Electrolyte Physiology

Quick Answer

Answer: Electrolyte physiology encompasses the regulation of sodium, potassium, calcium, magnesium, phosphate, and chloride ions that are essential for cellular function, neuromuscular activity, and acid-base balance. Sodium (135-145 mmol/L) is the primary extracellular cation controlling plasma osmolality via ADH, RAAS, and natriuretic peptides. Potassium (3.5-5.0 mmol/L) is 98% intracellular, with the Na-K-ATPase pump maintaining the resting membrane potential; shifts occur with insulin, catecholamines, and pH changes. Calcium exists as ionised (1.1-1.3 mmol/L, 45%) and protein-bound forms, regulated by PTH, vitamin D, and calcitonin, with critical effects on cardiac contractility and QT interval. Magnesium (0.7-1.0 mmol/L) is essential for enzymatic functions and influences potassium and calcium handling. Phosphate (0.8-1.5 mmol/L) is critical for ATP synthesis, 2,3-DPG, and cell membrane structure. Chloride is the major extracellular anion affecting acid-base balance through strong ion difference.

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CICM Exam Focus

What Examiners Expect

First Part Written SAQ:

• Draw and label the Na-K-ATPase pump mechanism (commonly examined)
• Explain the relationship between sodium, water, and ADH in hyponatraemia
• Describe factors causing intracellular-extracellular potassium shifts
• Outline the role of PTH, vitamin D, and calcitonin in calcium homeostasis
• Explain the renal handling of each major electrolyte by nephron segment

First Part Viva:

• Detailed knowledge of transport mechanisms at molecular level
• Integration of hormonal control systems (RAAS, ADH, PTH)
• ECG changes associated with electrolyte abnormalities
• Applied physiology to clinical scenarios

High-Yield Topics

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Key Points

1. Sodium is the primary determinant of extracellular osmolality; plasma Na+ reflects water balance rather than sodium balance (PMID: 25656170)

2. Na-K-ATPase maintains electrochemical gradients by pumping 3 Na+ out and 2 K+ into cells, consuming ~40% of resting cellular ATP (PMID: 12655054)

3. ADH (vasopressin) acts via V2 receptors to insert aquaporin-2 channels in collecting duct, enabling water reabsorption (PMID: 25656170)

4. Potassium is 98% intracellular (140-150 mmol/L ICF vs 3.5-5.0 mmol/L ECF), creating the -90mV resting membrane potential (PMID: 26189426)

5. Insulin and beta-agonists shift potassium intracellularly via Na-K-ATPase stimulation; acidosis causes potassium efflux (PMID: 15312219)

6. Ionised calcium (1.1-1.3 mmol/L) is the physiologically active fraction; each 1 g/L decrease in albumin lowers total calcium by 0.02 mmol/L (PMID: 24461462)

7. PTH increases serum calcium via bone resorption, renal reabsorption, and vitamin D activation (PMID: 16291984)

8. Magnesium deficiency causes refractory hypokalaemia (impairs Na-K-ATPase) and hypocalcaemia (impairs PTH secretion) (PMID: 25023984)

9. Phosphate is critical for ATP, 2,3-DPG, and membrane phospholipids; severe deficiency causes respiratory failure (PMID: 25023984)

10. Chloride is a strong ion affecting acid-base balance; hyperchloraemic acidosis occurs with 0.9% saline resuscitation (PMID: 11733215)

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Sodium Physiology

Normal Values and Distribution

Normal plasma sodium: 135-145 mmol/L (reference range varies slightly by laboratory)

Total body sodium: Approximately 60 mmol/kg (4,200 mmol in 70 kg adult)

Distribution:

Intracellular sodium concentration: 10-14 mmol/L (maintained low by Na-K-ATPase)

Key Concept: Plasma sodium concentration reflects the ratio of total body sodium to total body water, not absolute sodium content. Hyponatraemia typically reflects relative water excess rather than sodium deficit (PMID: 25656170).

Na-K-ATPase Pump

The Na-K-ATPase (sodium-potassium adenosine triphosphatase) is a P-type ATPase enzyme found in the plasma membrane of virtually all animal cells.

Structure (PMID: 12655054, 17855526):

• Alpha subunit (α): Catalytic subunit containing ATP binding site, Na+ and K+ binding sites, and phosphorylation site
• Beta subunit (β): Glycoprotein required for proper folding and membrane insertion
• Gamma subunit (γ/FXYD): Tissue-specific regulatory subunit modulating pump kinetics

Mechanism (E1-E2 Model):

1. E1 State: High affinity for Na+, low affinity for K+

   • 3 Na+ ions bind intracellularly
   • ATP binds to α subunit

2. Phosphorylation: ATP hydrolysis → ADP + Pi

   • Aspartate residue phosphorylated
   • Conformational change to E2 state

3. E2 State: Low affinity for Na+, high affinity for K+

   • 3 Na+ released extracellularly
   • 2 K+ bind extracellularly

4. Dephosphorylation:

   • Conformational change back to E1
   • 2 K+ released intracellularly
   • Cycle repeats (~100 cycles/second)

Stoichiometry: 3 Na+ out : 2 K+ in per ATP hydrolysed

Electrogenicity: Net export of one positive charge per cycle, contributing ~10 mV to resting membrane potential (PMID: 12655054)

Energy Consumption: Consumes 20-40% of cellular ATP at rest; up to 70% in neurons (PMID: 17855526)

Clinical Significance:

• Digoxin inhibits Na-K-ATPase by binding to extracellular K+ site
• Hypothermia reduces pump activity
• Insulin and catecholamines stimulate pump activity
• Hypokalaemia inhibits pump activity
• Magnesium is essential cofactor for ATPase activity

Regulation of Sodium Balance

Daily Sodium Turnover:

• Intake: 100-200 mmol/day (Western diet)
• Obligatory losses: Sweat (10-40 mmol/day), faeces (5 mmol/day)
• Renal excretion: 90-95% of intake (adjusted by RAAS)

Renal Sodium Handling by Nephron Segment:

Key Regulatory Hormones:

Antidiuretic Hormone (ADH/Vasopressin)

Synthesis and Storage (PMID: 25656170):

• Synthesised in magnocellular neurons of supraoptic and paraventricular nuclei of hypothalamus
• Transported via axons to posterior pituitary (neurohypophysis)
• Stored in neurosecretory vesicles

Release Stimuli:

Receptor Subtypes and Actions:

Aquaporin-2 Mechanism (PMID: 16239463):

1. ADH binds V2 receptor on basolateral membrane
2. Adenylyl cyclase activation → cAMP production
3. Protein kinase A activation
4. Aquaporin-2 vesicle phosphorylation
5. Vesicle trafficking to apical membrane
6. Fusion with membrane → water channels inserted
7. Water reabsorption down osmotic gradient into hypertonic medullary interstitium

Water Permeability:

• Without ADH: Collecting duct impermeable → dilute urine (50-100 mOsm/kg)
• With maximal ADH: Collecting duct highly permeable → concentrated urine (up to 1,200 mOsm/kg)

Renin-Angiotensin-Aldosterone System (RAAS)

Renin Release (PMID: 18723481):

Renin is secreted by juxtaglomerular (JG) cells of afferent arteriole in response to:

1. Decreased renal perfusion pressure: Intrarenal baroreceptors in afferent arteriole
2. Decreased NaCl delivery to macula densa: Sensed via NKCC2 transporter activity
3. Increased sympathetic activity: β1-adrenergic receptor stimulation
4. Low potassium: Inhibits renin; high potassium stimulates

RAAS Cascade:

Angiotensinogen (liver)
        ↓ Renin (kidney)
Angiotensin I (10 amino acids)
        ↓ ACE (lung, endothelium)
Angiotensin II (8 amino acids)
        ↓
    ┌───────────┼───────────┐
    ↓           ↓           ↓
  AT1         AT1         AT1
Vasocon-    Aldosterone  Proximal
striction   release      Na+ reab-
                         sorption

Angiotensin II Effects (PMID: 18723481):

Aldosterone (PMID: 15613620):

• Synthesised in zona glomerulosa of adrenal cortex
• Stimulated by: Angiotensin II, hyperkalaemia, ACTH (minor)
• Inhibited by: ANP, hypokalaemia, hyponatraemia (mild effect)

Aldosterone Mechanism:

1. Enters principal cells of collecting duct
2. Binds mineralocorticoid receptor (MR) in cytoplasm
3. Hormone-receptor complex translocates to nucleus
4. Transcription of target genes (effect takes 1-2 hours):
   • SGK1 (serum-glucocorticoid kinase)
   • ENaC subunits
   • Na-K-ATPase subunits
5. Net effect: Increased Na+ reabsorption, K+ and H+ secretion

Natriuretic Peptides (ANP, BNP)

Atrial Natriuretic Peptide (ANP) (PMID: 22536115):

• Secreted by atrial myocytes in response to atrial stretch
• 28 amino acid peptide stored in granules
• Plasma half-life: 2-3 minutes

B-type Natriuretic Peptide (BNP):

• Secreted by ventricular myocytes in response to wall stress
• 32 amino acid peptide
• Plasma half-life: 20 minutes
• Clinical marker of heart failure (NT-proBNP more stable)

Natriuretic Peptide Actions (via NPR-A receptor, cGMP):

Osmoreceptors and Thirst Mechanism

Osmoreceptors (PMID: 25656170):

Located in circumventricular organs lacking blood-brain barrier:

• Organum vasculosum of lamina terminalis (OVLT)
• Subfornical organ (SFO)
• Median preoptic nucleus

Mechanism:

• Cells shrink in hyperosmolar environment
• Mechanical stretch-activated channels (TRPV1, TRPV4) detect volume change
• Depolarisation triggers ADH release and thirst

Sensitivity:

• ADH release threshold: ~280-285 mOsm/kg
• Thirst threshold: ~295 mOsm/kg
• 1-2% change in osmolality detected

Set Point Modifiers:

• Pregnancy: Lower threshold (reset osmolstat)
• Ageing: Higher threshold, blunted response
• Hypovolaemia: Left-shifts relationship (ADH released at lower osmolality)

Thirst Mechanism:

• Perceived when osmolality exceeds ~295 mOsm/kg or significant hypovolaemia
• Mediated by anterior cingulate cortex and insula
• Inhibited by oropharyngeal sensors before absorption (anticipatory)
• Ultimate defense against hypernatraemia

Hyponatraemia Pathophysiology

Definition: Plasma sodium <135 mmol/L

Classification by Mechanism (PMID: 25656170):

1. Dilutional Hyponatraemia (Excess Water Relative to Sodium)

Causes:

• SIADH (Syndrome of Inappropriate ADH)
• Hypothyroidism (reduced free water excretion)
• Glucocorticoid deficiency (ADH disinhibition)
• Psychogenic polydipsia (overwhelms excretion capacity)
• Beer potomania (low solute intake)
• Post-surgical ADH release (pain, nausea, hypovolaemia)
• Drug-induced SIADH (SSRIs, carbamazepine, oxytocin)

Pathophysiology:

• ADH causes water retention via aquaporin-2
• Dilution of ECF sodium
• Total body sodium normal or increased
• ECF volume normal or expanded

SIADH Criteria (Bartter-Schwartz) (PMID: 25656170):

1. Plasma osmolality <275 mOsm/kg
2. Urine osmolality >100 mOsm/kg (inappropriately concentrated)
3. Urine sodium >30 mmol/L (if adequate sodium intake)
4. Euvolaemia (clinically)
5. Normal thyroid and adrenal function
6. No diuretics

2. Depletional Hyponatraemia (Sodium Loss > Water Loss)

Causes:

Pathophysiology:

• Primary sodium loss → hypovolaemia
• Hypovolaemia stimulates ADH release (non-osmotic, baroreceptor-mediated)
• Water retention dilutes remaining sodium
• Volume takes priority over osmolality regulation

Thiazide-Induced Hyponatraemia:

• Inhibits NCC in DCT
• Does NOT impair concentrating ability (unlike loop diuretics)
• Continued water reabsorption in collecting duct (if ADH present)
• Risk factors: elderly, female, low body weight

3. Redistributive Hyponatraemia

Causes:

• Hyperglycaemia: Each 5.5 mmol/L rise in glucose decreases Na+ by ~1.6-2.4 mmol/L (water shifts from ICF to ECF)
• Mannitol, glycine (irrigation fluids)
• Pseudohyponatraemia: Severe hyperlipidaemia, hyperproteinaemia (artefact with indirect ion-selective electrodes)

Corrected Sodium for Hyperglycaemia: Corrected Na+ = Measured Na+ + [1.6 × (Glucose - 5.5) / 5.5] mmol/L

Or simplified: Add 2.4 mmol/L per 5.5 mmol/L glucose above normal

Hypernatraemia Pathophysiology

Definition: Plasma sodium >145 mmol/L

Always indicates relative water deficit (hyperosmolar state)

Classification (PMID: 26189426):

Diabetes Insipidus (DI) (PMID: 29490171):

Pathophysiology of Central DI:

• Destruction of >80% magnocellular neurons or posterior pituitary
• No ADH release despite hyperosmolality
• Collecting duct remains impermeable
• Dilute urine (up to 20 L/day if unrestricted)
• Hypernatraemia if water intake inadequate

Nephrogenic DI Mechanisms:

• Lithium: Enters principal cells via ENaC, downregulates aquaporin-2 expression
• Hypercalcaemia: Reduces NKCC2 activity, impairs medullary gradient
• Hypokalaemia: Downregulates aquaporin-2, increases PGE2 (antagonises ADH)
• Genetic: V2 receptor mutations, aquaporin-2 mutations

Cerebral Adaptation to Hypernatraemia:

• Hours: Loss of electrolytes from brain cells (K+, Na+)
• Days: Generation of idiogenic osmoles (organic osmolytes: taurine, myo-inositol, glutamine)
• Rapid correction risks cerebral oedema (osmoles cannot be quickly eliminated)

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Potassium Physiology

Normal Values and Distribution

Normal plasma potassium: 3.5-5.0 mmol/L

Total body potassium: ~3,500 mmol (50-55 mmol/kg)

Distribution (PMID: 26189426):

This 30-40:1 gradient is maintained by the Na-K-ATPase pump and is essential for the resting membrane potential.

Resting Membrane Potential

Goldman-Hodgkin-Katz Equation (PMID: 26189426):

The resting membrane potential (Em) depends on the relative permeabilities and concentrations of K+, Na+, and Cl-:

Em = (RT/F) × ln{(PK[K+]o + PNa[Na+]o + PCl[Cl-]i) / (PK[K+]i + PNa[Na+]i + PCl[Cl-]o)}

Simplified (K+ dominant at rest):

At rest, membrane is 50-100 times more permeable to K+ than Na+:

Em ≈ -61 × log([K+]i/[K+]o) = -61 × log(140/4) = -90 mV (approximately)

Clinical Significance:

• Hypokalaemia hyperpolarises membrane (more negative) → decreased excitability initially, then paradoxical arrhythmias
• Hyperkalaemia depolarises membrane → initial increased excitability, then inexcitability (depolarisation block)

Renal Potassium Handling

Daily Potassium Balance:

• Intake: 40-120 mmol/day
• Renal excretion: 80-90% of intake
• Faecal excretion: 5-10 mmol/day (can increase in renal failure)

Nephron Segment Handling (PMID: 15312219):

Principal Cell K+ Secretion:

1. Basolateral Na-K-ATPase: Pumps K+ into cell, Na+ out
2. Apical ROMK channels: K+ exits down concentration gradient into lumen
3. Apical BK (big K) channels: Flow-activated, contribute to K+ secretion with high flow rates
4. Driving force: Lumen-negative potential (created by ENaC Na+ reabsorption)

Factors Affecting Renal K+ Excretion:

Factors Shifting Potassium Between Compartments

Transcellular shifts can rapidly alter plasma K+ without changing total body K+ (PMID: 15312219, 26189426):

Clinical Application - Insulin + Glucose for Hyperkalaemia:

• Insulin (10 units IV) with dextrose 50% (25-50g)
• Onset: 15-30 minutes
• Peak effect: 30-60 minutes
• Duration: 4-6 hours
• Expected K+ reduction: 0.5-1.2 mmol/L

Cardiac Effects of Potassium Abnormalities

Hypokalaemia (<3.5 mmol/L) (PMID: 26189426):

ECG Changes (in order of severity):

1. T wave flattening (earliest)
2. ST segment depression
3. U waves (after T wave, same direction)
4. Prolonged QT interval (QU interval)
5. Increased P wave amplitude
6. PR prolongation
7. Widened QRS (severe)

Arrhythmias:

• Atrial and ventricular ectopy
• Atrial fibrillation
• Torsades de pointes (especially with long QT)
• Ventricular tachycardia/fibrillation (severe cases)

Mechanism:

• Hyperpolarisation increases maximum diastolic potential
• Paradoxically, delayed afterdepolarisations and triggered activity
• Decreased K+ conductance prolongs repolarisation

Hyperkalaemia (>5.5 mmol/L) (PMID: 26189426):

ECG Changes (progressive):

Mechanism:

• Depolarisation of resting membrane potential
• Inactivation of sodium channels
• Slowed conduction velocity
• Eventually depolarisation block and asystole

Calcium Gluconate for Hyperkalaemia:

• 10 mL of 10% calcium gluconate IV over 2-3 minutes
• Does NOT lower K+ level
• Stabilises cardiac membrane by restoring difference between resting and threshold potential
• Onset: 1-3 minutes
• Duration: 30-60 minutes
• Repeat if ECG changes persist

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Calcium Physiology

Normal Values and Distribution

Total body calcium: ~1,000 g (25,000 mmol), 99% in bone

Plasma Calcium (PMID: 24461462):

Total calcium: 2.1-2.6 mmol/L

Protein Binding and pH Effects

Albumin Correction (PMID: 24461462):

Each 1 g/L decrease in albumin below 40 g/L decreases protein-bound calcium by ~0.02 mmol/L

Corrected Ca2+ = Measured Ca2+ + 0.02 × (40 - Albumin in g/L)

Example: Total Ca2+ 2.0 mmol/L, Albumin 20 g/L Corrected Ca2+ = 2.0 + 0.02 × (40 - 20) = 2.4 mmol/L

pH Effects:

• Acidosis: H+ competes for albumin binding sites → increased ionised Ca2+
• Alkalosis: Decreased H+ → more albumin binding sites available → decreased ionised Ca2+
• 0.1 pH unit change alters ionised Ca2+ by ~0.03-0.05 mmol/L

Clinical Implication: Hyperventilation-induced respiratory alkalosis can precipitate symptomatic hypocalcaemia (tetany) even with normal total calcium.

Hormonal Regulation

Parathyroid Hormone (PTH)

Synthesis (PMID: 16291984):

• 84 amino acid peptide synthesised by chief cells of parathyroid glands
• Stored in secretory granules
• Half-life: 2-4 minutes

Regulation:

• Calcium-sensing receptor (CaSR) on chief cells
• Low ionised Ca2+ → decreased CaSR activation → increased PTH release
• High ionised Ca2+ → increased CaSR activation → suppressed PTH

PTH Actions (Net Effect: ↑ Plasma Calcium):

PTH Receptor (PTH1R):

• G-protein coupled receptor
• Coupled to both Gs (cAMP) and Gq (IP3/DAG) pathways
• Present in bone and kidney

Vitamin D

Synthesis Pathway (PMID: 18689389):

Skin (UVB radiation)
7-dehydrocholesterol → Vitamin D3 (cholecalciferol)
                            ↓
Liver (25-hydroxylase, CYP2R1)
                            ↓
25(OH)D (calcidiol) - Major circulating form, half-life 2-3 weeks
                            ↓
Kidney (1α-hydroxylase, CYP27B1) - PTH stimulates, FGF23 inhibits
                            ↓
1,25(OH)2D (calcitriol) - Active hormone, half-life 4-6 hours

1,25(OH)2D Actions (Net Effect: ↑ Plasma Calcium and Phosphate):

Regulation:

• PTH: Stimulates 1α-hydroxylase
• Low phosphate: Stimulates 1α-hydroxylase
• FGF23: Inhibits 1α-hydroxylase
• 1,25(OH)2D: Negative feedback on its own synthesis

Calcitonin

Synthesis (PMID: 16291984):

• 32 amino acid peptide
• Secreted by parafollicular C cells of thyroid
• Released in response to hypercalcaemia

Actions (Net Effect: ↓ Plasma Calcium):

• Inhibits osteoclast activity (rapid, transient)
• Increases renal Ca2+ excretion
• Relatively weak effect compared to PTH and vitamin D

Clinical Significance:

• Calcitonin levels elevated in medullary thyroid carcinoma (tumour marker)
• Therapeutic use in acute hypercalcaemia (rapid but transient effect)
• Used in Paget's disease

Intestinal Calcium Absorption

Total daily calcium intake: 800-1,200 mg (20-30 mmol) Net absorption: 200-400 mg/day (25-35%)

Mechanisms (PMID: 18689389):

1. Transcellular (active, regulated):

• Predominant in duodenum and proximal jejunum
• Vitamin D-dependent
• Entry: TRPV6 channels (apical membrane)
• Cytosolic transport: Calbindin-D9k (binds Ca2+, prevents toxicity)
• Exit: Plasma membrane Ca2+-ATPase (PMCA1b), Na+/Ca2+ exchanger (NCX1)

2. Paracellular (passive, concentration-dependent):

• Throughout small intestine
• Driven by electrochemical gradient
• Predominates at high calcium intakes
• Vitamin D-independent

Renal Calcium Handling

Filtered load: ~250 mmol/day (ionised + complexed fractions) Urinary excretion: 2.5-7.5 mmol/day (1-3% of filtered)

Nephron Handling (PMID: 26131931):

TRPV5 Channel (distal tubule):

• Apical Ca2+ entry channel
• Regulated by PTH (increases open probability)
• Calbindin-D28k shuttles Ca2+ to basolateral membrane
• Exit via PMCA, NCX1 exchangers

Loop Diuretics and Calcium:

• Inhibit NKCC2 → abolish lumen-positive potential
• Reduced paracellular Ca2+ reabsorption in TAL
• Net effect: Increased Ca2+ excretion (calciuresis)
• Used therapeutically in acute hypercalcaemia

Thiazides and Calcium:

• Inhibit NCC in DCT → volume depletion
• Volume depletion → increased proximal Na+ and Ca2+ reabsorption
• Also direct stimulation of TRPV5 in DCT
• Net effect: Decreased Ca2+ excretion (hypocalciuria)
• Used in hypercalciuric stone disease

Cardiac Effects of Calcium

Hypocalcaemia (ionised <1.0 mmol/L):

ECG Changes:

• Prolonged QT interval (specifically QTc prolongation due to ST segment prolongation)
• T wave may be normal or inverted

Mechanism:

• Calcium affects phase 2 plateau of cardiac action potential
• Reduced Ca2+ influx prolongs plateau duration
• Risk of torsades de pointes with severe prolongation

Clinical Features:

• Neuromuscular: Paraesthesia, tetany, Chvostek's sign, Trousseau's sign
• Cardiovascular: Hypotension, heart failure
• Neurological: Seizures, altered mental status

Hypercalcaemia (ionised >1.3 mmol/L):

ECG Changes:

• Shortened QT interval (shortened ST segment)
• Widened T wave
• Osborn waves (J waves) in severe cases
• Bradycardia, heart block (severe)

Mechanism:

• Increased Ca2+ entry accelerates repolarisation
• Reduced action potential duration
• At extreme levels, conduction abnormalities

Clinical Features:

• "Stones, bones, groans, and psychiatric moans"
• Polyuria, polydipsia (nephrogenic DI-like effect)
• Constipation, nausea
• Weakness, fatigue
• Confusion, coma

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Magnesium Physiology

Normal Values and Distribution

Normal plasma magnesium: 0.7-1.0 mmol/L (1.7-2.4 mg/dL)

Total body magnesium: ~1,000 mmol (24 g)

Distribution (PMID: 25023984):

Plasma Fractions:

Physiological Functions

Magnesium is a cofactor for >300 enzymes (PMID: 25023984):

Renal Magnesium Handling

Filtered load: ~100 mmol/day Urinary excretion: 3-5 mmol/day (adjustable 0.5-70%)

Nephron Handling (PMID: 25023984):

TRPM6 Channel (DCT):

• Major apical Mg2+ entry pathway
• Hormone-regulated (EGF, insulin, oestrogen increase expression)
• Mutations cause familial hypomagnesaemia

Factors Increasing Renal Mg2+ Excretion:

Relationship with Calcium and Potassium

Magnesium and Calcium (PMID: 25023984):

• Mg2+ required for PTH secretion (severe hypomagnesaemia → functional hypoparathyroidism)
• Mg2+ also required for PTH action at receptor level
• Severe hypomagnesaemia → hypocalcaemia refractory to calcium replacement
• Must correct Mg2+ first

Magnesium and Potassium (PMID: 15312219):

• Mg2+ is cofactor for Na-K-ATPase
• Hypomagnesaemia → impaired K+ uptake into cells
• Also increases ROMK channel activity → K+ wasting
• Results in refractory hypokalaemia
• Must replace Mg2+ for K+ repletion to be effective

Clinical Pearl: Always check and correct magnesium when treating refractory hypokalaemia or hypocalcaemia.

Clinical Features of Magnesium Disorders

Hypomagnesaemia (<0.7 mmol/L):

Causes:

• Reduced intake (malnutrition, alcoholism, TPN without Mg2+)
• GI losses (diarrhoea, malabsorption, PPI use)
• Renal losses (diuretics, aminoglycosides, cisplatin, amphotericin B)
• Redistribution (refeeding, insulin therapy)

Clinical Features:

• Neuromuscular: Tremor, tetany, seizures
• Cardiovascular: Arrhythmias (especially torsades de pointes), ECG changes
• Metabolic: Refractory hypokalaemia, hypocalcaemia

ECG Changes:

• Prolonged PR interval
• Prolonged QT interval
• T wave flattening or inversion
• Predisposes to digoxin toxicity

Hypermagnesaemia (>1.0 mmol/L):

Causes:

• Renal failure (most common)
• Excessive administration (MgSO4 for pre-eclampsia, antacids, laxatives)
• Adrenal insufficiency

Clinical Features (progressive with increasing levels):

Treatment of Severe Hypermagnesaemia:

• IV calcium gluconate (antagonises Mg2+ at neuromuscular junction)
• IV fluids and frusemide (if renal function preserved)
• Dialysis (if renal failure)

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Phosphate Physiology

Normal Values and Distribution

Normal plasma phosphate: 0.8-1.5 mmol/L (varies with age; higher in children)

Total body phosphate: ~700 g (23,000 mmol)

Distribution (PMID: 25023984):

Plasma Forms:

• 85% as HPO42- and H2PO4- (at pH 7.4, ratio ~4:1)
• 10% protein-bound
• 5% complexed with Ca2+, Mg2+

Physiological Functions

Phosphate is essential for (PMID: 25023984):

Hormonal Regulation

PTH and Phosphate:

• Decreases proximal tubule phosphate reabsorption
• Net effect: Phosphaturia (lowers plasma phosphate)
• Increases bone resorption (releases phosphate, but net effect is phosphaturia)

Vitamin D and Phosphate:

• Increases intestinal phosphate absorption
• Increases renal phosphate reabsorption
• Net effect: Increases plasma phosphate

FGF23 (Fibroblast Growth Factor 23) (PMID: 28322636):

• 32 kDa peptide produced by osteocytes
• Released in response to high phosphate, high vitamin D
• Requires Klotho as co-receptor

FGF23 Actions:

• Decreases phosphate reabsorption in proximal tubule (via NaPi-IIa, NaPi-IIc)
• Inhibits 1α-hydroxylase → decreases 1,25(OH)2D synthesis
• Suppresses PTH secretion
• Net effect: Phosphaturia, decreased plasma phosphate

Phosphate Regulatory Axis:

High Phosphate Intake
        ↓
   ↑ FGF23 (osteocytes)
        ↓
   ↓ NaPi-IIa/IIc
   ↓ 1α-hydroxylase
        ↓
   Phosphaturia
   ↓ Intestinal absorption
        ↓
   Plasma phosphate normalises

Renal Phosphate Handling

Filtered load: ~200 mmol/day Urinary excretion: 20-50 mmol/day (adjustable)

Nephron Handling (PMID: 28322636):

NaPi-IIa Cotransporter:

• Apical membrane of proximal tubule
• 3 Na+ : 1 HPO42- (electrogenic)
• PTH causes endocytosis and degradation
• FGF23 also causes internalisation

Tubular Maximum for Phosphate (TmP/GFR):

• Maximum phosphate reabsorption rate normalised to GFR
• Normal: 0.8-1.35 mmol/L
• Reflects density of NaPi transporters
• TmP/GFR calculated from fasting phosphate and fractional excretion

Refeeding Syndrome

Definition: Life-threatening metabolic disturbance occurring when nutrition is reinitiated in malnourished patients (PMID: 18583681)

Pathophysiology:

During starvation:

• Glycogen stores depleted within 24-48 hours
• Fat and protein catabolism predominate
• Intracellular phosphate, potassium, magnesium depleted (lost in urine)
• Total body stores reduced despite normal plasma levels

Upon refeeding (especially with carbohydrate):

1. Insulin release → glucose uptake
2. Anabolic state → phosphorylation reactions
3. Massive intracellular shift of PO43-, K+, Mg2+
4. Precipitous drop in plasma levels

Clinical Consequences of Hypophosphataemia:

Risk Factors (NICE criteria):

• BMI <16 kg/m2
• Weight loss >15% in 3-6 months
• Little/no nutritional intake >10 days
• Low phosphate, potassium, or magnesium pre-feeding
• History of alcohol abuse, anorexia nervosa, malignancy

Prevention:

• Start feeds slowly (10-20 kcal/kg/day)
• Check and replace electrolytes before feeding
• Monitor PO43-, K+, Mg2+ daily for first week
• Thiamine supplementation (500 mg IV pre-feeding)
• Gradually increase calories over 4-7 days

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Chloride Physiology

Normal Values and Distribution

Normal plasma chloride: 96-106 mmol/L

Total body chloride: ~2,000 mmol

Distribution:

Chloride is the major extracellular anion, balancing sodium (major cation).

Renal Chloride Handling

Chloride handling parallels sodium in most nephron segments:

Strong Ion Physiology and Acid-Base

Stewart Approach (PMID: 11733215):

Plasma pH is determined by three independent variables:

1. PCO2 (respiratory component)
2. Strong Ion Difference (SID) (metabolic component)
3. Total weak acids (Atot) (albumin, phosphate)

Strong Ion Difference: SID = [Na+] + [K+] + [Ca2+] + [Mg2+] - [Cl-] - [lactate-] - [other anions]

Simplified: SID ≈ [Na+] - [Cl-] ≈ 38-42 mEq/L

Effect of Chloride on pH:

• High Cl- → Low SID → Acidosis
• Low Cl- → High SID → Alkalosis

Hyperchloraemic Metabolic Acidosis

Causes (PMID: 11733215, 23622821):

1. Saline Infusion:

• 0.9% NaCl: Na+ 154 mmol/L, Cl- 154 mmol/L (SID = 0)
• Plasma SID ~40 mEq/L
• Dilution of plasma SID → acidosis
• Effect proportional to volume infused

2. Renal Tubular Acidosis (RTA):

3. Gastrointestinal HCO3- Loss:

• Diarrhoea (HCO3--rich fluid)
• Pancreatic/biliary fistulae
• Ureterosigmoidostomy

4. Post-Hypocapnic State:

• Chronic respiratory alkalosis → renal HCO3- wasting
• If ventilation normalised rapidly → transient hyperchloraemic acidosis

Clinical Implications

0.9% Saline vs Balanced Crystalloids (PMID: 29485925, 29485926):

SMART Trial (2018): Balanced crystalloids vs saline in non-critically ill adults SALT-ED Trial (2018): Similar in ICU patients

Key Findings:

• Balanced crystalloids reduced composite of death, new RRT, or persistent renal dysfunction (MAKE30)
• Greatest benefit in sepsis, large-volume resuscitation
• Current recommendation: Prefer balanced crystalloids for routine resuscitation

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Clinical Applications - ICU Electrolyte Emergencies

Severe Hyponatraemia (<120 mmol/L)

Presentation:

• Confusion, seizures, coma
• Cerebral oedema (acute cases)
• Respiratory arrest

Pathophysiology of Cerebral Oedema:

• Acute hyponatraemia → ECF hypoosmolar relative to brain ICF
• Water enters brain cells → swelling
• Risk of uncal herniation

Management (PMID: 25656170):

If symptomatic (seizures, severe symptoms):

1. 3% Hypertonic Saline: 100-150 mL bolus over 10-20 minutes
2. Repeat if symptoms persist (up to 3 boluses)
3. Target: 4-6 mmol/L increase in first 1-2 hours
4. Do NOT exceed: 8-10 mmol/L in first 24 hours

Rate of Correction:

• Risk of osmotic demyelination syndrome (ODS) if corrected too rapidly
• High risk: Chronic hyponatraemia, hypokalaemia, alcoholism, malnutrition, liver disease
• If over-correction occurs: Lower Na+ with desmopressin + D5W

Osmotic Demyelination Syndrome (ODS):

• Demyelination of pons (central pontine myelinolysis) and/or extrapontine structures
• Presents 2-6 days after correction
• Features: Dysarthria, dysphagia, quadriparesis, "locked-in" syndrome
• Prevention: Slow correction, monitor Na+ frequently

Severe Hyperkalaemia (>6.5 mmol/L)

ECG Changes Present or K+ >6.5 mmol/L:

Immediate Management (PMID: 26189426):

Calcium Gluconate:

• Does NOT lower K+ level
• Restores normal gradient between resting and threshold potential
• Repeat if ECG changes persist
• Caution in digoxin toxicity (can precipitate arrhythmias)

Symptomatic Hypocalcaemia

Clinical Features:

• Perioral paraesthesia, carpopedal spasm
• Chvostek's sign (facial twitching with nerve tap)
• Trousseau's sign (carpal spasm with BP cuff inflation)
• Seizures, laryngospasm
• QT prolongation, arrhythmias

Management (PMID: 24461462):

Acute Symptomatic:

1. Calcium gluconate 10%: 10-20 mL (2.2-4.4 mmol elemental Ca2+) IV over 10-20 minutes
2. Continuous infusion: 0.5-2.0 mg/kg/hr elemental calcium
3. Monitor ionised calcium and ECG

Calcium gluconate vs Calcium chloride:

Also:

• Correct hypomagnesaemia (impairs PTH secretion and action)
• Oral calcium and vitamin D for maintenance
• Treat underlying cause

Severe Hypophosphataemia (<0.32 mmol/L)

Clinical Features:

• Respiratory failure (diaphragm weakness)
• Cardiac dysfunction, arrhythmias
• Encephalopathy, seizures
• Rhabdomyolysis, haemolysis
• Leukocyte/platelet dysfunction

Management (PMID: 25023984):

Severe (<0.32 mmol/L) or Symptomatic:

1. IV Phosphate: 0.08-0.16 mmol/kg over 6 hours
2. Potassium phosphate or sodium phosphate solutions
3. Monitor phosphate, calcium (precipitation risk), potassium
4. Maximum rate: 0.5 mmol/kg/day

Moderate (0.32-0.65 mmol/L):

• Oral replacement: Phosphate-Sandoz (500 mg = 16 mmol phosphate) 1-2 tabs TDS
• Cow's milk (30 mmol/L phosphate)

Caution:

• Rapid IV phosphate can cause hypocalcaemia (precipitation)
• Risk of metastatic calcification if calcium-phosphate product >4.4 (mmol/L)2

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Australian and New Zealand Context

Indigenous Health Considerations

Aboriginal and Torres Strait Islander Populations (PMID: 19619041):

Higher Prevalence of:

• Chronic kidney disease (3-5 times higher)
• Type 2 diabetes (2-3 times higher)
• Hypertension (1.5-2 times higher)
• Electrolyte disturbances related to CKD

Contributing Factors:

• Remote community challenges (access to healthcare, dialysis)
• Food insecurity (reduced fresh food availability)
• High salt intake (processed foods)
• Reduced access to clean water in some communities
• Lower rates of CKD diagnosis and treatment

Cultural Considerations:

• Involve Aboriginal Health Workers (AHWs) and Aboriginal Liaison Officers (ALOs)
• Family-centred decision making (extended family involvement)
• "Sorry Business" may affect attendance and treatment adherence
• Preference for "Country" (returning to homelands when seriously ill)
• Communication through yarning (conversational storytelling approach)

Māori Health (New Zealand):

• Higher rates of diabetes and CKD
• Whānau (extended family) involvement in health decisions
• Respect for kaumātua (elders) in communication
• Tikanga (cultural practices) may influence treatment acceptance
• Te Tiriti o Waitangi obligations for equitable care

Remote and Rural Challenges

Retrieval Medicine Considerations:

• Limited laboratory access (point-of-care testing critical)
• Delayed transport (hours to tertiary centre)
• Telemedicine consultation for electrolyte emergencies
• Pre-hospital treatment protocols
• RFDS (Royal Flying Doctor Service) involvement

Point-of-Care Electrolytes:

• iSTAT, ABL analysers provide rapid results
• Essential for remote emergency departments
• Calibration and quality control challenges

ANZICS Guidelines

Fluid Resuscitation (ANZICS CORE):

• Preference for balanced crystalloids over 0.9% saline
• Albumin 4% considered equivalent to crystalloid for resuscitation
• Avoid starches (hydroxyethyl starch) due to AKI risk

Electrolyte Replacement Protocols:

• Institutional protocols for standardised replacement
• Weight-based dosing for potassium, magnesium, phosphate
• Monitoring intervals during replacement

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SAQ Practice Questions

SAQ 1: Hypokalaemia Pathophysiology (15 marks)

Question: A 45-year-old woman is admitted to ICU with severe vomiting for 5 days. Her investigations show:

• Serum K+ 2.2 mmol/L
• pH 7.52
• PaCO2 45 mmHg
• HCO3- 36 mmol/L
• Urine K+ 45 mmol/L (spot sample)

(a) Explain the pathophysiological mechanisms causing hypokalaemia in this patient. (6 marks)

(b) Describe the expected ECG changes with this degree of hypokalaemia. (3 marks)

(c) Outline why this patient may have refractory hypokalaemia despite potassium replacement. What additional electrolyte should be checked? (3 marks)

(d) Calculate the expected potassium deficit and describe your replacement strategy. (3 marks)

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Model Answer:

(a) Mechanisms of Hypokalaemia (6 marks)

1. Gastrointestinal Losses (1 mark):

• Gastric fluid K+ content: 5-10 mmol/L (relatively low)
• Vomiting causes volume depletion and metabolic alkalosis
• Direct GI loss is minor contributor

2. Renal Potassium Wasting (3 marks):

• Volume depletion activates RAAS → aldosterone → K+ secretion
• Increased distal Na+ delivery (from HCO3- reaching collecting duct) → increased K+ secretion
• Metabolic alkalosis: Bicarbonaturia requires cation (Na+, K+) for electrical balance
• High urine K+ (45 mmol/L) confirms renal wasting

3. Transcellular Shift (2 marks):

• Metabolic alkalosis causes H+ efflux from cells for buffering
• K+ enters cells for electroneutrality
• Worsens measured plasma K+ beyond total body deficit

(b) ECG Changes (3 marks)

At K+ 2.2 mmol/L (severe hypokalaemia):

• T wave flattening or inversion (1 mark)
• ST segment depression (0.5 mark)
• Prominent U waves (>1 mm, after T wave) (0.5 mark)
• Prolonged QT interval (actually QU interval) (0.5 mark)
• Risk of atrial and ventricular arrhythmias, torsades de pointes (0.5 mark)

(c) Refractory Hypokalaemia (3 marks)

Check Magnesium (1 mark):

• Hypomagnesaemia commonly coexists with hypokalaemia
• Vomiting causes Mg2+ losses

Mechanism of Magnesium Effect (2 marks):

• Mg2+ is essential cofactor for Na-K-ATPase
• Hypomagnesaemia → impaired cellular K+ uptake
• Hypomagnesaemia also increases ROMK channel activity → increased renal K+ secretion
• Must correct Mg2+ for effective K+ repletion

(d) Replacement Strategy (3 marks)

Deficit Estimation (1 mark):

• Each 1 mmol/L decrease below 4 mmol/L ≈ 100-200 mmol total body deficit
• K+ 2.2 = ~180-360 mmol deficit (plus ongoing losses)

Replacement (2 marks):

• IV potassium chloride: Maximum 20 mmol/hour via central line (10 mmol/hour peripheral)
• Oral supplementation if tolerating (slow-K, Span K)
• Monitor K+ every 2-4 hours during replacement
• Replace magnesium simultaneously: MgSO4 20 mmol IV over 4-6 hours
• Target K+ >4.0 mmol/L before reducing monitoring frequency

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SAQ 2: Sodium Regulation and ADH (15 marks)

Question: A 60-year-old man is admitted with small cell lung cancer and serum sodium 118 mmol/L. He is asymptomatic. Investigations show:

• Plasma osmolality 248 mOsm/kg
• Urine osmolality 580 mOsm/kg
• Urine sodium 65 mmol/L

(a) What is the likely diagnosis? Explain the pathophysiology. (4 marks)

(b) Describe the normal physiological mechanisms that regulate ADH secretion. (5 marks)

(c) Explain why rapid correction of hyponatraemia is dangerous. What is the target rate of correction? (4 marks)

(d) Outline your management plan for this patient. (2 marks)

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Model Answer:

(a) Diagnosis and Pathophysiology (4 marks)

Diagnosis: SIADH secondary to small cell lung cancer (1 mark)

Evidence:

• Hypotonic hyponatraemia (plasma osmolality 248 mOsm/kg) (0.5 mark)
• Inappropriately concentrated urine (580 mOsm/kg when should be maximally dilute) (0.5 mark)
• Urine sodium >30 mmol/L (inappropriate natriuresis) (0.5 mark)
• Clinical euvolaemia (no signs of hyper/hypovolaemia) (0.5 mark)

Pathophysiology (1 mark):

• Small cell lung carcinoma secretes ectopic ADH (tumour cells have neuroendocrine features)
• Autonomous ADH release independent of osmolality
• Continued water reabsorption despite hypoosmolality
• Dilutional hyponatraemia with normal/expanded ECF volume

(b) Normal ADH Regulation (5 marks)

Synthesis and Storage (1 mark):

• Synthesised in magnocellular neurons of supraoptic and paraventricular nuclei of hypothalamus
• Transported via axons to posterior pituitary
• Stored in neurosecretory vesicles

Osmotic Regulation (2 marks):

• Osmoreceptors in OVLT and SFO (circumventricular organs, no BBB)
• Detect changes in plasma osmolality via mechanosensitive channels (TRPV1, TRPV4)
• Threshold for ADH release: ~280-285 mOsm/kg
• Sensitivity: 1-2% change in osmolality detected
• Rising osmolality → increasing ADH release → water retention

Volume/Pressure Regulation (1 mark):

• Baroreceptors in carotid sinus, aortic arch, left atrium
• Detect volume depletion or hypotension
• Less sensitive: Requires 10-15% volume change
• Overrides osmotic control (volume takes priority)

Other Stimuli (1 mark):

• Nausea (potent stimulus)
• Pain, stress
• Angiotensin II (potentiates ADH release)
• Drugs (morphine, SSRIs)

(c) Dangers of Rapid Correction (4 marks)

Osmotic Demyelination Syndrome (ODS) (2 marks):

• Also called central pontine myelinolysis (CPM)
• Demyelination of pons and/or extrapontine structures
• Occurs when brain cells shrink rapidly due to rising ECF osmolality
• Presents 2-6 days after correction: dysarthria, dysphagia, quadriparesis, "locked-in" syndrome
• Often irreversible

Why It Occurs (1 mark):

• In chronic hyponatraemia, brain adapts by losing organic osmolytes (taurine, myo-inositol, glutamine)
• These cannot be replaced rapidly when osmolality rises
• Brain cells shrink → oligodendrocyte damage → demyelination

Safe Correction Rate (1 mark):

• Maximum 8-10 mmol/L in first 24 hours
• Maximum 8 mmol/L per 24 hours in high-risk patients
• If asymptomatic: 4-6 mmol/L per 24 hours preferred
• Monitor sodium 4-6 hourly during active correction

(d) Management Plan (2 marks)

Acute Management (1 mark):

• Fluid restriction: 1-1.5 L/day (first-line for euvolaemic SIADH)
• Stop any medications causing SIADH
• Monitor sodium 4-6 hourly

If Fluid Restriction Fails (1 mark):

• Salt tablets + frusemide ("vaptan" effect)
• Tolvaptan (V2 receptor antagonist): Specialist use, monitor closely for rapid correction
• Demeclocycline (induces nephrogenic DI): Rarely used now
• Treat underlying cause: Chemotherapy for SCLC

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Viva Scenarios

Viva Scenario 1: Na-K-ATPase and Membrane Potential

Setting: First Part Physiology Viva

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Examiner: "You have a critically ill patient who has developed severe hypokalaemia (K+ 2.0 mmol/L) and the ECG shows significant changes. Let's discuss the underlying physiology. Can you describe the Na-K-ATPase pump and its role in maintaining cell membrane potential?"

Candidate: "The Na-K-ATPase, or sodium-potassium pump, is a P-type ATPase enzyme found in the plasma membrane of virtually all animal cells. It consists of alpha, beta, and gamma subunits. The alpha subunit is the catalytic subunit containing the ATP binding site, sodium and potassium binding sites, and the phosphorylation site. The beta subunit is required for proper folding and membrane insertion. The gamma subunit is tissue-specific and modulates pump kinetics."

Examiner: "Good. Describe the mechanism of the pump."

Candidate: "The pump operates through an E1-E2 conformational cycle. In the E1 state, the pump has high affinity for sodium and low affinity for potassium. Three sodium ions bind intracellularly, and ATP binds to the alpha subunit. ATP is then hydrolysed to ADP, with phosphorylation of an aspartate residue. This causes a conformational change to the E2 state, which has low affinity for sodium and high affinity for potassium. Three sodium ions are released extracellularly, and two potassium ions bind. Dephosphorylation then occurs, returning the pump to the E1 state, and two potassium ions are released intracellularly. The cycle repeats approximately 100 times per second."

Examiner: "What is the stoichiometry and what does this mean for the membrane?"

Candidate: "The stoichiometry is 3 sodium out for 2 potassium in, per ATP hydrolysed. This creates a net export of one positive charge per cycle, making the pump electrogenic. This contributes approximately 10 millivolts to the resting membrane potential, with the inside of the cell being more negative."

Examiner: "How does this relate to the resting membrane potential calculation?"

Candidate: "The resting membrane potential is primarily determined by the potassium gradient, as described by the Goldman-Hodgkin-Katz equation. At rest, the membrane is approximately 50-100 times more permeable to potassium than sodium. Using a simplified Nernst equation for potassium, with intracellular concentration of 140 mmol/L and extracellular of 4 mmol/L, the equilibrium potential is about minus 90 millivolts. The actual resting potential is slightly less negative (around minus 70 to minus 90 millivolts) because of small sodium permeability and the electrogenic contribution of the Na-K-ATPase."

Examiner: "Now, with hypokalaemia, what happens to this potential and why is this clinically important?"

Candidate: "In hypokalaemia, the extracellular potassium concentration decreases. Using the Nernst equation, this increases the potassium equilibrium potential, making it more negative. The cell membrane becomes hyperpolarised, meaning a larger stimulus is required to reach threshold potential. Initially, this decreases cellular excitability. However, paradoxically, hypokalaemia causes cardiac arrhythmias through several mechanisms.

First, the hyperpolarisation delays repolarisation, prolonging the action potential duration and QT interval. Second, the reduced potassium conductance leads to delayed afterdepolarisations and triggered activity. Third, there's heterogeneity of repolarisation across the myocardium, creating substrate for re-entry. This explains the U waves, QT prolongation, and risk of torsades de pointes we see clinically."

Examiner: "Excellent. What factors can shift potassium intracellularly in this patient?"

Candidate: "Several factors stimulate the Na-K-ATPase or affect potassium distribution:

Insulin stimulates the pump via the PI3K pathway, shifting potassium intracellularly. This is why we give insulin with glucose for acute hyperkalaemia treatment.

Beta-2 agonists such as salbutamol stimulate the pump via cAMP, also shifting potassium intracellularly. Adrenaline has dual effects with beta-2 causing intracellular shift and alpha causing hepatic potassium release.

Alkalosis causes potassium to enter cells in exchange for hydrogen ions leaving for buffering. The rule of thumb is 0.6 mmol/L decrease in potassium per 0.1 increase in pH, though this varies.

Hyperosmolality causes water to leave cells, increasing intracellular potassium concentration, which then drives potassium efflux.

In this patient, if they have metabolic alkalosis from vomiting or respiratory alkalosis, this would worsen the hypokalaemia through transcellular shift."

Examiner: "The patient is also found to have low magnesium. How does this affect potassium?"

Candidate: "Magnesium is an essential cofactor for the Na-K-ATPase. Without adequate magnesium, the pump cannot function properly, so potassium cannot be effectively pumped into cells.

Additionally, hypomagnesaemia increases the activity of ROMK channels in the collecting duct, leading to increased renal potassium wasting.

This creates a situation of refractory hypokalaemia that cannot be corrected until magnesium is replaced. Clinical guidelines recommend checking and replacing magnesium in any patient with hypokalaemia, especially if resistant to potassium replacement."

Examiner: "Good. How much potassium replacement would you give and how would you monitor?"

Candidate: "The potassium deficit can be estimated roughly. Each 1 mmol/L decrease below 4 mmol/L represents approximately 100-200 mmol total body deficit. So a potassium of 2.0 would represent a deficit of 200-400 mmol, though ongoing losses must be considered.

For replacement, intravenous potassium chloride is preferred for severe hypokalaemia. The maximum safe rate via peripheral IV is 10 mmol per hour, or 20 mmol per hour via central line with cardiac monitoring. Concentrated solutions above 40 mmol per litre are irritant and risk phlebitis peripherally.

I would also replace magnesium simultaneously, typically 20 mmol of magnesium sulphate over 4-6 hours.

Monitoring should include ECG continuously, potassium levels every 2-4 hours during active replacement, and assessment of clinical status. The target is potassium above 4.0 mmol/L before reducing monitoring intensity."

Examiner: "Thank you. That concludes this station."

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Viva Scenario 2: Calcium Regulation and Hormonal Control

Setting: First Part Physiology Viva

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Examiner: "A patient in ICU has ionised calcium of 0.85 mmol/L with QT prolongation on ECG. Let's discuss calcium physiology. First, describe the different forms of calcium in plasma and why ionised calcium is the important measurement."

Candidate: "Total plasma calcium is normally 2.1 to 2.6 mmol/L and exists in three forms.

First, ionised or free calcium represents 45-50% of total calcium, with a concentration of 1.1-1.3 mmol/L. This is the physiologically active fraction that affects neuromuscular function, cardiac contractility, enzyme activity, and coagulation.

Second, protein-bound calcium represents 40-45%, bound predominantly to albumin (80%) and globulins (20%). This fraction is inactive.

Third, complexed calcium represents 5-10%, bound to anions such as citrate, phosphate, and sulphate. This is also inactive.

Ionised calcium is the important measurement because it reflects the physiologically active fraction. Total calcium can be misleading in critically ill patients who commonly have hypoalbuminaemia, acid-base disturbances, or receive citrated blood products."

Examiner: "How does pH affect ionised calcium?"

Candidate: "pH affects the binding of calcium to albumin. Each albumin molecule has multiple binding sites for calcium that are pH-dependent.

In acidosis, hydrogen ions compete for albumin binding sites, displacing calcium. This increases the ionised fraction even if total calcium is unchanged.

In alkalosis, there are fewer hydrogen ions competing, so more calcium binds to albumin. This decreases the ionised fraction.

The approximate effect is 0.03 to 0.05 mmol/L change in ionised calcium per 0.1 pH unit change. This explains why hyperventilation-induced respiratory alkalosis can precipitate symptomatic hypocalcaemia in patients with borderline calcium levels, manifesting as perioral tingling, carpopedal spasm, or tetany."

Examiner: "Good. Describe the hormonal regulation of calcium, focusing on PTH."

Candidate: "Calcium homeostasis is primarily regulated by parathyroid hormone, vitamin D, and to a lesser extent calcitonin.

Parathyroid hormone is an 84 amino acid peptide synthesised by the chief cells of the parathyroid glands. It has a short half-life of 2-4 minutes. PTH secretion is regulated by the calcium-sensing receptor on chief cells. Low ionised calcium decreases CaSR activation, which stimulates PTH release. High calcium does the opposite.

PTH acts on three target organs. In bone, PTH activates osteoclasts indirectly through RANK-ligand expression by osteoblasts, leading to bone resorption and calcium release. In the kidney, PTH increases calcium reabsorption in the distal tubule by activating TRPV5 channels. It decreases phosphate reabsorption in the proximal tubule. It also stimulates 1-alpha-hydroxylase, which converts 25-hydroxyvitamin D to the active 1,25-dihydroxyvitamin D or calcitriol.

The net effect of PTH is to increase plasma calcium and decrease plasma phosphate."

Examiner: "How does vitamin D contribute to calcium regulation?"

Candidate: "Vitamin D3, or cholecalciferol, is synthesised in the skin from 7-dehydrocholesterol upon UVB exposure, or obtained from dietary sources. It undergoes two hydroxylation steps.

First, 25-hydroxylation in the liver by CYP2R1 produces 25-hydroxyvitamin D or calcidiol. This is the major circulating form with a half-life of 2-3 weeks and is measured clinically to assess vitamin D status.

Second, 1-alpha-hydroxylation in the kidney by CYP27B1 produces 1,25-dihydroxyvitamin D or calcitriol. This is the active hormone with a half-life of 4-6 hours. This step is stimulated by PTH and inhibited by FGF23.

Calcitriol acts on nuclear vitamin D receptors in target tissues. In the intestine, it increases calcium absorption by upregulating TRPV6 calcium channels and calbindin. In bone, it promotes mineralisation but at high levels cooperates with PTH for resorption. In the parathyroid glands, it suppresses PTH gene transcription as negative feedback.

The net effect of vitamin D is to increase both calcium and phosphate absorption, maintaining adequate minerals for bone mineralisation."

Examiner: "This patient has hypocalcaemia. What are the potential causes in an ICU patient?"

Candidate: "Hypocalcaemia in ICU has multiple potential causes.

First, hypoparathyroidism may occur post-thyroidectomy or parathyroidectomy if the parathyroid glands are damaged or removed. Hungry bone syndrome can occur after parathyroidectomy when bone rapidly takes up calcium.

Second, vitamin D deficiency is common in ICU patients due to malnutrition, liver disease, kidney disease, or lack of sun exposure. Critical illness may also accelerate vitamin D catabolism.

Third, citrated blood products bind ionised calcium. Massive transfusion or CRRT with citrate anticoagulation are common causes in ICU. The liver normally metabolises citrate to bicarbonate, but in hepatic dysfunction, citrate accumulates.

Fourth, acute pancreatitis causes calcium soap formation as free fatty acids bind calcium.

Fifth, alkalosis shifts calcium to the protein-bound fraction, reducing ionised calcium.

Sixth, sepsis and critical illness cause multifactorial hypocalcaemia through reduced PTH secretion, vitamin D resistance, and increased calcitonin.

Seventh, hypomagnesaemia impairs both PTH secretion and its peripheral action, causing functional hypoparathyroidism."

Examiner: "How would you manage this patient's hypocalcaemia with QT prolongation?"

Candidate: "This is symptomatic or severe hypocalcaemia requiring immediate treatment.

For acute replacement, I would give calcium gluconate 10%, 10-20 mL intravenously over 10-20 minutes. This provides 2.2-4.4 mmol of elemental calcium. I would use calcium gluconate rather than calcium chloride for peripheral administration as calcium chloride causes severe tissue necrosis if extravasated.

If ongoing replacement is needed, I would start a continuous infusion at 0.5-2.0 mg/kg/hour of elemental calcium, monitoring ionised calcium every 2-4 hours.

Simultaneously, I would check and correct magnesium, as hypomagnesaemia causes refractory hypocalcaemia by impairing PTH secretion and action.

I would investigate the underlying cause with PTH level, 25-hydroxyvitamin D, phosphate, magnesium, and albumin. If vitamin D deficient, I would replace with cholecalciferol or calcitriol depending on kidney function.

I would monitor ECG for QT normalisation and watch for complications of calcium replacement including hypercalcaemia and precipitation if given with phosphate or bicarbonate."

Examiner: "What about the cardiac effects of hypocalcaemia - why does it prolong the QT?"

Candidate: "Calcium affects the cardiac action potential, particularly phase 2 or the plateau phase.

During phase 2, calcium enters cardiomyocytes through L-type calcium channels. This calcium influx triggers calcium-induced calcium release from the sarcoplasmic reticulum, producing contraction.

In hypocalcaemia, the reduced extracellular calcium concentration decreases the driving force for calcium entry through L-type channels. This prolongs the plateau phase as more time is needed to achieve adequate calcium influx for repolarisation to begin.

On the ECG, this manifests as ST segment prolongation, which lengthens the QT interval. The T wave morphology is usually preserved, distinguishing hypocalcaemia from hypokalaemia where the QT prolongation is due to U waves.

Severe QT prolongation predisposes to torsades de pointes, particularly if there are other QT-prolonging factors such as drugs, hypokalaemia, or hypomagnesaemia."

Examiner: "Thank you. That concludes this station."

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