Arterial blood gas analysis

Principle

• pH: glass electrode (potentiometric) against a reference electrode.
• pCO2: Severinghaus electrode, a pH electrode behind a CO2-permeable membrane with bicarbonate buffer; CO2 diffusion changes the buffer pH.
• pO2: Clark electrode (amperometric); O2 is reduced at a platinum cathode and the current is proportional to pO2.
• Calculated: bicarbonate (Henderson–Hasselbalch), base excess and, unless co-oximetry is used, oxygen saturation.

Specimen

• Arterial blood (usually radial artery; perform modified Allen test first) in a syringe with dry balanced lithium heparin.
• Expel air bubbles immediately, cap, mix gently.
• Analyse plastic syringes at room temperature within about 30 minutes; do not ice plastic syringes.

Results

• Typical arterial values: pH 7.35–7.45; pCO2 35–45 mmHg (4.7–6.0 kPa); pO2 80–100 mmHg (10.6–13.3 kPa) on room air; HCO3 22–26 mmol/L; base excess –2 to +2 mmol/L; SaO2 >95%. Local ranges apply.
• Interpretation: check pH (acidaemia/alkalaemia), then pCO2 (respiratory) and HCO3 (metabolic) to find the primary disorder, then assess compensation and anion gap.

Quality control and pitfalls

• Air bubbles: pO2 moves toward about 150 mmHg and pCO2 falls, raising pH.
• Delay: continued cell metabolism lowers pO2 and pH and raises pCO2; worse with high white cell or platelet counts.
• Excess liquid heparin dilutes the sample, lowering pCO2 and HCO3.
• Venous contamination gives falsely low pO2.
• Analysers measure at 37 °C; temperature correction is optional per local policy.
• Run QC at three levels and follow calibration checks; participate in EQA.

Clinical use

• Assessment of oxygenation, ventilation and acid–base status in critical care, respiratory failure, DKA and sepsis.