchnic perfusion: Avoid excessive vasoconstrictors

Intra-Abdominal Hypertension and Abdominal Compartment Syndrome

Intra-abdominal hypertension (IAH) and abdominal compartment syndrome (ACS) have profound effects on GI physiology. [76,77]

Definitions:

• Normal IAP: 5-7 mmHg
• IAH: Sustained IAP ≥12 mmHg
• ACS: IAP ≥20 mmHg with new organ dysfunction

Grading (WSACS):

GI Effects:

• Reduced mesenteric blood flow → mucosal ischaemia, bacterial translocation
• Reduced gastric perfusion → stress ulceration
• Increased gastric residuals → aspiration risk
• Reduced portal flow → hepatic dysfunction
• Impaired lymphatic drainage → oedema

Management:

• Non-operative: Nasogastric decompression, rectal tubes, prokinetics, diuretics, neuromuscular blockade
• Operative: Decompressive laparotomy if ACS with organ dysfunction

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Special Populations

Indigenous Health Considerations (Aboriginal and Torres Strait Islander)

Epidemiology:

• Higher rates of peptic ulcer disease and H. pylori infection
• Increased incidence of inflammatory bowel disease in some communities
• Higher rates of gastrointestinal cancers
• Limited access to specialist gastroenterology services in remote areas

Cultural Considerations:

• Dietary practices: Traditional foods may differ from "standard" enteral formulas
• Family involvement: Extended family involvement in care decisions
• Communication: Use of Aboriginal Health Workers (AHWs) and Aboriginal Liaison Officers (ALOs)
• End-of-life care: Cultural practices around death and dying
• Health literacy: Clear, simple explanations; visual aids

ICU-Specific Issues:

• Late presentation with GI emergencies
• Higher surgical mortality
• Challenges with post-discharge follow-up
• Need for culturally appropriate nutritional support [78]

Māori Health Considerations

Epidemiology:

• Higher rates of gastrointestinal cancers
• Increased H. pylori prevalence
• Higher rates of hepatitis B-related liver disease
• Health inequities in access to GI services

Cultural Concepts:

• Whānau (family): Central to care; collective decision-making
• Tikanga Māori (Māori customs): Respect for tapu (sacredness) of the body
• Whare tapa whā model: Four dimensions of health (physical, mental, spiritual, family)

Clinical Implications:

• Involve whānau in discussions about enteral feeding, procedures
• Respect cultural practices around food and eating
• Access to Māori spiritual advisors (kaumātua, tohunga) as requested
• Clear communication about GI procedures and interventions [79]

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Progressive Difficulty Assessments

Basic Level Questions

1. Name the two plexuses of the enteric nervous system and state their primary functions. Answer: Myenteric (Auerbach's) plexus - controls motility; Submucosal (Meissner's) plexus - controls secretion.

2. What are the three main stimulants of gastric acid secretion? Answer: Gastrin (G cells), histamine (ECL cells via H2 receptors), acetylcholine (vagus nerve via M3 receptors).

3. State the mechanism of action of proton pump inhibitors. Answer: Irreversible inhibition of H+/K+-ATPase (proton pump) on parietal cell apical membrane.

4. What is the primary route of long-chain fatty acid absorption? Answer: Lymphatic (chylomicrons enter lacteals → mesenteric lymphatics → thoracic duct → systemic circulation).

5. Name three components of the gut barrier. Answer: Mucus layer, epithelial cells with tight junctions, secretory IgA, antimicrobial peptides.

Intermediate Level Questions

1. Describe the phases of the migrating motor complex and explain its clinical significance. Answer: Phase I (quiescence, 45-60 min), Phase II (intermittent contractions, 30-45 min), Phase III (intense rhythmic contractions, 5-10 min - "housekeeper wave"), Phase IV (transition). Clinical significance: Clears debris and bacteria during fasting; disruption in critical illness leads to bacterial overgrowth and feeding intolerance.

2. Explain how secretin and CCK regulate pancreatic secretion. Answer: Secretin (from S cells, triggered by duodenal acid) stimulates ductal cell bicarbonate secretion. CCK (from I cells, triggered by fatty acids and amino acids) stimulates acinar cell enzyme secretion and gallbladder contraction.

3. Describe the pathophysiology of bacterial translocation in critical illness. Answer: Splanchnic hypoperfusion → mucosal ischaemia → tight junction dysfunction + dysbiosis (antibiotics) + mucosal atrophy (bowel rest) → impaired barrier function → passage of bacteria and endotoxin to mesenteric lymph and portal blood → systemic inflammation and sepsis.

4. Explain why shunted blood in cirrhosis leads to elevated ammonia levels. Answer: Normally, ammonia from gut bacteria is carried via portal blood to liver for detoxification via urea cycle. In cirrhosis, portosystemic shunting bypasses the liver, allowing ammonia to enter systemic circulation directly. Combined with reduced hepatocyte mass for urea synthesis, this leads to hyperammonaemia and hepatic encephalopathy.

Exam Level Questions

1. A critically ill patient on vasopressors develops bloody diarrhoea and elevated lactate. Describe the pathophysiology and outline your investigation and management approach. Answer: This suggests non-occlusive mesenteric ischaemia (NOMI). Pathophysiology: Prolonged splanchnic vasoconstriction in shock + vasopressor effects → mucosal ischaemia → barrier failure, bloody diarrhoea, lactate elevation. Investigation: CT angiography (exclude occlusive disease), lactate trend, ABG (metabolic acidosis). Management: Optimise cardiac output, reduce vasopressors if possible, avoid enteral feeding, consider angiography with papaverine infusion, surgical consultation (may require laparotomy if peritonitis develops).

2. Discuss the physiological rationale for using prokinetic agents in critical illness and compare the mechanisms of metoclopramide and erythromycin. Answer: Rationale: Critical illness causes gastroparesis (50-80%) due to opioids, catecholamines, hyperglycaemia, and inflammation → feeding intolerance, aspiration risk. Metoclopramide: D2 receptor antagonist (removes dopamine inhibition of GI motility), 5-HT4 agonist (stimulates ACh release from myenteric neurons) → coordinated antroduodenal contractions. Erythromycin: Motilin receptor agonist → mimics Phase III MMC → gastric emptying. Both develop tachyphylaxis; combination may be more effective.

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SAQ Practice

SAQ 1: Gastric Acid Secretion (15 marks)

Question: A 65-year-old woman is admitted to ICU with severe sepsis requiring mechanical ventilation. She is started on stress ulcer prophylaxis.

(a) Describe the mechanism of gastric acid secretion by the parietal cell. (4 marks) (b) Outline the regulation of gastric acid secretion, including the three phases and the major stimulants and inhibitors. (5 marks) (c) Compare the mechanisms of action, efficacy, and potential adverse effects of proton pump inhibitors and H2 receptor antagonists in the ICU setting. (4 marks) (d) Describe the pathophysiology of stress ulceration in critically ill patients and outline which patients should receive prophylaxis. (2 marks)

Model Answer:

(a) Mechanism of Gastric Acid Secretion (4 marks)

The parietal cell secretes hydrochloric acid at a concentration of 150 mmol/L (pH 1-2), representing a 3-million-fold H+ concentration gradient against plasma.

Key steps:

1. H+/K+-ATPase (proton pump): Located on apical (canalicular) membrane; exchanges H+ for K+ using ATP; rate-limiting step for acid secretion (1 mark)
2. Cl- secretion: Chloride channels on apical membrane secrete Cl- into lumen; Cl- enters cell via basolateral Cl-/HCO3- exchanger (1 mark)
3. HCO3- generation: Carbonic anhydrase catalyses: H2O + CO2 → H+ + HCO3-; H+ secreted apically; HCO3- exits basolaterally (alkaline tide) (1 mark)
4. Stimulation mechanism: Receptor activation (H2, M3, CCK-B) → fusion of tubulovesicular membrane containing H+/K+-ATPase with canalicular membrane → increased pump expression on secretory surface (1 mark)

(b) Regulation of Gastric Acid Secretion (5 marks)

Three phases:

• Cephalic phase (30%): Triggered by sight, smell, taste of food; mediated by vagus nerve releasing ACh; also stimulates gastrin release via GRP (1 mark)
• Gastric phase (60%): Triggered by gastric distension and amino acids/peptides in antrum; mediated by gastrin release from G cells, local ACh (1 mark)
• Intestinal phase (10%): Triggered by duodenal distension and amino acids; intestinal gastrin release; primarily inhibitory phase (enterogastric reflex, secretin) (1 mark)

Stimulants (classic triad):

• Gastrin: From G cells; acts via CCK-B receptors; primarily stimulates ECL histamine release
• Histamine: From ECL cells; acts via H2 receptors on parietal cells; potentiates gastrin and ACh
• Acetylcholine: From vagus and ENS; acts via M3 receptors; stimulates acid and histamine/gastrin release (1 mark)

Inhibitors:

• Somatostatin: From D cells; inhibits G cells, ECL cells, and parietal cells
• Prostaglandins (PGE2): Inhibit parietal cells, stimulate mucus/bicarbonate
• Secretin: From S cells; released by duodenal acid; inhibits gastric acid
• Low gastric pH: Negative feedback on gastrin release (1 mark)

(c) PPI vs H2RA Comparison (4 marks)

(1 mark for mechanism comparison, 1 mark for efficacy comparison)

Adverse effects:

• PPIs: C. difficile infection (controversial), CAP risk (controversial), hypomagnesaemia, vitamin B12 deficiency, altered drug absorption (clopidogrel interaction debated)
• H2RAs: Tachyphylaxis, confusion/delirium (particularly cimetidine), thrombocytopenia, drug interactions (cimetidine CYP450 inhibition) (1 mark)

ICU evidence: SUP-ICU trial (2018) showed pantoprazole vs placebo had no mortality difference; PPIs may provide marginally better protection against clinically significant bleeding (1 mark)

(d) Stress Ulceration Pathophysiology and Prophylaxis (2 marks)

Pathophysiology: Splanchnic hypoperfusion → mucosal ischaemia → reduced mucus and bicarbonate secretion → back-diffusion of H+ into mucosa → epithelial injury. Reperfusion generates ROS → further damage. Impaired prostaglandin-mediated cytoprotection in critical illness exacerbates injury. (1 mark)

Prophylaxis indications (ASHP criteria):

• Mechanical ventilation >48 hours
• Coagulopathy (platelets <50, INR >1.5, PTT >2× normal)
• Additional risk factors: Burns >35% TBSA, head injury, spinal cord injury, sepsis, high-dose steroids, history of GI bleeding (1 mark)

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SAQ 2: GI Motility and Ileus (15 marks)

Question: A 58-year-old man is day 5 post-emergency laparotomy for perforated diverticulitis. He is developing abdominal distension and high nasogastric aspirates despite attempted commencement of enteral feeding.

(a) Describe the control of gastrointestinal motility, including the role of the enteric nervous system and extrinsic innervation. (4 marks) (b) Describe the migrating motor complex, including its phases, regulatory mechanisms, and function. (3 marks) (c) Explain the pathophysiology of postoperative ileus and list six contributing factors in this patient. (4 marks) (d) Outline your pharmacological and non-pharmacological management strategies for this patient. (4 marks)

Model Answer:

(a) Control of GI Motility (4 marks)

Enteric Nervous System (ENS):

• "Second brain" with ~100 million neurons
• Myenteric (Auerbach's) plexus: Between circular and longitudinal muscle layers; primary control of motility; coordinates peristalsis and segmentation (1 mark)
• Submucosal (Meissner's) plexus: Controls secretion and blood flow; modulates epithelial function (0.5 mark)
• Operates autonomously but modulated by extrinsic innervation

Neurotransmitters:

• Excitatory: Acetylcholine, substance P, serotonin (5-HT)
• Inhibitory: VIP, nitric oxide (NO), ATP
• Peristalsis requires coordinated contraction behind bolus (ACh) and relaxation ahead (NO, VIP) (0.5 mark)

Extrinsic Innervation:

• Parasympathetic (vagus, pelvic nerves): Generally stimulatory; increases motility and secretion; "rest and digest" (1 mark)
• Sympathetic (T5-L2): Generally inhibitory; decreases motility; contracts sphincters; vasoconstriction; "fight or flight" predominates in stress/shock (1 mark)

(b) Migrating Motor Complex (3 marks)

Phases:

• Phase I (45-60 min): Quiescence, minimal contractions
• Phase II (30-45 min): Irregular, intermittent contractions
• Phase III (5-10 min): Intense, rhythmic contractions (11-12/min) - "housekeeper wave"
• Phase IV (0-5 min): Transition to Phase I or fed pattern
• Cycle duration: 90-120 minutes (1 mark)

Regulation:

• Motilin: Primary hormone; released from duodenal M cells during Phase III; stimulates gastric and duodenal contraction
• Vagal innervation: Required for normal MMC; vagotomy disrupts pattern
• Erythromycin: Motilin receptor agonist; therapeutic application (1 mark)

Function:

• Clears residual debris and bacteria during fasting
• Prevents bacterial overgrowth in small intestine
• Abolished by feeding (conversion to digestive pattern)
• Absence in critical illness contributes to bacterial overgrowth, malabsorption, feeding intolerance (1 mark)

(c) Pathophysiology and Contributing Factors (4 marks)

Pathophysiology of postoperative ileus:

• Surgical manipulation → local inflammatory response → macrophage activation → iNOS upregulation → NO release → smooth muscle relaxation
• Sympathetic activation → catecholamine release → inhibits ENS and smooth muscle
• Opioid analgesia → μ-receptor activation in ENS → reduced ACh release → reduced motility
• Bowel oedema → impaired neuromuscular transmission
• Electrolyte disturbance → impaired smooth muscle contractility (2 marks)

Contributing factors in this patient (6 required):

1. Surgical manipulation and peritoneal inflammation (peritonitis)
2. Opioid analgesia (μ-receptor mediated)
3. Sepsis/systemic inflammation
4. Catecholamine/vasopressor use
5. Electrolyte disturbance (hypokalaemia, hypomagnesaemia)
6. Hyperglycaemia
7. Residual anaesthetic effects
8. Immobility
9. Possible intra-abdominal abscess/collection
10. Bowel oedema (2 marks for 6 factors)

(d) Management Strategies (4 marks)

Pharmacological:

• Metoclopramide 10 mg IV 6-8 hourly: D2 antagonist, 5-HT4 agonist; improves gastric emptying
• Erythromycin 100-250 mg IV 8 hourly: Motilin receptor agonist; potent prokinetic; tachyphylaxis limits use to 3-5 days
• Consider combination therapy for synergistic effect
• Neostigmine 2 mg IV (for acute colonic pseudo-obstruction/Ogilvie syndrome only; requires cardiac monitoring) (2 marks)

Non-pharmacological:

• Correct electrolytes: K+ >4 mmol/L, Mg2+ >0.8 mmol/L, Ca2+, PO4
• Optimise glycaemic control: Target glucose <10 mmol/L
• Minimise opioids: Multimodal analgesia (paracetamol, regional techniques, ketamine)
• Early mobilisation: Ambulation if possible; physiotherapy
• Nasogastric decompression: Reduces distension, prevents aspiration
• Consider post-pyloric feeding: Jejunal tube if gastroparesis refractory
• Exclude mechanical obstruction: CT if clinical concern
• Bowel regimen: Stool softeners, stimulant laxatives when appropriate (2 marks)

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Viva Scenarios

Viva 1: Gastric Acid Secretion and Stress Ulcer Prophylaxis

Examiner: Tell me about the mechanism of gastric acid secretion.

Candidate: Gastric acid is secreted by parietal cells in the gastric fundus. The key enzyme is H+/K+-ATPase, or the proton pump, located on the apical membrane. This pump uses ATP to exchange intracellular H+ for luminal K+, generating a massive concentration gradient - the lumen reaches pH 1-2 while blood pH is 7.4.

The H+ ions come from intracellular carbonic anhydrase catalysing CO2 + H2O → H+ + HCO3-. The bicarbonate exits via a basolateral Cl-/HCO3- exchanger, creating the postprandial "alkaline tide" in venous blood. Chloride is secreted through apical chloride channels to combine with H+ forming HCl.

Examiner: How is acid secretion regulated?

Candidate: Regulation occurs through three phases - cephalic, gastric, and intestinal - and involves three main stimulants forming the "classic triad": gastrin, histamine, and acetylcholine.

Gastrin is released from G cells in the antrum in response to amino acids and gastric distension. It acts primarily by stimulating ECL cells to release histamine, which then acts on H2 receptors on parietal cells.

Histamine from ECL cells is the key amplifier - it potentiates the effects of both gastrin and acetylcholine. The H2 receptor activates adenylyl cyclase, increasing cAMP.

Acetylcholine from the vagus nerve acts on M3 muscarinic receptors, increasing intracellular calcium. It also stimulates gastrin and histamine release.

The main inhibitor is somatostatin from D cells, which inhibits all three stimulants. Low gastric pH provides negative feedback on gastrin release.

Examiner: Which drugs target this pathway?

Candidate: The major drug classes are:

Proton pump inhibitors like omeprazole and pantoprazole irreversibly inhibit H+/K+-ATPase. They're prodrugs that accumulate in the acidic parietal cell canaliculus and are converted to active form. Because inhibition is irreversible, effects last 24-48 hours until new pumps are synthesised. They reduce acid secretion by 90-99%.

H2 receptor antagonists like famotidine competitively block histamine at H2 receptors. They reduce acid secretion by 50-70% but are subject to tachyphylaxis with continuous use.

Examiner: What's the current evidence for stress ulcer prophylaxis in ICU?

Candidate: The major recent trial is SUP-ICU from 2018, which compared pantoprazole 40mg daily to placebo in ICU patients at risk of GI bleeding. The primary outcome was 90-day mortality, which showed no significant difference. There was a small reduction in clinically significant GI bleeding with PPIs, but the absolute rates were low - around 2.5% with pantoprazole versus 4.2% with placebo.

Current guidelines recommend prophylaxis for high-risk patients - specifically those on mechanical ventilation for more than 48 hours or with coagulopathy. Other risk factors include burns, head injury, sepsis, and high-dose steroids.

The debate continues about PPIs versus H2RAs - there's concern that acid suppression may increase C. difficile infection and pneumonia risk, though the evidence is conflicting.

Examiner: If a patient develops significant GI bleeding despite prophylaxis, how would you manage them?

Candidate: I'd follow a structured approach:

First, resuscitate - establish good IV access, give blood products guided by haemodynamics and haemoglobin, correct coagulopathy with FFP, platelets, and consider tranexamic acid.

Second, high-dose PPI therapy - typically a PPI bolus followed by continuous infusion to maintain gastric pH above 6.

Third, early endoscopy within 24 hours for diagnosis and therapy - endoscopic haemostasis with clips, thermal coagulation, or injection therapy.

Fourth, if endoscopic therapy fails, consider interventional radiology with angiographic embolisation, or surgical intervention as a last resort.

Finally, address modifiable risk factors - optimise coagulation, avoid NSAIDs, consider the role of ongoing stress from the underlying critical illness.

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Viva 2: Gut Barrier Function and Bacterial Translocation

Examiner: Describe the structure of the gut barrier.

Candidate: The gut barrier is a multi-layered defence system that permits nutrient absorption while excluding pathogens. It has four main components:

First, the mucus layer - a gel-like barrier secreted by goblet cells containing MUC2 mucin. The inner layer is dense and largely bacteria-free, while the outer layer is colonised by commensals.

Second, the epithelial layer - a single layer of columnar cells including enterocytes, goblet cells, Paneth cells (which secrete antimicrobial peptides), and enteroendocrine cells. This layer has remarkable turnover, replacing itself every 3-5 days.

Third, tight junctions between epithelial cells - protein complexes including claudins, occludins, and zonula occludens proteins that seal the paracellular space. Different claudin subtypes create either sealing or pore-forming functions.

Fourth, immune components including secretory IgA, antimicrobial peptides like defensins, and the gut-associated lymphoid tissue.

Examiner: What is bacterial translocation and why is it clinically important?

Candidate: Bacterial translocation is the passage of viable bacteria or bacterial products like endotoxin from the intestinal lumen to normally sterile sites - including mesenteric lymph nodes, portal blood, and ultimately systemic circulation.

It's clinically important because it can trigger and perpetuate systemic inflammation. The gut has been called the "motor of MODS" because barrier failure allows gut-derived inflammatory mediators to reach the systemic circulation.

In critical illness, multiple factors promote translocation: splanchnic hypoperfusion causes mucosal ischaemia and tight junction dysfunction; antibiotics disrupt the normal microbiome reducing colonisation resistance; bowel rest leads to mucosal atrophy and reduced secretory IgA.

The consequences include portal endotoxaemia which activates hepatic Kupffer cells releasing cytokines, direct bacteraemia leading to sepsis, and the propagation of SIRS and multi-organ dysfunction.

Examiner: How does splanchnic hypoperfusion contribute to barrier failure?

Candidate: Splanchnic circulation receives about 25% of cardiac output, but in shock states, blood is redistributed away from the gut to maintain perfusion to heart and brain.

The gut is particularly vulnerable because splanchnic vasoconstriction is disproportionately greater than in other vascular beds. Additionally, the mucosa has higher metabolic demands and the villus tip is in a watershed zone with counter-current exchange making it susceptible to hypoxia.

When hypoperfusion occurs:

• Epithelial cells become ATP-depleted, causing tight junction dysfunction
• The glycocalyx is damaged
• Mucus production decreases
• Enterocyte apoptosis increases
• Antimicrobial peptide secretion is impaired

Then when reperfusion occurs, ischaemia-reperfusion injury generates reactive oxygen species through xanthine oxidase activation, neutrophils are recruited, complement is activated, and paradoxically more damage occurs.

Examiner: What strategies might prevent or minimise bacterial translocation in ICU patients?

Candidate: The key strategies include:

Early enteral nutrition - this is probably the most important intervention. Enteral feeding maintains epithelial integrity, stimulates mucus and IgA production, and preserves the normal microbiome. Even small volumes of trophic feeding have beneficial effects on the gut.

Avoiding unnecessary bowel rest - if the gut works, use it. Parenteral nutrition should be reserved for patients with true intestinal failure.

Optimising splanchnic perfusion - this means adequate fluid resuscitation and judicious use of vasopressors. There's ongoing debate about which vasopressors are least harmful to splanchnic circulation.

Antimicrobial stewardship - minimising unnecessary antibiotics preserves the microbiome and colonisation resistance.

Selective digestive decontamination - this is controversial but involves topical and systemic antibiotics to reduce pathogenic gram-negative colonisation while preserving anaerobes. It reduces VAP but there are concerns about antibiotic resistance.

Probiotics - there's limited evidence in ICU populations, with some benefit shown for VAP prevention with specific strains, but concerns about safety in immunocompromised patients.

Glutamine supplementation - this was previously recommended as glutamine is essential for enterocyte metabolism, but the REDOXS trial showed harm with high-dose IV glutamine in critically ill patients, so routine supplementation is no longer recommended.

Examiner: Excellent. That covers the key points well.

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ZO-1, ZO-2, ZO-3), JAMs - seal paracellular space

12. Q: What percentage of cardiac output goes to splanchnic circulation? A: ~25% (1500-2000 mL/min)

13. Q: What is non-occlusive mesenteric ischaemia (NOMI)? A: Mesenteric ischaemia without arterial/venous occlusion, caused by prolonged splanchnic vasoconstriction in low-flow states

14. Q: What is bacterial translocation? A: Passage of viable bacteria or bacterial products (endotoxin) from intestinal lumen to normally sterile sites

15. Q: What stimulates secretin release? A: Duodenal pH <4.5 (from S cells in duodenal mucosa)

16. Q: What are the main actions of CCK? A: Gallbladder contraction, sphincter of Oddi relaxation, pancreatic enzyme secretion, delayed gastric emptying, satiety

17. Q: What is the incretin effect? A: Greater insulin response to oral glucose vs IV glucose at same blood glucose level, mediated by GLP-1 and GIP

18. Q: What are short-chain fatty acids (SCFAs)? A: Bacterial fermentation products of dietary fibre: butyrate (colonocyte fuel), propionate, acetate

19. Q: What are the major risk factors for stress ulcer prophylaxis? A: Mechanical ventilation >48 hours, coagulopathy (platelets <50, INR >1.5)

20. Q: What is the gut surface area? A: ~32 m² (size of a tennis court), amplified 600× by folds, villi, and microvilli

Clinical Reasoning (15 cards)

21. Q: Why does erythromycin work as a prokinetic? A: Motilin receptor agonist → mimics Phase III of MMC → stimulates gastric and duodenal contractions

22. Q: Why does fat slow gastric emptying? A: CCK release → pyloric contraction + fundal relaxation; enterogastric reflex; matches delivery to digestive capacity

23. Q: How do PPIs differ from H2RAs in mechanism? A: PPIs irreversibly inhibit H+/K+-ATPase (90-99% acid suppression, 24-48h duration); H2RAs competitively block H2 receptors (50-70%, 6-12h duration, tachyphylaxis)

24. Q: Why does bowel rest promote bacterial translocation? A: Mucosal atrophy, reduced sIgA production, decreased mucus, impaired tight junctions, loss of trophic effects of enteral nutrients

25. Q: Why are medium-chain triglycerides (MCTs) useful in fat malabsorption? A: Absorbed directly to portal blood (not requiring chylomicron formation/lymphatic drainage) - bypasses bile salt and lipase requirements

26. Q: How does hyperglycaemia impair GI motility? A: Delayed gastric emptying begins at glucose >10 mmol/L; mechanism involves reduced vagal tone and direct smooth muscle effects

27. Q: Why does vomiting cause hypochloraemic metabolic alkalosis? A: Loss of gastric H+ and Cl- → metabolic alkalosis; Cl- loss prevents renal HCO3- excretion (contraction alkalosis); K+ loss from secondary hyperaldosteronism

28. Q: Why does diarrhoea cause hyperchloraemic metabolic acidosis? A: Loss of intestinal HCO3- (secreted in exchange for Cl- in colon) → net loss of base → metabolic acidosis with normal anion gap

29. Q: How does sepsis impair gut barrier function? A: Splanchnic hypoperfusion, cytokine-mediated tight junction dysfunction, oxidative stress, reduced mucus production, dysbiosis from antibiotics

30. Q: Why might post-pyloric feeding be beneficial in gastroparesis? A: Bypasses gastric stasis, allowing nutrient delivery to functioning small bowel; reduces aspiration risk from high gastric residuals

31. Q: How do opioids cause ileus? A: μ-receptor activation on myenteric neurons → reduced ACh release → decreased smooth muscle contraction → slowed transit

32. Q: Why is the villus tip vulnerable to ischaemia? A: Counter-current exchange between villus arteriole and venule creates oxygen gradient; tip receives lowest oxygen delivery (watershed zone)

33. Q: What is the rationale for early enteral nutrition? A: Maintains epithelial integrity, stimulates mucus/IgA production, preserves microbiome, prevents mucosal atrophy, reduces bacterial translocation

34. Q: How does Ogilvie syndrome differ from mechanical obstruction? A: Ogilvie = acute colonic pseudo-obstruction (functional, no mechanical cause); may respond to neostigmine (acetylcholinesterase inhibitor) vs surgical intervention for mechanical obstruction

35. Q: Why does terminal ileum resection cause fat malabsorption? A: Loss of bile salt reabsorption (ASBT transporter) → depleted bile salt pool → inadequate micelle formation → steatorrhoea

Guidelines and Evidence (15 cards)

36. Q: What did the SUP-ICU trial (2018) show? A: Pantoprazole vs placebo in ICU patients: no mortality difference; small reduction in clinically significant GI bleeding (2.5% vs 4.2%); PMID: 29668344

37. Q: What are ASHP criteria for stress ulcer prophylaxis? A: Major: Mechanical ventilation >48h, coagulopathy. Additional: Burns >35% TBSA, head/spinal injury, sepsis, hepatic/renal failure, high-dose steroids, GI bleeding history

38. Q: What is the evidence for metoclopramide + erythromycin combination? A: Synergistic effect on gastric emptying; may be more effective than either alone for gastroparesis (limited RCT evidence)

39. Q: What is the half-life of gastric emptying for solids vs liquids? A: Liquids: 15-20 minutes; Solids: 60-120 minutes

40. Q: What MMC phase duration should you know for exams? A: Cycle 90-120 min; Phase I: 45-60 min; Phase II: 30-45 min; Phase III: 5-10 min; Phase IV: 0-5 min

41. Q: What is the prevalence of gastroparesis in mechanically ventilated patients? A: 50-80%

42. Q: What is the normal gastric acid output? A: Basal: 2-5 mmol/hour; Maximal (stimulated): 20-40 mmol/hour; Volume: 2-3 L/day

43. Q: What is the gastric pH and H+ concentration? A: pH 1-2; H+ concentration ~150 mmol/L

44. Q: What neostigmine dose is used for Ogilvie syndrome? A: 2 mg IV slowly with cardiac monitoring (risk of bradycardia)

45. Q: What is the bile acid pool size and cycling frequency? A: Pool: 2-4 g; Cycles: 6-12 times/day; Daily secretion: 12-36 g (recycled); Faecal loss: 5% (~500-600 mg/day)

46. Q: What are the major GI fluid volumes? A: Input: Oral 2L, saliva 1.5L, gastric 2-3L, bile 0.5-1L, pancreas 1-2L, intestinal 1-2L = 8-10L total; Absorbed: 8-9L; Stool: 0.1-0.2L

47. Q: What is the clinical significance of Qs/Qt >30% in shunt? A: Severe shunt; associated with increased mortality; refractory to supplemental oxygen; may require ECMO if refractory

48. Q: What defines intra-abdominal hypertension and abdominal compartment syndrome? A: IAH: Sustained IAP ≥12 mmHg; ACS: IAP ≥20 mmHg with new organ dysfunction

49. Q: What is the prevalence of stress-related mucosal lesions on endoscopy? A: 75-100% of ICU patients within 24 hours (endoscopic lesions); Clinically significant bleeding: 1.5-6%

50. Q: What is the mortality associated with clinically significant stress ulcer bleeding? A: 2-4 fold increased mortality compared to patients without bleeding

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