Interpret the arterial blood gas (ABG) values, including pH, partial pressure of carbon dioxide (PaCO₂), partial pressure of oxygen (PaO₂), bicarbonate (HCO₃⁻), and any other relevant measurements such as base excess or lactate.

How to Interpret an Arterial Blood Gas (ABG)

Use a systematic stepwise approach evaluating pH first, then PaCO₂ and HCO₃⁻ to determine the primary acid-base disturbance, followed by assessment of compensation and oxygenation status.

Systematic Interpretation Algorithm

Step 1: Assess the pH (Normal: 7.35-7.45)

• pH < 7.35 = Acidemia
• pH > 7.45 = Alkalemia
• pH 7.35-7.45 = Normal (but may still have compensated disorder)

Step 2: Determine the Primary Disturbance Using RoMe Technique

The RoMe mnemonic provides a simple framework 1:

• Respiratory = Opposite: pH and PaCO₂ move in opposite directions

  • Low pH + High PaCO₂ = Respiratory acidosis
  • High pH + Low PaCO₂ = Respiratory alkalosis

• Metabolic = Equal: pH and HCO₃⁻ move in the same direction

  • Low pH + Low HCO₃⁻ = Metabolic acidosis
  • High pH + High HCO₃⁻ = Metabolic alkalosis

Step 3: Evaluate PaCO₂ (Normal: 35-45 mmHg or 4.7-6.0 kPa)

• PaCO₂ > 45 mmHg (>6.0 kPa) = Hypercapnia (respiratory acidosis or compensation for metabolic alkalosis)
• PaCO₂ < 35 mmHg (<4.7 kPa) = Hypocapnia (respiratory alkalosis or compensation for metabolic acidosis)

Step 4: Evaluate HCO₃⁻ (Normal: 22-26 mEq/L)

• HCO₃⁻ < 22 mEq/L = Metabolic acidosis or compensation for respiratory alkalosis
• HCO₃⁻ > 26 mEq/L = Metabolic alkalosis or compensation for respiratory acidosis

Step 5: Determine Compensation Status 2

• Uncompensated: Only one parameter (PaCO₂ or HCO₃⁻) is abnormal
• Partially compensated: Both PaCO₂ and HCO₃⁻ are abnormal, pH remains abnormal
• Fully compensated: Both PaCO₂ and HCO₃⁻ are abnormal, pH returns to normal range (7.35-7.45)

Step 6: Assess Oxygenation (Normal PaO₂: 80-100 mmHg)

• PaO₂ < 60 mmHg = Hypoxemia requiring intervention
• Always interpret PaO₂ in context of FiO₂ (inspired oxygen concentration) 3
• Calculate P/F ratio (PaO₂/FiO₂) if available; <300 suggests acute respiratory failure

Step 7: Evaluate Additional Parameters

Base Excess (Normal: -2 to +2 mEq/L)

• Reflects metabolic component independent of respiratory changes
• BE < -2 = Metabolic acidosis
• BE > +2 = Metabolic alkalosis
• Base excess is a strong early predictor of mortality in critically ill patients 4

Lactate (Normal: <2 mmol/L)

• Lactate > 4 mmol/L indicates tissue hypoperfusion and is associated with increased mortality 4, 5
• Lactate is the strongest predictor of poor neurological outcomes in cardiac arrest patients 4

Critical Clinical Thresholds

Acute Hypercapnic Respiratory Failure

NIV should be initiated when pH <7.35, PaCO₂ ≥6.5 kPa (49 mmHg), and respiratory rate >23 breaths/min persist after one hour of optimal medical therapy 6. For PaCO₂ between 6.0-6.5 kPa, consider NIV based on clinical context 6.

Prognostic Markers

• pH < 7.25 is a time-dependent predictor of mortality, particularly in males within the first 12 hours of trauma 5
• BE < -5.5 mmol/L predicts mortality starting at 6-12 hours post-injury 5
• Combined assessment of pH, base excess, and lactate provides superior prognostic information for both mortality and neurological outcomes 4

Common Pitfalls to Avoid

1. Ignoring compensation: A "normal" pH doesn't mean normal physiology—look for compensatory changes in PaCO₂ and HCO₃⁻
2. Missing mixed disorders: When PaCO₂ and HCO₃⁻ move in unexpected directions, suspect combined metabolic and respiratory disturbances 6
3. Overlooking oxygenation: Don't focus solely on acid-base status; hypoxemia requires immediate attention
4. Neglecting clinical context: ABG interpretation must align with patient presentation, not exist in isolation 3
5. Temperature correction controversy: Most modern practice uses uncorrected values at 37°C for consistency 3

Sex-Specific Considerations

Males demonstrate more extreme pH and lactate values at admission compared to females in trauma settings 5. Time-dependent predictive thresholds for pH, BE, and lactate differ by sex, with males showing earlier predictive value for pH abnormalities 5.