Renal / Respiratory

Primary Acid–Base Disturbances

Metabolic & respiratory — compensation, causes, calculations

Primary Acid–Base Disturbances
Feature	High-AG metabolic acidosis	Normal-AG metabolic acidosis	Respiratory acidosis	Respiratory alkalosis
Mechanism
Primary disturbance	↓HCO₃⁻ from added acid (AG expanded)	↓HCO₃⁻ with Cl⁻ gain (AG normal)	↑PaCO₂ (alveolar hypoventilation)	↓PaCO₂ (hyperventilation)
Assessment
Expected compensation	Winter: expected PaCO₂ ≈ 1.5×HCO₃⁻ + 8 (±2)	Same Winter relation applies	Acute: HCO₃⁻ +1 per 10 mmHg PaCO₂; Chronic: +3.5–4 per 10	Acute: HCO₃⁻ −2 per 10; Chronic: −4–5 per 10
Traps & pitfalls
Common ICU causes	Lactic (shock, metformin), ketoacidosis (DM/alcohol), toxins (methanol, ethylene glycol, salicylate), renal failure	GI HCO₃⁻ loss (diarrhea, fistula/high-output stoma), RTAs, acetazolamide, large-volume saline (hyperchloremic)	Sedatives/opioids, neuromuscular weakness (GBS/MG/high cord), COPD exacerbation, obesity hypoventilation, fatigue/auto-PEEP in late ARDS	Pain/anxiety, hypoxemia, early sepsis, PE, liver failure, salicylates, over-ventilation
Assessment
Key calculations	AG = Na − (Cl + HCO₃⁻); ΔΔ gap to expose a second (normal-AG) disorder; osmolar gap if toxin suspected; correct AG for albumin (+~2.5 per 1 g/dL below 4)	Urine anion gap; ΔΔ (low ratio suggests combined HAGMA+NAGMA)	Compare measured HCO₃⁻ vs expected → exposes superimposed metabolic disorder	Same expected-compensation comparison
Traps & pitfalls
Common traps	Hypoalbuminemia masks a high AG; D-lactate not detected on standard lactate assay	Post-saline acidosis misread as worsening AKI acidosis	Rising post-intubation PaCO₂ from AUTO-PEEP mistaken for worsening disease	Near-normal pH from mixed disturbances hides severity; severe alkalemia → arrhythmia, ↓cerebral flow, ↓ionized Ca²⁺

Comparing: High-AG metabolic acidosis

Mechanism · Primary disturbance

↓HCO₃⁻ from added acid (AG expanded)

Assessment · Expected compensation

Winter: expected PaCO₂ ≈ 1.5×HCO₃⁻ + 8 (±2)

Traps & pitfalls · Common ICU causes

Lactic (shock, metformin), ketoacidosis (DM/alcohol), toxins (methanol, ethylene glycol, salicylate), renal failure

Assessment · Key calculations

AG = Na − (Cl + HCO₃⁻); ΔΔ gap to expose a second (normal-AG) disorder; osmolar gap if toxin suspected; correct AG for albumin (+~2.5 per 1 g/dL below 4)

Traps & pitfalls · Common traps

Hypoalbuminemia masks a high AG; D-lactate not detected on standard lactate assay

For healthcare-professional education. Clinical decisions require patient-specific assessment and local protocols.

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