1. Introduction
Arterial blood gas analysis provides rapid information about oxygenation, ventilation and acid-base status. Interpretation is most accurate when performed systematically and in the context of the patient''s clinical condition and inspired oxygen concentration.
2. Technical checks and oxygenation
Start by confirming the sample type, inspired oxygen concentration and any important clinical context such as asthma, COPD, sepsis or reduced conscious level. Check the pH, partial pressure of oxygen, oxygen saturation and lactate. Interpret oxygenation in light of the FiO2 rather than using the PaO2 in isolation.
3. Acid-base assessment
Determine whether there is acidaemia or alkalaemia using the pH. Then assess PaCO2 and bicarbonate or base excess to decide whether the primary disturbance is respiratory or metabolic. If PaCO2 changes in the opposite direction to pH, the primary problem is respiratory. If bicarbonate changes in the same direction as pH, the primary problem is metabolic.
4. Compensation and mixed disorders
Assess whether compensation is appropriate. Respiratory disorders are compensated metabolically by the kidneys, whereas metabolic disorders are compensated through changes in ventilation. If compensation is absent, excessive or the pH is near normal despite several abnormalities, consider a mixed acid-base disorder.
5. Further analysis and summary
If metabolic acidosis is present, calculate the anion gap when the necessary electrolytes are available and consider lactate. Summarise the primary disorder, degree of compensation, oxygenation status and any likely cause. State whether there is type 1 or type 2 respiratory failure if relevant.
Initial checks
Acid-base
Interpretation
Oxygenation
Additional analysis
Summary