⚗️ Chapter 93: Metabolic Acidosis and Metabolic Alkalosis

Anion gap (AG) · Delta gap (AG excess/HCO₃ deficit) · Winter's formula (expected PaCO₂ = 1.5×HCO₃ + 8 ± 2) · High AG vs normal AG acidosis · Lactic acidosis (type A/B) · Stewart's strong ion difference (SID) · Metabolic alkalosis: chloride-responsive vs resistant · Urine chloride

🔍 Core Concepts: Metabolic Acidosis & Alkalosis

📌 Metabolic Acidosis (pH <7.35)
High AG: MUDPILES (methanol, uraemia, DKA, propylene glycol, INH, lactate, ethylene glycol, salicylates). Normal AG: HARDASS (hyperalimentation, acetazolamide, RTA, diarrhoea, Addison's, saline, spironolactone).
📊 Winter's Formula
Expected PaCO₂ = (1.5 × HCO₃) + 8 ± 2. If measured PaCO₂ < expected → respiratory alkalosis; > expected → respiratory acidosis (mixed disorder).
📐 Anion Gap & Delta Gap
AG = Na + K - (Cl + HCO₃). Normal 8-16 (with K) or 10-20. Correct AG for albumin (↑2.5 per 1 g/dL ↓albumin). Delta gap = (AG - 12)/(24 - HCO₃). Simple acidosis: delta 1-2.
🩸 Lactic Acidosis
Type A (hypoxia): shock, sepsis. Type B (normal O2 delivery): drugs, liver disease, mitochondrial disorders. D-lactic acidosis: short bowel syndrome.
⚖️ Metabolic Alkalosis (pH >7.45)
Chloride-responsive (UCl <20): vomiting, NG suction. Treat with NS + KCl. Chloride-resistant (UCl >20): mineralocorticoid excess, Bartter. Treat underlying.
⚠️ Stewart's Strong Ion Difference (SID)
SID = Na + K + Ca + Mg - (Cl + lactate). Normal 40-42. Low SID → acidosis (e.g., saline infusion → hyperchloraemic acidosis).

🩺 Stepwise Approach: Metabolic Acidosis

1
Confirm acidosis & assess compensation (Winter's formula)
pH <7.35, HCO₃ low. Calculate expected PaCO₂ = 1.5×HCO₃ + 8 ± 2. If measured PaCO₂ matches, simple metabolic acidosis. If higher → mixed respiratory acidosis; if lower → respiratory alkalosis.
2
Calculate anion gap (correct for albumin)
AG = Na + K - (Cl + HCO₃). Normal 8-16 (with K). If low albumin, add 2.5 × (4 - albumin). High AG → MUDPILES. Normal AG → HARDASS.
3
Calculate delta gap (AG excess/HCO₃ deficit)
Delta = (AG-12)/(24-HCO₃). If ~1-2, pure high AG acidosis. If <1, coexisting normal AG acidosis (HCO₃ loss). If >2, coexisting metabolic alkalosis.
4
Treat underlying cause
Lactic acidosis: fluids, vasopressors, treat sepsis. DKA: insulin/fluids. Toxins: dialysis, antidotes. Diarrhoea: fluids, repletion.
5
Consider bicarbonate (cautious use)
Indicated only if pH <7.15-7.20 with severe haemodynamic compromise. Avoid in DKA, lactic acidosis unless life-threatening. Dose: 1-2 mEq/kg over 30-60 min.
6
Metabolic alkalosis management
Check urine Cl. If <20 mEq/L: chloride-responsive → NS + KCl. If >20: chloride-resistant → treat underlying (spironolactone for hyperaldosteronism, stop diuretics).