Acid-Base Interpretation

By VeriScorePrep Team · 6 min

Arterial blood gas interpretation is a core skill with a simple, repeatable approach.

Normal values

• pH: 7.35–7.45
• PaCO2: 35–45 mmHg
• HCO3: 22–26 mEq/L

The steps

1. Look at pH. Below 7.35 = acidosis; above 7.45 = alkalosis.
2. Determine the source. If pH and PaCO2 move in opposite directions, respiratory; if pH and HCO3 move in the same direction, metabolic.
3. Check compensation. The body attempts to normalize pH:

• Respiratory acidosis: kidneys retain HCO3 (over days).
• Respiratory alkalosis: kidneys excrete HCO3 (over days).
• Metabolic acidosis: lungs blow off CO2 (Kussmaul breathing, minutes to hours).
• Metabolic alkalosis: lungs retain CO2 (less reliable).

1. Determine if compensation is partial or full. Full compensation returns pH to the normal range.
2. Look for a mixed disorder if the compensation formula does not match (e.g., Winter's formula: expected PaCO2 = 1.5 × HCO3 + 8 ± 2 in metabolic acidosis).
3. Assess oxygenation with PaO2 and SaO2 separately from acid-base status.

Common patterns

• Respiratory acidosis: COPD, hypoventilation, opiate overdose — high CO2, low pH.
• Respiratory alkalosis: hyperventilation, anxiety, pain, sepsis early — low CO2, high pH.
• Metabolic acidosis: DKA, lactic acidosis, renal failure, diarrhea — low HCO3, low pH.
• Metabolic alkalosis: vomiting, NG suction, diuretics — high HCO3, high pH.