Phosphate and Magnesium Disorders in the ICU

Quick Answer

Phosphate and Magnesium Disorders are among the most common electrolyte abnormalities in critically ill patients, with hypophosphatemia occurring in 20-40% and hypomagnesemia in 20-65% of ICU admissions. These disorders are frequently encountered together, particularly in refeeding syndrome, alcoholism, and DKA treatment.

Key Clinical Features:

• Hypophosphatemia: Muscle weakness, respiratory failure, rhabdomyolysis, haemolysis, cardiac dysfunction
• Hyperphosphatemia: Metastatic calcification, hypocalcaemia, renal failure, tumour lysis syndrome
• Hypomagnesemia: Torsades de Pointes, refractory hypokalaemia, refractory hypocalcaemia, seizures
• Hypermagnesemia: Areflexia, respiratory depression, bradycardia, cardiac arrest

Emergency Management:

1. Identify and treat underlying cause (refeeding, DKA treatment, diuretics, alcoholism)
2. Replace phosphate IV for severe deficiency (<0.32 mmol/L) - 10-20 mmol over 6-12 hours
3. Replace magnesium IV for arrhythmias or severe deficiency - 8-16 mmol over 2-4 hours
4. Calcium gluconate 10% IV for symptomatic hypermagnesemia (10-20 mL)
5. Monitor for and prevent refeeding syndrome in malnourished patients

ICU Mortality: Severe hypophosphatemia 30-50%, hypomagnesemia with arrhythmias 20-40%

Must-Know Facts:

• Phosphate is essential for ATP synthesis, 2,3-DPG, and cellular energy metabolism
• Magnesium is a cofactor for >300 enzymatic reactions including Na-K-ATPase
• Both disorders cause refractory hypokalaemia and hypocalcaemia if not corrected
• Refeeding syndrome is the most common cause of combined deficiency in ICU
• CRRT causes significant phosphate and magnesium losses requiring supplementation

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CICM Exam Focus

What Examiners Expect

Second Part Written (SAQ):

Common SAQ stems:

• "A 48yo male with chronic alcoholism is admitted to ICU with pneumonia. On Day 3, he develops respiratory failure requiring intubation. Investigations show PO4 0.28 mmol/L, Mg 0.45 mmol/L, K 2.8 mmol/L. Outline the pathophysiology and management."
• "A 62yo female with anorexia nervosa (BMI 14) is admitted for nutritional rehabilitation. Outline your approach to preventing refeeding syndrome."
• "A patient develops Torsades de Pointes on the cardiac monitor. ECG shows QTc 580ms. List the causes and outline immediate management."
• "Discuss the renal handling of phosphate and the hormonal regulation by PTH and FGF-23."

Expected depth:

• Cellular functions of phosphate (ATP, 2,3-DPG, phospholipids, bone mineral)
• Magnesium as enzyme cofactor and membrane stabiliser
• Renal tubular handling and hormonal regulation
• Recognition of refeeding syndrome risk factors and NICE criteria
• IV replacement protocols with dosing, monitoring, and complications
• Interpretation of calcium-phosphate product and metastatic calcification risk
• TdP management algorithm with magnesium, overdrive pacing
• CRRT effects on phosphate and magnesium balance

Second Part Hot Case:

Typical presentations:

• Day 5 post-admission malnourished patient with new respiratory failure (refeeding)
• Post-DKA treatment with weakness and failure to wean from ventilation
• Chronic alcoholic with seizures and cardiac arrhythmias
• Post-parathyroidectomy with hypocalcaemia and hypophosphataemia
• CKD patient with hyperphosphatemia and pruritus

Examiners assess:

• Systematic A-E examination approach
• Recognition of muscle weakness patterns (proximal, respiratory)
• ECG interpretation (QT prolongation, arrhythmias)
• Assessment of nutritional status (BMI, anthropometry)
• Fluid status and dialysis access if applicable
• Medication review (diuretics, PPIs, amphotericin)
• Family communication regarding prognosis

Second Part Viva:

Expected discussion areas:

• Phosphate homeostasis: PTH, FGF-23, vitamin D axis
• Magnesium renal handling and factors affecting reabsorption
• Pathophysiology of refeeding syndrome (insulin surge, intracellular shift)
• TLS prevention and management (rasburicase, dialysis)
• IV replacement protocols and monitoring
• CRRT and electrolyte losses
• Indigenous health considerations (malnutrition, alcoholism)

Examiner expectations:

• Safe, consultant-level decision-making
• Evidence-based practice (NICE refeeding guidelines)
• Understanding of cellular physiology
• Holistic patient management
• Indigenous health awareness

Common Mistakes

• Failing to check phosphate and magnesium in patients at risk of refeeding syndrome
• Using oral replacement when IV is required (severe deficiency, ileus)
• Not recognising that hypomagnesemia causes refractory hypokalaemia
• Forgetting to supplement thiamine before glucose in alcoholics/malnourished
• Replacing potassium without correcting magnesium (futile)
• Over-rapid phosphate replacement causing hypocalcaemia
• Not monitoring calcium during phosphate replacement
• Missing hyperphosphatemia in tumour lysis syndrome
• Underestimating CRRT losses of phosphate and magnesium

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Key Points

Must-Know Facts

1. Phosphate Distribution: 85% in bone, 14% intracellular, only 1% extracellular; serum levels do not reflect total body stores; intracellular shift can rapidly deplete serum phosphate (PMID: 25559582).

2. Magnesium Distribution: 50-60% in bone, 40% intracellular, only 1% extracellular (0.3% of total body magnesium is in serum); low serum Mg indicates severe deficiency (PMID: 26510880).

3. ATP Synthesis: Phosphate is essential for oxidative phosphorylation; severe deficiency causes cellular energy failure, explaining the multi-organ dysfunction (muscle, cardiac, RBC, WBC, platelets).

4. 2,3-DPG: Phosphate depletion reduces 2,3-DPG, shifting oxygen-haemoglobin dissociation curve leftward, impairing tissue oxygen delivery despite normal PaO2 and Hb (PMID: 7192306).

5. Magnesium as Cofactor: Required for >300 enzymatic reactions including Na-K-ATPase (explains refractory hypokalaemia), adenylate cyclase, and all phosphorylation reactions.

6. Refeeding Syndrome: Carbohydrate intake triggers insulin release, driving phosphate, magnesium, potassium into cells; most dangerous in first 72 hours of refeeding (NICE CG32, PMID: 28859143).

7. TdP and Magnesium: IV magnesium (8 mmol bolus) is first-line treatment for Torsades de Pointes regardless of serum magnesium level (PMID: 33167609).

8. Refractory Hypokalaemia: If potassium remains low despite aggressive replacement, check and correct magnesium first; Mg is required for Na-K-ATPase function (PMID: 8757393).

9. Calcium-Phosphate Product: Ca × PO4 product >4.4 mmol²/L² (55 mg²/dL²) increases risk of metastatic calcification; target <4.0 mmol²/L² in CKD (KDIGO 2017, PMID: 28167285).

10. CRRT Losses: Standard CRRT removes 10-30 mmol phosphate and 5-15 mmol magnesium daily; routine supplementation required in most CRRT patients (PMID: 20877948).

Memory Aids

REFEEDING - Risk Factors for Refeeding Syndrome:

• R: Recent weight loss >15% in 3-6 months
• E: Eating disorders (anorexia nervosa, bulimia)
• F: Fasting/starvation >10 days
• E: Elderly with reduced intake
• E: Enteral/parenteral nutrition initiation
• D: Drug abuse (alcoholism)
• I: Inflammatory bowel disease, malabsorption
• N: Neoplasm (cancer cachexia)
• G: Glucose infusion before thiamine

PHOSPHATE - Functions of Phosphate:

• P: Phospholipids (cell membranes)
• H: High-energy bonds (ATP, ADP)
• O: Oxygen delivery (2,3-DPG)
• S: Signal transduction (phosphorylation)
• P: Protein synthesis
• H: Hydrogen buffering (intracellular buffer)
• A: Acid-base (urinary buffer)
• T: Teeth and bone (hydroxyapatite)
• E: Energy metabolism (oxidative phosphorylation)

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Definition and Epidemiology

Definitions

Phosphate Disorders:

Magnesium Disorders:

Epidemiology

International Data:

Hypophosphatemia:

• ICU incidence: 20-40% of admissions (PMID: 26510880)
• Severe hypophosphatemia: 5-10% of ICU patients
• Highest in sepsis (30-40%), post-DKA (15-25%), refeeding (80-100% if not prevented)
• Associated with increased ICU mortality (OR 1.5-2.0) and prolonged mechanical ventilation (PMID: 25055376)

Hyperphosphatemia:

• ICU incidence: 10-20% of admissions
• Most common in AKI/CKD (60-80% of ESKD patients), tumour lysis syndrome (70-90%), rhabdomyolysis (50-70%)
• Independently associated with mortality in AKI (PMID: 20164458)

Hypomagnesemia:

• ICU incidence: 20-65% of admissions (varies with definition and population) (PMID: 26510880)
• Most common electrolyte disorder in ICU after hypokalaemia
• Higher in sepsis, cardiac surgery, and patients receiving diuretics
• Associated with increased ICU mortality, arrhythmias, and prolonged ventilation (PMID: 24393802)

Hypermagnesemia:

• ICU incidence: 5-10% (predominantly iatrogenic or renal failure)
• Rare except in renal failure, obstetric magnesium infusions, magnesium-containing medications

Australian/New Zealand Data:

• ANZICS-APD data: Electrolyte disorders present in 40-50% of ICU admissions
• Indigenous Australians: Higher rates of malnutrition, alcoholism, and CKD predispose to electrolyte disorders
• Remote/rural populations: Delayed presentation may increase severity at ICU admission
• Refeeding syndrome incidence: 0.8-2.0% of all hospital admissions, 10-20% of malnourished patients (PMID: 28859143)

High-Risk Populations:

• Aboriginal and Torres Strait Islander peoples: 2-3x higher rates of malnutrition, alcoholism, and CKD
• Māori: Elevated rates of diabetes, CKD, and nutritional disorders
• Alcoholics: Combined phosphate, magnesium, and thiamine deficiency extremely common
• Eating disorder patients: 15-30% develop refeeding syndrome if not managed appropriately
• Cancer patients: Tumour lysis syndrome, malnutrition, chemotherapy effects
• Elderly: Malnutrition, polypharmacy (diuretics, PPIs), reduced renal function

Mortality:

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Applied Basic Sciences

This section bridges First Part basic sciences with Second Part clinical practice

Phosphate Physiology

Distribution and Normal Homeostasis:

Total body phosphate: 500-800 g (16-26 mol) in 70 kg adult

• Bone: 85% (as hydroxyapatite Ca10(PO4)6(OH)2)
• Intracellular: 14% (ATP, phospholipids, nucleic acids)
• Extracellular: 1% (serum phosphate, <1 mmol/L)

Serum phosphate exists in three forms:

• Ionised (free): 55% (physiologically active, filterable)
• Protein-bound: 10% (mainly albumin)
• Complexed: 35% (with calcium, magnesium, sodium)

Daily Phosphate Balance:

• Dietary intake: 800-1400 mg/day (26-45 mmol)
• GI absorption: 60-70% in small intestine (enhanced by vitamin D)
• Renal excretion: 600-900 mg/day (matches net absorption)
• Bone turnover: Balanced resorption and deposition

Renal Handling of Phosphate:

Filtration: 100% of non-protein-bound phosphate filtered at glomerulus (5,000-7,000 mg/day)

Reabsorption: 80-90% reabsorbed in proximal tubule

• Sodium-phosphate cotransporters (NaPi-IIa, NaPi-IIc): Located in proximal tubule brush border
• Type IIa (SLC34A1): Major transporter, regulated by PTH and FGF-23
• Type IIc (SLC34A3): Minor contributor, mutations cause HHRH

Hormonal Regulation:

PTH (Parathyroid Hormone) - Phosphaturic:

• Increases phosphate excretion by downregulating NaPi-IIa
• Internalisation and degradation of NaPi-IIa transporters
• Net effect: Decreased reabsorption, increased urinary phosphate
• Also increases calcium reabsorption in distal tubule (PMID: 24558296)

FGF-23 (Fibroblast Growth Factor-23) - Phosphaturic:

• Secreted by osteocytes in response to phosphate load
• Requires Klotho as co-receptor
• Decreases NaPi-IIa and NaPi-IIc expression
• Suppresses 1-alpha-hydroxylase, reducing calcitriol production
• Elevated very early in CKD (before hyperphosphataemia) (PMID: 24558296)

1,25-Dihydroxyvitamin D (Calcitriol) - Phosphate Retention:

• Increases intestinal phosphate absorption
• Increases renal phosphate reabsorption (minor effect)
• Stimulates bone resorption (releases phosphate)

Cellular Functions of Phosphate:

1. Energy Metabolism (ATP):

• Adenosine triphosphate (ATP) contains 3 phosphate groups
• High-energy phosphate bonds store cellular energy
• ADP + Pi → ATP in oxidative phosphorylation
• Severe phosphate deficiency → ATP depletion → cellular energy failure (PMID: 25559582)

2. Oxygen Delivery (2,3-DPG):

• 2,3-Diphosphoglycerate binds haemoglobin, facilitating oxygen release
• Normal 2,3-DPG: Right-shifted oxygen-haemoglobin dissociation curve
• Low phosphate → Low 2,3-DPG → Left shift → Impaired tissue oxygen delivery
• Explains tissue hypoxia despite normal PaO2 and Hb (PMID: 7192306)

3. Phospholipid Membranes:

• Phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine
• Maintain cell membrane integrity and fluidity
• Phosphate deficiency → Membrane instability → Haemolysis, rhabdomyolysis

4. Signal Transduction:

• Protein phosphorylation/dephosphorylation regulates enzyme activity
• Kinases and phosphatases require phosphate
• Second messenger systems (cAMP, IP3) depend on phosphate

5. Nucleic Acids:

• DNA and RNA contain phosphodiester backbone
• ATP, GTP, CTP, UTP for nucleotide synthesis
• Phosphate deficiency → Impaired DNA replication, protein synthesis

6. Intracellular Buffer:

• H2PO4⁻/HPO4²⁻ buffer system (pKa 6.8)
• Major intracellular buffer, contributes to urinary buffering
• Titratable acid in distal nephron

Magnesium Physiology

Distribution and Normal Homeostasis:

Total body magnesium: 21-28 g (880-1,160 mmol) in 70 kg adult

• Bone: 50-60% (bound to hydroxyapatite, slowly exchangeable)
• Intracellular: 38-40% (muscle, soft tissues; second most abundant intracellular cation)
• Extracellular: 1-2% (serum magnesium)

Serum magnesium exists in three forms:

• Ionised (free): 55-60% (physiologically active)
• Protein-bound: 30-35% (mainly albumin)
• Complexed: 10-15% (with citrate, phosphate, bicarbonate)

Daily Magnesium Balance:

• Dietary intake: 300-400 mg/day (12-16 mmol)
• GI absorption: 30-50% in small intestine (passive and active transport via TRPM6/7)
• Renal excretion: 100-150 mg/day (4-6 mmol)
• Fecal losses: 150-200 mg/day

Renal Handling of Magnesium:

Filtration: 70-80% of serum magnesium is ultrafilterable (ionised + complexed)

Reabsorption:

• Proximal tubule: 10-20% (paracellular, passive, solvent drag)
• Thick ascending limb of Henle (TAL): 60-70% (paracellular, claudin-16/19 dependent)
• Distal convoluted tubule (DCT): 5-10% (transcellular, active via TRPM6)

Regulation:

Cellular Functions of Magnesium:

1. Enzyme Cofactor (>300 Reactions):

• Na-K-ATPase: Maintains transmembrane ion gradients (explains refractory hypokalaemia)
• Adenylate cyclase: cAMP generation
• Phospholipases, kinases, enolase: Glycolysis and oxidative phosphorylation
• RNA/DNA polymerases: Nucleic acid synthesis
• ATPases: All ATP-dependent reactions require Mg-ATP complex (PMID: 26510880)

2. Membrane Stability:

• Stabilises neuronal and cardiac cell membranes
• Blocks calcium channels, reduces calcium influx
• Deficiency → Membrane hyperexcitability → Seizures, arrhythmias, tetany

3. PTH Secretion:

• Magnesium required for PTH secretion from parathyroid glands
• Hypomagnesemia → Impaired PTH release → Functional hypoparathyroidism
• Explains refractory hypocalcaemia in severe hypomagnesemia (PMID: 8757393)

4. Cardiovascular Function:

• Modulates vascular smooth muscle tone
• Antiarrhythmic effects (stabilises cardiac membranes)
• Blocks NMDA receptors (neuroprotective, antiseizure)

5. Neuromuscular Transmission:

• Reduces presynaptic acetylcholine release
• Antagonises postsynaptic calcium effects
• High Mg → Neuromuscular blockade (therapeutic in eclampsia, TdP)

Interactions Between Phosphate and Magnesium

Common Causes of Combined Deficiency:

• Refeeding syndrome (insulin-driven intracellular shift of both)
• Chronic alcoholism (malnutrition, renal wasting)
• DKA treatment (insulin shifts both intracellularly)
• Chronic diarrhoea/malabsorption
• CRRT (continuous removal of both)
• Diuretics (loop and thiazides)

Shared Pathophysiology:

• Both cause ATP depletion → Cellular energy failure
• Both cause membrane instability → Haemolysis, rhabdomyolysis
• Both contribute to cardiac dysfunction
• Both required for normal neuromuscular function

Correction Priorities:

• Correct magnesium first if potassium is also low (Mg required for K correction)
• Monitor calcium during phosphate replacement (phosphate binds calcium)
• Replace both in refeeding syndrome along with potassium and thiamine

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Hypophosphatemia

Aetiology

Classification of Causes:

1. Decreased Intestinal Absorption:

• Malnutrition, starvation, anorexia nervosa
• Malabsorption syndromes (coeliac disease, short bowel)
• Phosphate-binding antacids (aluminium hydroxide, calcium carbonate)
• Vitamin D deficiency (reduced intestinal absorption)
• Chronic diarrhoea

2. Increased Renal Losses:

• Hyperparathyroidism (primary and secondary)
• Vitamin D deficiency with secondary hyperparathyroidism
• Fanconi syndrome (generalised proximal tubular dysfunction)
• Inherited phosphate-wasting disorders (XLH, HHRH)
• Post-renal transplant phosphaturia
• Osmotic diuresis (DKA, hyperglycaemia)
• Diuretics (acetazolamide, loop diuretics - mild)

3. Intracellular Shift (Most Common in ICU):

4. Alcoholism (Multifactorial):

• Malnutrition, poor dietary intake
• Renal phosphate wasting (direct ethanol effect)
• Antacid use (for gastritis)
• Vitamin D deficiency
• Intracellular shift on refeeding

Clinical Features

Graded by Severity:

Detailed Clinical Features:

Neuromuscular (Most clinically significant):

• Proximal muscle weakness (difficulty rising from chair, climbing stairs)
• Respiratory muscle weakness → Failure to wean from mechanical ventilation
• Diaphragmatic weakness → Hypercapnic respiratory failure
• Rhabdomyolysis with CK elevation (can be massive, >100,000 U/L)
• Ileus, dysphagia

Cardiac:

• Reduced myocardial contractility
• Cardiomyopathy (reversible with correction)
• Arrhythmias (less common than with magnesium disorders)
• Heart failure with pulmonary oedema

Haematological:

• Haemolysis: Red cell ATP depletion → Membrane rigidity → Intravascular haemolysis
• WBC dysfunction: Impaired phagocytosis, chemotaxis, oxidative burst (PMID: 6329907)
• Platelet dysfunction: Impaired aggregation, thrombocytopenia
• Increased 2,3-DPG initially, then decreased with severe depletion

Neurological:

• Irritability, confusion, delirium
• Paraesthesias
• Seizures (rare)
• Coma (severe deficiency)
• Central pontine myelinolysis (rare, usually with rapid correction of hyponatraemia)

ICU Management of Hypophosphatemia

General Principles:

• Identify and treat underlying cause
• Assess severity and symptoms
• Choose oral vs IV replacement based on severity and GI function
• Monitor for complications of replacement (hypocalcaemia)
• Prevent recurrence (address underlying cause)

Indications for IV Replacement:

• Severe hypophosphatemia (<0.32 mmol/L)
• Symptomatic hypophosphatemia (weakness, respiratory failure)
• Unable to tolerate oral intake (ileus, NPO)
• Rapid correction required (weaning failure, rhabdomyolysis)

IV Phosphate Replacement Protocol:

Preparation:

• Available as potassium phosphate (Potassium Dihydrogen Phosphate/Phosphoric Acid) or sodium phosphate
• 1 mmol phosphate = 31 mg elemental phosphorus
• Potassium phosphate: Contains 1.5-2 mmol K+ per mmol PO4 (monitor potassium)
• Sodium phosphate: Use if hyperkalaemic or at risk

Dosing by Severity (PMID: 25055376):

Administration:

• Central line preferred for concentrations >40 mmol/L (peripheral vein irritation)
• Maximum concentration: 0.4 mmol/mL (40 mmol in 100 mL)
• Maximum rate: 7 mmol/hour (reduces risk of hypocalcaemia)
• Dilute in 0.9% saline or 5% dextrose (not Hartmann's - contains calcium)

Monitoring During Replacement:

• Serum phosphate: Every 4-6 hours until stable, then every 6-12 hours
• Serum calcium: Every 4-6 hours (phosphate binds calcium, risk of hypocalcaemia)
• Serum potassium: If using potassium phosphate (contains K+)
• Magnesium: Often deficient concurrently
• Renal function: Adjust dose in renal impairment

Red Flags During Replacement:

• Symptomatic hypocalcaemia (tetany, seizures, prolonged QT)
• Hyperkalaemia (if using potassium phosphate)
• Soft tissue calcite deposition (if replacing in hyperphosphatemia - avoid)

Oral Replacement (For mild-moderate, functioning GI tract):

• Phosphate Sandoz (500 mg = 16 mmol phosphate): 1-2 tablets TDS
• Neutral phosphate solution: 15-30 mmol TDS
• Maximum oral dose: 100 mmol/day (limited by GI tolerance - diarrhoea)
• Absorption reduced by concurrent calcium, antacids, food

Refeeding Syndrome Prevention Protocol (NICE CG32, PMID: 28859143):

High Risk Criteria (One or more):

• BMI <16 kg/m²
• Unintentional weight loss >15% in 3-6 months
• Little or no nutritional intake for >10 days
• Low levels of K, PO4, or Mg prior to feeding

Extremely High Risk (Two or more of):

• BMI <18.5 kg/m²
• Unintentional weight loss >10% in 3-6 months
• Little or no nutritional intake for >5 days
• History of alcohol abuse, drugs (insulin, chemotherapy, antacids, diuretics)

NICE Refeeding Protocol:

1. Before feeding: Check and correct K, PO4, Mg, thiamine
2. Thiamine: 200-300 mg oral or 100 mg IV TDS for 3 days (BEFORE glucose)
3. Start low: 10 kcal/kg/day (5 kcal/kg if extremely high risk)
4. Advance slowly: Increase by 5-10 kcal/kg/day over 4-7 days
5. Supplement electrolytes: K 2-4 mmol/kg/day, PO4 0.3-0.6 mmol/kg/day, Mg 0.2-0.4 mmol/kg/day
6. Monitor: Electrolytes daily for first 7 days, ECG if symptomatic
7. Fluid restriction: 20-30 mL/kg/day initially (avoid sodium overload)

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Hyperphosphatemia

Aetiology

Classification of Causes:

1. Decreased Renal Excretion (Most Common):

• Acute kidney injury
• Chronic kidney disease G4-5 (eGFR <30)
• Hypoparathyroidism (reduced PTH-mediated phosphaturia)
• Pseudohypoparathyroidism (PTH resistance)
• Bisphosphonate therapy
• Acromegaly

2. Increased Phosphate Load:

3. Transcellular Shift (Out of Cells):

• Metabolic acidosis (H+ exchange for PO4)
• Diabetic ketoacidosis (despite total body depletion)
• Respiratory acidosis
• Catabolic states (trauma, sepsis, burns)

4. Pseudohyperphosphatemia:

• Paraproteinaemia (multiple myeloma)
• Hyperlipidaemia
• Hyperbilirubinaemia
• Haemolysed sample (laboratory artefact)

Tumour Lysis Syndrome (TLS)

Definition (Cairo-Bishop Criteria, PMID: 15182052):

Laboratory TLS (≥2 of the following within 3 days before or 7 days after chemotherapy):

• Uric acid ≥476 μmol/L or 25% increase from baseline
• Potassium ≥6.0 mmol/L or 25% increase
• Phosphate ≥1.45 mmol/L (adults) or 25% increase
• Calcium ≤1.75 mmol/L or 25% decrease

Clinical TLS: Laboratory TLS plus one or more of:

• AKI (creatinine ≥1.5× ULN)
• Cardiac arrhythmia or sudden death
• Seizures

High-Risk Malignancies:

• Acute lymphoblastic leukaemia (ALL)
• Burkitt lymphoma
• High-grade non-Hodgkin lymphoma
• Acute myeloid leukaemia with high WBC count
• Chronic lymphocytic leukaemia with high WBC
• Bulky solid tumours with rapid response to treatment

TLS Prevention (PMID: 18372509):

• Aggressive IV hydration (3 L/m²/day or 150-200 mL/hour)
• Allopurinol 100-300 mg TDS (started 1-2 days before chemotherapy)
• Rasburicase 0.2 mg/kg IV (for high-risk patients - converts uric acid to allantoin)
• Avoid potassium and phosphate in IV fluids
• Monitor electrolytes every 6-8 hours
• Early RRT if AKI develops

TLS Treatment:

• Aggressive hydration with 0.9% saline
• Rasburicase for uric acid (contraindicated in G6PD deficiency)
• Calcium gluconate for symptomatic hypocalcaemia
• Phosphate binders (aluminium hydroxide, sevelamer)
• RRT for severe AKI, refractory electrolyte abnormalities

Calcium-Phosphate Product

Concept:

• Calcium × Phosphate product indicates risk of metastatic calcification
• When product exceeds solubility threshold, precipitation occurs in soft tissues
• Critical in CKD patients with chronic hyperphosphatemia

Calculation:

• Ca (mmol/L) × PO4 (mmol/L) = Product (mmol²/L²)
• Alternative: Ca (mg/dL) × PO4 (mg/dL) = Product (mg²/dL²)
• Conversion: 1 mmol/L Ca = 4 mg/dL; 1 mmol/L PO4 = 3.1 mg/dL

Target (KDIGO 2017, PMID: 28167285):

• Ca-PO4 product <4.4 mmol²/L² (<55 mg²/dL²)
• Many guidelines recommend <4.0 mmol²/L² (<50 mg²/dL²)

Sites of Metastatic Calcification:

• Vascular calcification (coronary arteries, peripheral vessels)
• Cardiac valve calcification (aortic, mitral)
• Periarticular calcite deposits
• Soft tissue calcification (pulmonary, corneal)
• Calciphylaxis (calcific uremic arteriolopathy) - rare but devastating

ICU Management of Hyperphosphatemia

General Principles:

• Identify and treat underlying cause
• Reduce phosphate intake
• Enhance phosphate elimination
• Prevent and treat complications (hypocalcaemia, soft tissue calcification)

Acute Management:

1. Discontinue Phosphate Sources:

• Stop phosphate-containing IV fluids, parenteral nutrition
• Hold phosphate supplements
• Avoid phosphate-containing enemas/laxatives

2. Dietary Restriction (Limited role in acute ICU setting):

• Phosphate restriction 800-1000 mg/day
• Limit dairy, meat, cola drinks, nuts
• Dietitian involvement for chronic management

3. Phosphate Binders (Bind dietary phosphate in GI tract):

4. Renal Replacement Therapy (Most Effective for Acute Severe Hyperphosphatemia):

• Indications: TLS, rhabdomyolysis with AKI, refractory hyperphosphatemia
• CRRT: Continuous removal, 10-30 mmol phosphate removed daily
• Intermittent HD: 40-60 mmol phosphate removed per 4-hour session
• CVVHDF preferred for continuous control

5. Volume Expansion (Mild effect):

• Saline loading may increase phosphate excretion (if renal function preserved)
• Limited efficacy compared to dialysis

Chronic Management in CKD (KDIGO 2017, PMID: 28167285):

• Target PO4 toward normal range (not specified upper limit)
• Use phosphate binders with meals
• Non-calcium binders preferred if hypercalcaemia risk
• Dietary phosphate restriction
• Consider dialysis prescription optimization (longer, more frequent sessions)

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Hypomagnesemia

Aetiology

Classification of Causes:

1. Gastrointestinal Losses:

• Chronic diarrhoea (most common GI cause)
• Malabsorption syndromes (coeliac disease, short bowel, pancreatitis)
• Proton pump inhibitors (PPIs) - Chronic use impairs Mg absorption (PMID: 21913641)
• Nasogastric suction
• Fistulas
• Bariatric surgery
• Inflammatory bowel disease

2. Renal Losses (Most Common in ICU):

3. Alcoholism (Multifactorial, Most Common Cause Overall):

• Malnutrition, poor dietary intake
• Chronic diarrhoea
• Renal Mg wasting (direct ethanol effect)
• Pancreatitis (saponification, malabsorption)
• Often combined with phosphate and thiamine deficiency

4. Redistribution/Intracellular Shift:

• Refeeding syndrome (insulin-driven shift)
• DKA treatment (insulin therapy)
• Hungry bone syndrome (post-parathyroidectomy)
• Acute pancreatitis (saponification in fat necrosis)
• Catecholamine excess

5. Endocrine Causes:

• Hyperaldosteronism (increased renal Mg excretion)
• Hyperthyroidism
• Hyperparathyroidism (paradoxically may cause Mg wasting)
• SIADH (dilutional)

6. Genetic Disorders (Rare):

• Gitelman syndrome (SLC12A3 mutation)
• Bartter syndrome (various mutations)
• FHHNC (familial hypomagnesemia with hypercalciuria and nephrocalcinosis)
• HSH (hypomagnesemia with secondary hypocalcaemia)

Clinical Features

Graded by Severity:

Cardiovascular Manifestations (Most Clinically Important):

ECG Changes:

• Prolonged QT interval (major concern)
• Prolonged PR interval
• Widened QRS complex
• T wave flattening or inversion
• U waves (prominent)
• ST segment depression

Arrhythmias:

• Torsades de Pointes (TdP): Polymorphic VT with QT prolongation - LIFE-THREATENING
• Atrial fibrillation/flutter
• Supraventricular tachycardia
• Ventricular premature beats
• Ventricular fibrillation
• Enhanced digoxin toxicity

Neuromuscular Manifestations:

• Muscle weakness
• Tremor (fine tremor, especially fingers)
• Fasciculations
• Positive Chvostek and Trousseau signs (shared with hypocalcaemia)
• Tetany
• Seizures

Electrolyte Effects:

• Refractory Hypokalaemia: Mg required for Na-K-ATPase function; K replacement futile until Mg corrected (PMID: 8757393)
• Refractory Hypocalcaemia: Mg required for PTH secretion; functional hypoparathyroidism

ICU Management of Hypomagnesemia

General Principles:

• Identify and treat underlying cause
• Assess severity and presence of arrhythmias
• Replace magnesium (IV for severe, oral for mild)
• Correct associated electrolyte abnormalities
• Monitor cardiac rhythm

Indications for IV Replacement:

• Severe hypomagnesemia (<0.40 mmol/L)
• Symptomatic (arrhythmias, seizures, tetany)
• Torsades de Pointes (regardless of serum Mg level)
• Eclampsia/pre-eclampsia prophylaxis
• Unable to tolerate oral intake
• Digoxin toxicity with arrhythmias

IV Magnesium Replacement Protocol:

Preparation:

• Magnesium sulfate (MgSO4·7H2O): 1 g = 4 mmol Mg
• Magnesium chloride (MgCl2): 1 g = 5 mmol Mg
• Magnesium sulfate more commonly available in Australia

Dosing by Clinical Scenario (PMID: 26510880):

Administration:

• Peripheral line: Maximum 0.3 mmol/mL (20 mmol in 100 mL)
• Central line: Higher concentrations acceptable
• Dilute in 0.9% saline or 5% dextrose (compatible with both)
• Maximum rate (non-emergency): 1 mmol/minute (4 mmol over 4 minutes)

Monitoring During Replacement:

• Serum magnesium: Every 4-6 hours until stable
• ECG/cardiac monitoring: Continuous if arrhythmias present
• Deep tendon reflexes: Loss of reflexes indicates toxicity
• Respiratory rate: Depression indicates toxicity
• Blood pressure: Hypotension with rapid infusion
• Urine output: Renal excretion is primary elimination route

Red Flags (Magnesium Toxicity):

• Loss of deep tendon reflexes (Mg >4 mmol/L)
• Respiratory depression (Mg >5 mmol/L)
• Bradycardia, hypotension (Mg >5 mmol/L)
• Cardiac arrest (Mg >7.5 mmol/L)
• Antidote: Calcium gluconate 10% 10-20 mL IV

Oral Replacement (For mild-moderate, functioning GI tract):

• Magnesium aspartate: 500-1000 mg TDS (10-20 mmol/day)
• Magnesium orotate: 500 mg TDS
• Magnesium oxide: Poorly absorbed (10-15%), causes diarrhoea
• Magnesium glycinate: Better tolerated, less diarrhoea

Torsades de Pointes (TdP) Management

Definition: Polymorphic ventricular tachycardia with characteristic "twisting of the points" around the isoelectric line, occurring in the context of QT prolongation.

Causes of Acquired Long QT (Mnemonic: DRUGS):

• D: Drugs (antiarrhythmics, antibiotics, antipsychotics, antiemetics)
• R: Repolarisation abnormalities (ischaemia, cardiomyopathy)
• U: Underfilled heart (hypovolaemia, heart failure)
• G: Glucose (hypoglycaemia) and electrolytes (hypoK, hypoMg, hypoCa)
• S: Slow heart rate (bradycardia prolongs QT)

Common QT-Prolonging Drugs in ICU:

• Antiarrhythmics: Amiodarone, sotalol, procainamide, quinidine
• Antibiotics: Macrolides (azithromycin, erythromycin), fluoroquinolones (ciprofloxacin, moxifloxacin)
• Antipsychotics: Haloperidol, droperidol, quetiapine
• Antiemetics: Ondansetron (high doses), metoclopramide
• Antidepressants: Tricyclics, SSRIs (citalopram, escitalopram)
• Others: Methadone, chloroquine, hydroxychloroquine

Immediate Management of TdP (PMID: 33167609):

1. Magnesium Sulfate (First-Line, Regardless of Serum Mg):

• Dose: 8 mmol (2 g) IV push over 1-2 minutes
• Repeat: 8 mmol if persistent after 5-10 minutes
• Maintenance: 8-16 mmol/hour infusion
• Mechanism: Suppresses early afterdepolarisations, shortens QT

2. Correct Other Electrolytes:

• Potassium: Target 4.0-4.5 mmol/L (supplement aggressively)
• Calcium: Replace if low

3. Overdrive Pacing (If Magnesium Fails):

• Temporary transvenous pacing at 90-110 bpm
• Shortens QT interval, suppresses pause-dependent TdP
• Particularly useful for bradycardia-dependent TdP

4. Isoprenaline (If Pacing Not Available):

• Start 1-2 mcg/min, titrate to HR 90-110 bpm
• Increases heart rate, shortens QT
• Avoid if congenital long QT syndrome suspected

5. Stop Offending Drugs:

• Identify and discontinue all QT-prolonging medications
• Review drug interactions

6. Defibrillation:

• If patient pulseless or TdP deteriorates to VF
• Unsynchronised shock 200J biphasic

7. Avoid:

• Class Ia, Ic, III antiarrhythmics (worsen QT prolongation)
• Amiodarone (prolongs QT, may worsen TdP - controversial)

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Hypermagnesemia

Aetiology

Primary Mechanism: Almost always due to impaired renal excretion (kidneys can excrete massive Mg loads if function normal)

1. Renal Failure (Most Common):

• Acute kidney injury
• Chronic kidney disease G4-5 (eGFR <30)
• Most common in patients receiving Mg-containing medications

2. Iatrogenic (Common in Hospital Setting):

3. Endocrine:

• Hypothyroidism (reduced renal excretion)
• Addison's disease (reduced renal excretion)
• Milk-alkali syndrome

4. Other:

• Lithium therapy (reduces renal Mg excretion)
• Massive tissue breakdown (theoretical)
• Familial hypocalciuric hypercalcaemia (rare)

Clinical Features

Graded by Serum Magnesium Level:

Detailed Clinical Features:

Neuromuscular:

• Hyporeflexia (earliest sign, important monitoring parameter)
• Areflexia (indicates significant toxicity)
• Muscle weakness, flaccid paralysis
• Respiratory muscle weakness → Respiratory failure
• Lethargy, confusion, coma

Cardiovascular:

• Vasodilation (flushing, warmth, hypotension)
• Bradycardia
• Prolonged PR interval, widened QRS
• First-degree, then higher-degree AV block
• Complete heart block
• Asystole

Other:

• Nausea, vomiting
• Ileus
• Urinary retention
• Pupillary dilation

ICU Management of Hypermagnesemia

General Principles:

• Stop all magnesium-containing medications
• Assess severity and symptoms
• Calcium gluconate for immediate cardiac protection
• Enhance magnesium elimination
• Supportive care for complications

Immediate Management:

1. Stop Magnesium Sources:

• Discontinue IV magnesium infusions immediately
• Hold Mg-containing antacids, laxatives
• Review parenteral nutrition

2. Calcium Gluconate (Immediate Cardiac Protection):

• Dose: 10-20 mL of 10% calcium gluconate IV over 5-10 minutes
• Mechanism: Calcium antagonises Mg effects at neuromuscular junction and heart
• Repeat every 5-10 minutes until reflexes return
• Does NOT lower serum Mg, only provides temporary protection
• Duration of effect: 30-60 minutes

3. Enhance Elimination:

If Renal Function Preserved:

• IV saline diuresis: 0.9% saline 150-250 mL/hour
• Loop diuretics: Furosemide 40-80 mg IV (increases renal Mg excretion)
• Target urine output >2 mL/kg/hour

If Renal Function Impaired (Most Cases):

• Haemodialysis: Most effective, removes Mg rapidly
• CRRT: For haemodynamically unstable patients
• Dialysate Mg should be low (0.25-0.5 mmol/L)

4. Supportive Care:

• Airway protection if obtunded
• Mechanical ventilation if respiratory failure
• Vasopressors if hypotension unresponsive to calcium
• Temporary pacing if complete heart block

5. Monitor:

• Serum magnesium every 1-2 hours until improving
• Deep tendon reflexes (bedside indicator of severity)
• Continuous cardiac monitoring
• Respiratory rate and effort
• Serum calcium (if giving calcium)

Special Situation: Eclampsia/Pre-Eclampsia Treatment

Therapeutic hypermagnesemia is intentionally induced for seizure prophylaxis:

• Target Mg: 2.0-3.5 mmol/L (4-7 mEq/L)
• Monitor: DTRs (present), respiratory rate (>12/min), urine output (>25 mL/hr)
• Antidote readily available: Calcium gluconate 10% 10 mL

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ICU-Specific Considerations

Refeeding Syndrome

Definition: Potentially fatal shifts in fluids and electrolytes that may occur in malnourished patients when nutritional repletion is initiated. Characterised by hypophosphatemia, hypomagnesemia, hypokalaemia, thiamine deficiency, and fluid retention.

Pathophysiology (PMID: 28859143):

Starvation State:

• Glycogen stores depleted within 24-72 hours
• Body shifts to gluconeogenesis, then ketogenesis
• Fat and protein become primary fuel sources
• Insulin secretion minimal
• Intracellular electrolytes depleted but serum levels maintained

Refeeding (Carbohydrate Introduction):

• Glucose stimulates insulin secretion
• Insulin drives glucose, phosphate, potassium, magnesium into cells
• Phosphate consumed for ATP synthesis, phosphorylated intermediates
• Sudden drop in serum electrolytes
• Increased metabolic demands exceed supply

Clinical Consequences:

• Cardiac: Arrhythmias (hypoK, hypoMg), heart failure (fluid retention, thiamine deficiency)
• Respiratory: Respiratory failure (diaphragm weakness from hypoPO4)
• Neurological: Wernicke's encephalopathy (thiamine deficiency), seizures
• Haematological: Haemolysis, WBC dysfunction

NICE Guidelines Prevention Protocol (CG32, PMID: 28859143):

Risk Stratification:

• See criteria in Hypophosphatemia section above
• BMI <16: Extremely high risk
• Weight loss >15% in 3-6 months: High risk
• Little intake >10 days: High risk

Prevention Protocol:

1. Thiamine 200-300 mg oral or 100 mg IV TDS before and during first 10 days
2. Balanced multivitamin/trace element supplement
3. Start nutrition at 10 kcal/kg/day (5 kcal/kg if BMI <14 or negligible intake >15 days)
4. Increase slowly over 4-7 days to target (25-30 kcal/kg/day)
5. Supplement: K 2-4 mmol/kg/day, PO4 0.3-0.6 mmol/kg/day, Mg 0.2-0.4 mmol/kg/day
6. Restrict sodium (1 mmol/kg/day) and fluid (20-30 mL/kg/day)
7. Monitor electrolytes daily for first 7 days

CRRT and Electrolyte Management

Phosphate Removal by CRRT (PMID: 20877948):

• CRRT removes phosphate continuously by diffusion and convection
• Sieving coefficient: ~1.0 (freely filtered)
• Daily removal: 10-30 mmol (varies with CRRT dose and serum level)
• 80-90% of CRRT patients develop hypophosphatemia without supplementation

Phosphate Replacement in CRRT:

• Routine supplementation recommended
• Options:
  • Add to replacement fluid (15-30 mmol per 5 L bag)
  • Add to dialysate
  • Separate IV infusion
• Commercial phosphate-containing replacement solutions available
• Monitor serum PO4 every 12-24 hours

Magnesium Removal by CRRT:

• Magnesium freely filtered (sieving coefficient ~0.8-1.0 for ionised)
• Daily removal: 5-15 mmol
• Standard dialysate/replacement Mg: 0.5-0.75 mmol/L (may need adjustment)
• Hypomagnesemia common if not monitored

Magnesium Replacement in CRRT:

• Ensure dialysate/replacement contains adequate Mg (0.5-0.75 mmol/L)
• Additional IV supplementation often required
• Monitor serum Mg every 12-24 hours

Post-DKA Electrolyte Management

Phosphate in DKA (PMID: 8243819):

Initial Presentation:

• Serum phosphate may be normal or high (acidosis drives PO4 out of cells)
• Total body phosphate severely depleted (osmotic diuresis)

During Insulin Treatment:

• Insulin drives phosphate into cells
• Serum phosphate drops, often to severe levels (<0.32 mmol/L)
• Peak drop usually 6-12 hours after starting treatment

Replacement:

• Monitor phosphate every 2-4 hours during DKA treatment
• Replace if <0.65 mmol/L and patient can receive IV
• Potassium phosphate: Addresses both K and PO4 depletion
• Typical dose: 10-20 mmol PO4 over 6-12 hours
• Monitor calcium (phosphate replacement can precipitate hypocalcaemia)

Magnesium in DKA:

• Also depleted by osmotic diuresis
• Monitor and replace alongside phosphate
• Often overlooked but important for K correction

Indigenous Health Considerations

Aboriginal and Torres Strait Islander Populations:

Risk Factors for Electrolyte Disorders:

• Higher rates of chronic alcoholism (2-3x) - leads to combined PO4/Mg deficiency
• Higher rates of malnutrition and food insecurity
• Higher rates of diabetes and DKA
• Higher rates of CKD (5-10x) - hyperphosphatemia
• Delayed presentation due to remote location - more severe at diagnosis
• Cultural factors affecting dietary intake

Management Considerations:

• Aboriginal Health Worker (AHW) involvement for communication
• Aboriginal Liaison Officer (ALO) for family support
• Cultural safety considerations (family decision-making, sorry business)
• Health literacy assessment - visual aids, interpreter services
• Discharge planning with community health services
• Follow-up in Aboriginal Community Controlled Health Services

Māori Health Considerations:

• Similar elevated rates of diabetes, CKD, and nutritional disorders
• Whānau (family) involvement in care decisions
• Māori Health Worker involvement
• Tikanga (cultural protocols) consideration
• Hauora (holistic health) approach

Remote/Rural Considerations:

• Limited laboratory access may delay electrolyte monitoring
• Retrieval considerations (RFDS, state retrieval services)
• Telemedicine consultation with tertiary centres
• Extended monitoring post-discharge
• Community health nurse follow-up

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Prognosis and Outcome Measures

Mortality

Hypophosphatemia:

• Mild-moderate: Minimal independent mortality impact
• Severe (<0.32 mmol/L): Associated with 30-50% ICU mortality (PMID: 25055376)
• Respiratory failure from hypophosphatemia: High mortality if not recognised
• Refeeding syndrome with cardiac events: 40-60% mortality

Hyperphosphatemia:

• In AKI: Independent predictor of mortality (OR 1.2-1.5 per mmol/L increase) (PMID: 20164458)
• In TLS: 20-30% mortality with multiorgan failure
• In CKD: Chronic elevation associated with cardiovascular mortality

Hypomagnesemia:

• Associated with increased ICU mortality (OR 1.5-2.0) (PMID: 24393802)
• TdP without treatment: Near 100% mortality
• With appropriate treatment: Excellent prognosis if underlying cause addressed

Hypermagnesemia:

• Mild-moderate: Low mortality with cessation of source
• Severe (>5 mmol/L) with cardiac arrest: 30-50% mortality
• Iatrogenic in hospital: Usually reversible with appropriate treatment

Morbidity

Functional Recovery:

• Most electrolyte-related weakness fully reversible with correction
• Rhabdomyolysis: 10-30% may develop AKI requiring RRT
• Refeeding syndrome: Prolonged rehabilitation may be required
• Cardiac arrhythmias: Full recovery expected if survived

ICU Length of Stay:

• Severe hypophosphatemia: Associated with 2-4 day increased LOS
• Failure to wean due to hypophosphatemia: Prolonged mechanical ventilation
• TdP requiring intervention: Additional 2-5 day ICU stay

Prognostic Factors

Good Prognostic Factors:

• Early recognition and treatment
• Reversible underlying cause (DKA, refeeding, drugs)
• Preserved renal function
• No cardiac arrest from arrhythmia
• Appropriate electrolyte monitoring in high-risk patients

Poor Prognostic Factors:

• Delayed diagnosis
• Cardiac arrest from TdP or hypermagnesemia
• Underlying malignancy (TLS)
• Irreversible renal failure requiring long-term dialysis
• Severe rhabdomyolysis with myoglobinuric AKI
• Multiple organ failure

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SAQ Practice

SAQ 1: Refeeding Syndrome Prevention

Time Allocation: 10 minutes Total Marks: 20

Stem: A 38-year-old female with a history of anorexia nervosa is admitted to ICU following aspiration pneumonia requiring intubation and mechanical ventilation. She weighs 42 kg (BMI 15.5 kg/m²) and has had minimal oral intake for the past 3 weeks.

Observations on arrival:

• HR: 52 bpm
• BP: 88/54 mmHg
• Temperature: 37.8°C
• Mechanically ventilated: FiO2 0.5, PEEP 8

Investigations:

• Na 138, K 3.2, Cl 102, HCO3 22 mmol/L
• Urea 2.1, Creatinine 42 μmol/L
• Phosphate 0.72 mmol/L
• Magnesium 0.62 mmol/L
• Albumin 28 g/L

Question 1.1 (8 marks) List the risk factors for refeeding syndrome in this patient and explain the pathophysiology of electrolyte changes that occur during refeeding.

Question 1.2 (6 marks) Outline your nutritional management strategy for this patient over the first 7 days.

Question 1.3 (6 marks) List the monitoring parameters and complications you would anticipate.

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Model Answer

Question 1.1 (8 marks)

Risk Factors for Refeeding Syndrome (4 marks):

• BMI 15.5 kg/m² (<16 = extremely high risk) (1 mark)
• Minimal oral intake for >10 days (3 weeks) (1 mark)
• History of anorexia nervosa (eating disorder) (1 mark)
• Already depleted electrolytes: K 3.2, PO4 0.72, Mg 0.62 mmol/L (1 mark)

Pathophysiology (4 marks):

• During starvation, body shifts to fat and protein catabolism; intracellular electrolytes are depleted but serum levels maintained by cellular release (1 mark)
• Carbohydrate intake stimulates insulin secretion (1 mark)
• Insulin drives glucose, phosphate, potassium, and magnesium into cells for glycolysis and ATP synthesis (1 mark)
• Rapid intracellular shift causes precipitous drop in serum electrolytes, particularly phosphate; phosphate consumed for ATP synthesis and phosphorylated glycolytic intermediates; leads to cellular energy failure, cardiac dysfunction, respiratory muscle weakness (1 mark)

Question 1.2 (6 marks)

Nutritional Strategy (6 marks):

Before Commencing Nutrition (2 marks):

• Give thiamine 100 mg IV TDS (before glucose to prevent Wernicke's) (1 mark)
• Correct electrolytes: K to >4.0 mmol/L, PO4 to >0.8 mmol/L, Mg to >0.7 mmol/L (1 mark)

Initiating Nutrition (2 marks):

• Start at 5-10 kcal/kg/day (extremely high risk, so 5 kcal/kg = 210 kcal/day) (1 mark)
• Use enteral nutrition if GI tract functional; consider NG tube post-aspiration (1 mark)

Advancement (2 marks):

• Increase by 5 kcal/kg/day every 24-48 hours if electrolytes stable (1 mark)
• Target 25-30 kcal/kg/day by Day 7 (1,050-1,260 kcal/day) (1 mark)

Electrolyte Supplementation:

• PO4 0.3-0.6 mmol/kg/day (12-25 mmol/day)
• K 2-4 mmol/kg/day (84-168 mmol/day)
• Mg 0.2-0.4 mmol/kg/day (8-16 mmol/day)
• Restrict Na and fluid initially

Question 1.3 (6 marks)

Monitoring Parameters (3 marks):

• Electrolytes (K, PO4, Mg, Ca) daily for first 7 days (1 mark)
• Fluid balance, daily weight; restrict to 20-30 mL/kg/day initially (1 mark)
• Cardiac monitoring: ECG, continuous telemetry for arrhythmias (1 mark)

Anticipated Complications (3 marks):

• Cardiac: Arrhythmias (TdP from hypoMg, VT from hypoK), heart failure from fluid overload and thiamine deficiency (1 mark)
• Respiratory: Failure to wean, diaphragmatic weakness from hypophosphatemia (1 mark)
• Neurological: Wernicke's encephalopathy (confusion, ataxia, ophthalmoplegia) if thiamine not given (1 mark)

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SAQ 2: Torsades de Pointes with Hypomagnesemia

Time Allocation: 10 minutes Total Marks: 20

Stem: A 68-year-old male is Day 4 post-ICU admission for community-acquired pneumonia. He has been treated with ceftriaxone and azithromycin. He has a history of chronic heart failure (EF 35%) and takes furosemide 80 mg BD at home.

The nurse calls you urgently as the patient has become unresponsive with the following rhythm on the monitor:

[Description: Polymorphic ventricular tachycardia with characteristic "twisting" morphology. HR approximately 220 bpm. Preceding rhythm shows prolonged QT interval (QTc 580 ms).]

Observations during event:

• HR: 220 bpm
• BP: Not recordable
• SpO2: 78%
• GCS: 3

Recent investigations (2 hours ago):

• K 3.1 mmol/L
• Mg 0.52 mmol/L
• Ca (corrected) 2.18 mmol/L
• QTc on admission: 480 ms

Question 2.1 (6 marks) Identify the rhythm and list the causes of this arrhythmia in this patient.

Question 2.2 (8 marks) Outline your immediate management of this patient in the first 10 minutes.

Question 2.3 (6 marks) What strategies would you implement to prevent recurrence?

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Model Answer

Question 2.1 (6 marks)

Rhythm Identification (2 marks):

• Torsades de Pointes (TdP) - polymorphic ventricular tachycardia in the context of prolonged QT interval (2 marks)

Causes in This Patient (4 marks):

• Hypomagnesemia (Mg 0.52 mmol/L): From chronic furosemide use (1 mark)
• Hypokalaemia (K 3.1 mmol/L): From chronic furosemide use, prolongs QT (1 mark)
• QT-prolonging drug: Azithromycin (macrolide antibiotic) (1 mark)
• Underlying cardiac disease: Cardiomyopathy with EF 35%, pre-existing QTc 480 ms (1 mark)

Question 2.2 (8 marks)

Immediate Management (8 marks):

Recognition and Response (1 mark):

• Call for help, cardiac arrest team
• Check pulse - if pulseless, this is cardiac arrest

If Pulseless (2 marks):

• Start CPR
• Defibrillate 200J biphasic (unsynchronised for VF/pulseless VT) (1 mark)
• Continue CPR between shocks (1 mark)

Magnesium Sulfate (2 marks):

• Give 8 mmol (2 g) IV push over 1-2 minutes regardless of pulse (1 mark)
• Can repeat in 5-10 minutes if TdP persists; maintenance infusion 8-16 mmol/hour (1 mark)

Correct Other Electrolytes (1 mark):

• Potassium: Give 20-40 mmol IV, target K 4.0-4.5 mmol/L

If TdP Persists After Magnesium (1 mark):

• Overdrive pacing (temporary transvenous) at 90-110 bpm to suppress pause-dependent TdP

Airway and Breathing (1 mark):

• If not already, secure airway and provide ventilation

Question 2.3 (6 marks)

Prevention of Recurrence (6 marks):

Stop Offending Drugs (2 marks):

• Cease azithromycin immediately (QT prolongation) (1 mark)
• Switch to non-QT-prolonging antibiotic (e.g., continue ceftriaxone monotherapy, or add doxycycline) (1 mark)

Electrolyte Management (2 marks):

• Maintain K 4.0-4.5 mmol/L (1 mark)
• Maintain Mg >0.8 mmol/L (higher target given history) (1 mark)

Medication Review and Monitoring (2 marks):

• Review all medications for QT-prolonging potential (1 mark)
• Daily ECG with QTc measurement; discontinue any new QT-prolonging drugs if QTc >500 ms (1 mark)
• Consider cardiology review for device therapy if underlying substrate

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Viva Scenarios

Viva 1: Electrolyte Panel Interpretation in Refeeding Syndrome

Stem: You are called to review a 52-year-old male who was admitted 48 hours ago with alcoholic hepatitis. He has been started on enteral nutrition via NG tube. The nurse is concerned about today's blood results.

Opening Question:

Examiner: "Here are today's results. What concerns you?"

Expected Answer: "This pattern of falling electrolytes - potassium, phosphate, and magnesium - over the first 48 hours of nutritional therapy is highly suggestive of refeeding syndrome. This is particularly concerning given:

• The phosphate has dropped from 0.85 to 0.38 mmol/L, which is now in the severe range
• The patient's background of alcoholism, which is a major risk factor for malnutrition and refeeding
• All three electrolytes are falling simultaneously, consistent with insulin-driven intracellular shift

This is a medical emergency requiring immediate intervention."

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Follow-up Question 1:

Examiner: "Why does potassium remain low despite what you expect would be standard ICU care?"

Expected Answer: "Potassium replacement is likely to be refractory until we correct the magnesium deficiency. This is because:

• Magnesium is essential for the function of Na-K-ATPase, the pump that maintains intracellular potassium
• In hypomagnesemia, the Na-K-ATPase cannot function normally, leading to intracellular potassium depletion and increased renal potassium wasting through ROMK channels
• Studies have shown that hypomagnesemia causes 'refractory hypokalaemia' - potassium replacement alone will be ineffective
• We need to correct magnesium first, or at least concurrently, for potassium correction to be successful"

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Follow-up Question 2:

Examiner: "How would you manage this patient right now?"

Expected Answer: "I would take immediate action on several fronts:

1. Stop or Reduce Nutrition:

• Halt or significantly reduce the enteral feeding rate
• This prevents further insulin-driven intracellular shift
• Can restart at lower rate (10 kcal/kg/day) once electrolytes stabilising

2. Thiamine Supplementation:

• Give thiamine 100 mg IV TDS if not already given
• Essential to prevent Wernicke's encephalopathy, especially in alcoholics
• Must give before any glucose

3. Electrolyte Replacement:

• Magnesium: 16-32 mmol IV over 4-6 hours (0.42 is severe)
• Phosphate: 20-30 mmol IV over 6-12 hours (0.38 is severe) - use potassium phosphate to address both
• Potassium: Additional KCl as needed, target >4.0 mmol/L

4. Monitoring:

• Cardiac monitoring for arrhythmias
• Recheck electrolytes in 4-6 hours
• Continue daily monitoring for next 7 days

5. Fluid Management:

• Restrict fluids to 20-30 mL/kg/day
• Restrict sodium to prevent fluid overload"

---

Follow-up Question 3:

Examiner: "The patient's potassium is now 4.2 mmol/L after your interventions. However, his wife asks why this happened and whether it will happen again. How do you explain this?"

Expected Answer: "I would explain to the family in plain language:

'Your husband developed a condition called refeeding syndrome. When someone hasn't eaten properly for a while - which can happen with heavy drinking - the body adapts by using different energy sources. When we start feeding again, the body suddenly needs certain minerals - phosphate, magnesium, and potassium - to use the food for energy.

These minerals rush into the cells so fast that the levels in the blood drop dangerously low. This can cause serious problems including heart rhythm problems and muscle weakness.

We've now replaced these minerals and slowed down the feeding. We'll restart nutrition more gradually - the body needs time to readjust. We'll be monitoring his blood tests very closely over the next week to make sure this doesn't happen again.

This is a known risk with restarting nutrition after someone hasn't been eating well, and we now know to be extra careful. The good news is that with proper management, we expect him to make a full recovery from this.'"

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Viva 2: Post-DKA Electrolyte Management

Stem: A 28-year-old Aboriginal male from a remote community in the Northern Territory was brought to your tertiary ICU by RFDS after presenting to his local clinic with DKA. He has Type 1 diabetes diagnosed 10 years ago. He is now 18 hours into treatment and the medical registrar asks for your advice on electrolyte management.

Opening Question:

Examiner: "The registrar shows you these results. What is your interpretation?"

Expected Answer: "This demonstrates the classic electrolyte evolution during DKA treatment:

Potassium: Started high at 6.2 mmol/L due to acidosis driving K out of cells, but the total body K was actually depleted from osmotic diuresis. As we corrected acidosis with insulin and fluids, K moved back into cells and now we have severe hypokalaemia at 2.9 mmol/L.

Phosphate: Also started high-normal at 1.32 mmol/L due to acidosis, but again total body was depleted. With insulin treatment, phosphate is consumed for ATP synthesis during glycolysis and moves intracellularly. At 0.31 mmol/L, this is now severe hypophosphatemia.

Magnesium: Similarly depleted at presentation from osmotic diuresis, now severely low at 0.44 mmol/L.

All three are at dangerous levels requiring urgent IV replacement. The phosphate in particular is concerning as it may cause respiratory muscle weakness and failure to wean if he's been ventilated."

---

Follow-up Question 1:

Examiner: "Walk me through your immediate management plan."

Expected Answer: "This requires urgent but careful electrolyte replacement:

1. Potassium Replacement:

• K 2.9 is critically low with arrhythmia risk
• Give 40 mmol KCl IV over 2-4 hours via central line
• Maximum rate 20 mmol/hour with cardiac monitoring
• BUT: This won't be effective until we correct magnesium

2. Magnesium Replacement (Priority):

• Give 16-24 mmol magnesium sulfate IV over 4-6 hours
• Magnesium is required for Na-K-ATPase function
• Must correct for potassium replacement to work

3. Phosphate Replacement:

• PO4 0.31 is severe; give 20-30 mmol IV over 6-12 hours
• Use potassium phosphate solution (addresses K and PO4 together)
• Contains ~1.5-2 mmol K per mmol PO4 - account for this in K replacement
• Maximum rate 7 mmol phosphate/hour to avoid hypocalcaemia

4. Monitoring:

• Continuous cardiac monitoring
• Recheck electrolytes in 4-6 hours
• Monitor calcium (phosphate binds calcium)
• Assess respiratory muscle strength if ventilated - hypophosphatemia may impair weaning

5. Continue DKA Protocol:

• Insulin infusion to close anion gap
• Maintenance IV fluids"

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Follow-up Question 2:

Examiner: "This patient is Aboriginal from a remote community. Are there any specific considerations for his ongoing care?"

Expected Answer: "Yes, there are important cultural, social, and healthcare system considerations:

Cultural Considerations:

• Involve the Aboriginal Health Worker (AHW) and Aboriginal Liaison Officer (ALO) early
• Family involvement in discussions - Aboriginal culture often involves family/community in healthcare decisions
• Use interpreters if English is not first language
• Consider cultural safety - ensure staff are culturally competent
• If patient is a Traditional Owner, may have specific obligations requiring consideration

Healthcare System Factors:

• Remote community challenges: Limited access to regular diabetes care, pathology, specialist services
• Coordinate with remote area nurses and Aboriginal Community Controlled Health Services for follow-up
• Ensure discharge planning includes connection to community-based diabetes services
• Consider reasons for DKA - was this related to access to insulin, refrigeration, healthcare?
• Telehealth follow-up may be appropriate given distance

Social Determinants:

• Food security may affect diet and nutrition
• Housing conditions may affect medication storage
• Transport to appointments may be challenging
• Indigenous Australians have higher rates of Type 1 and Type 2 diabetes

Preventive Care:

• Diabetes education with culturally appropriate materials
• Sick day management education
• Blood glucose monitoring education
• Connection to Indigenous chronic disease programs
• Close the Gap initiatives"

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Follow-up Question 3:

Examiner: "The family asks if they need to worry about this happening again. How do you counsel them?"

Expected Answer: "I would have this conversation with the family present, with an Aboriginal Health Worker to assist if available, and in culturally appropriate language:

'Thank you for coming in. [Patient's name] has been very unwell with a problem where his diabetes got out of control. This happens when there isn't enough insulin in the body, and the body starts to break down fat for energy which creates acid in the blood.

While he was unwell, his body lost a lot of important minerals - like phosphate and magnesium - through passing urine frequently. We're now putting those back carefully.

This can happen again if diabetes isn't controlled well. The best way to prevent it is:

• Taking insulin regularly as prescribed
• Checking blood sugar levels
• Having a plan for sick days - when to take extra insulin or get help
• Making sure insulin is stored properly (keeping it cool)
• Coming to the clinic if feeling unwell

We'll make sure [patient's name] is connected with the diabetes team at the community clinic. They can help with regular checkups and support.

Do you have any questions? Is there anything that makes it hard for [patient's name] to manage his diabetes at home that we should know about?'

I would ensure follow-up is arranged with culturally appropriate services and address any barriers to care identified."