Understanding Acid-Base Physiology
The body tightly regulates blood pH between 7.35 and 7.45 through the lungs and kidneys. The lungs control carbon dioxide (CO₂) elimination, while the kidneys manage bicarbonate (HCO₃⁻) reabsorption and hydrogen ion excretion. When either organ fails, acid-base disturbance occurs.
- Respiratory acidosis: pH < 7.35 with elevated PCO₂ (> 45 mmHg), caused by hypoventilation or CO₂ retention.
- Respiratory alkalosis: pH > 7.45 with low PCO₂ (< 35 mmHg), caused by hyperventilation.
- Metabolic acidosis: pH < 7.35 with low HCO₃⁻ (< 22 mEq/L), reflecting loss of bicarbonate or accumulation of acids.
- Metabolic alkalosis: pH > 7.45 with elevated HCO₃⁻ (> 26 mEq/L), from bicarbonate excess or acid loss.
Mixed disturbances—when two primary disorders coexist—complicate interpretation and require careful analysis of expected compensatory responses.
Arterial Blood Gas Sampling and Interpretation
Arterial blood samples, typically drawn from the radial artery, provide direct measurement of pH, PCO₂, and PaO₂. Unlike pulse oximetry, which estimates oxygen saturation, ABG gives absolute partial pressures and acid-base status.
Standard reference ranges for adults are:
- pH: 7.35–7.45
- PCO₂: 35–45 mmHg
- PaO₂: 75–100 mmHg (on room air at sea level)
- HCO₃⁻: 22–26 mEq/L
Venous or capillary samples are less reliable for oxygenation assessment but can indicate pH and CO₂ in resource-limited settings. Sample handling is critical: delays, exposure to air, or elevated temperature cause falsely elevated PCO₂ and low PaO₂.
Anion Gap Calculation
The anion gap quantifies unmeasured anions and is essential for categorizing metabolic acidosis. A high anion gap (> 16 mEq/L) suggests organic acid accumulation (lactate, ketones, methanol, ethylene glycol). A normal anion gap indicates hyperchloremic metabolic acidosis, often from diarrhea or renal tubular dysfunction.
Anion Gap = Sodium − (Chloride + Bicarbonate)
Corrected AG = AG + 2.5 × (4.4 − Albumin)
Sodium (Na⁺)— Serum sodium concentration in mEq/L; typically 136–145 mEq/L.Chloride (Cl⁻)— Serum chloride concentration in mEq/L; typically 98–107 mEq/L.Bicarbonate (HCO₃⁻)— Serum bicarbonate concentration in mEq/L; typically 22–26 mEq/L.Albumin— Serum albumin in g/dL; used to correct for protein effect on anion gap (normal ≈ 4.4 g/dL).
Common Pitfalls in ABG Interpretation
Misinterpreting acid-base disorders is common without careful attention to patterns and compensation.
- Ignoring respiratory compensation — When metabolic acidosis is present, expect appropriate hyperventilation (low PCO₂). If PCO₂ is normal or elevated despite low pH and HCO₃⁻, concurrent respiratory acidosis exists—a medical emergency requiring urgent intervention.
- Forgetting to correct anion gap for albumin — Hypoalbuminemia lowers the anion gap. A patient with sepsis and a measured AG of 14 might have a corrected AG of 17 if albumin is severely depleted, revealing a high-AG metabolic acidosis masked by low serum protein.
- Confusing compensatory responses with primary disorders — In respiratory acidosis, the kidneys compensate by retaining HCO₃⁻. However, compensation takes 12–24 hours; if a patient with acute hypercapnia has HCO₃⁻ of 26, it reflects the primary respiratory process, not metabolic alkalosis.
- Sampling errors skewing results — Arterial samples exposed to air lose CO₂ (falsely lowering PCO₂) and gain oxygen. Venous admixture, delay in icing the sample, or patient movement during collection causes erroneous values that mislead clinical assessment.
Clinical Application and Case Example
Consider an 78-year-old patient presenting with altered mental status and laboured breathing. ABG shows: pH 7.22, PCO₂ 58 mmHg, HCO₃⁻ 24 mEq/L, Na⁺ 138 mEq/L, Cl⁻ 102 mEq/L. Anion gap = 138 − (102 + 24) = 12 mEq/L (normal).
The low pH and high PCO₂ indicate primary respiratory acidosis. The normal anion gap and near-normal HCO₃⁻ suggest this is acute (< 12 hours), as chronic hypercapnia would show elevated HCO₃⁻ from renal compensation. The cause might be oversedation, neuromuscular weakness, or severe COPD exacerbation. Immediate management focuses on improving ventilation rather than treating metabolic disorder.
This stepwise interpretation—assessing pH, identifying the primary process (respiratory vs. metabolic), calculating anion gap, and evaluating compensation—prevents diagnostic error and directs appropriate treatment.