Fluid, Electrolyte & Acid–Base Disorders

Acidosis

•    High Anion Gap Metabolic Acidosis

o    Due to accumulation of unmeasured acids

o    Causes:

    Lactic acidosis (shock, hypoxia, metformin toxicity)

    Ketoacidosis (diabetic, alcoholic, starvation)

    Renal failure (↓ excretion of acids)

    Toxins: methanol, ethylene glycol, salicylates

o    Anion gap = [Na⁺] – ([Cl⁻] + [HCO₃⁻]); normal = 8–12 mmol/L

•    Normal Anion Gap (Hyperchloraemic) Metabolic Acidosis

o    Causes:

    Diarrhoea: loss of HCO₃⁻

    Renal Tubular Acidosis (RTA)

    Ileal conduit, acetazolamide

Alkalosis

•    Metabolic Alkalosis

o    Causes:

    Vomiting or nasogastric suction (H⁺ loss)

    Diuretics (loop/thiazides)

    Hyperaldosteronism (↑ H⁺ secretion)

•    Respiratory Alkalosis

o    Due to hyperventilation:

    Anxiety, hypoxia, sepsis, pregnancy

    PaCO₂ ↓ → compensatory ↓ HCO₃⁻

Renal Tubular Acidosis (RTA)

•    Type 1 (Distal RTA)

o    Impaired H⁺ secretion → ↑ urine pH (>5.5), hypokalaemia

o    Nephrolithiasis common due to alkaline urine

o    Causes: autoimmune (e.g. Sjögren’s, SLE), amphotericin B

•    Type 2 (Proximal RTA)

o    Impaired HCO₃⁻ reabsorption

o    Urine pH initially >5.5, then <5.5 as HCO₃⁻ depletes

o    Causes: Fanconi syndrome, multiple myeloma, carbonic anhydrase inhibitors

•    Type 4 RTA

o    Hypoaldosteronism or aldosterone resistance

o    ↓ Na⁺ reabsorption and ↓ K⁺/H⁺ excretion → hyperkalaemic acidosis

o    Seen in diabetes, ACEi/ARBs, heparin

Potassium Disorders

•    Hypokalaemia

o    Symptoms: weakness, cramps, arrhythmias (U waves)

o    Causes:

    GI losses: diarrhoea, vomiting

    Renal losses: diuretics, hyperaldosteronism

    Intracellular shift: insulin, β-agonists, alkalosis

•    Hyperkalaemia

o    ECG: peaked T waves, widened QRS, sine wave pattern

o    Causes:

    CKD, ACEi/ARBs, spironolactone

    Cell lysis: rhabdomyolysis, haemolysis, tumour lysis

    Acidosis, Addison’s disease

o    Emergency Rx: calcium gluconate, insulin + glucose, salbutamol, dialysis

Sodium Disorders

•    Hyponatraemia

o    Symptoms: nausea, confusion, seizures if rapid onset

o    Causes:

    SIADH (euvolaemic)

    Heart failure, cirrhosis, nephrotic syndrome (hypervolaemic)

    Adrenal insufficiency, hypothyroidism

o    Correct slowly to avoid central pontine myelinolysis (CPM)

•    Hypernatraemia

o    Usually due to water loss > Na⁺ loss

o    Causes:

    Dehydration, osmotic diuresis

    Diabetes insipidus (DI)

o    Treat with slow rehydration (risk of cerebral oedema)

Polyuria

•    Cranial Diabetes Insipidus (DI)

o    ↓ ADH secretion → dilute urine

o    Causes: idiopathic, head trauma, pituitary surgery

o    Responds to desmopressin

•    Nephrogenic DI

o    Renal unresponsiveness to ADH

o    Causes: lithium, hypercalcaemia, hypokalaemia

o    No response to desmopressin

Calcium and Magnesium Disorders

•    Calcium

o    Hypocalcaemia: tetany, seizures, prolonged QT

    Causes: hypoparathyroidism, vitamin D deficiency, CKD

o    Hypercalcaemia: "bones, stones, groans, psychiatric overtones"

    Causes: hyperparathyroidism, malignancy, sarcoidosis

•    Magnesium

o    Hypomagnesaemia: ↑ neuromuscular excitability, associated with low K⁺ and Ca²⁺

Hypermagnesaemia: rare; may cause hypotonia, bradycardia, cardiac arrest (e.g. in renal failure)