Partial pressure of carbon dioxide (pCO₂)

What it measures

pCO₂ reflects the amount of carbon dioxide dissolved in blood and is controlled by breathing. It defines the respiratory component of acid–base balance and the adequacy of ventilation.

Specimen and preparation

• Specimen: Arterial blood in a heparinised (balanced lithium heparin) syringe
• Tube: Other top
• Preparation: Expel air bubbles, cap immediately, mix well and analyse within 30 minutes (ideally 15) at room temperature in a plastic syringe; do not ice plastic syringes, and use a glass syringe on ice only if longer delay is unavoidable. Record FiO₂ and temperature.

Reference range

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

• Hypoventilation (COPD, opioids, sedation)
• Neuromuscular disease and chest wall disorders
• Compensation for metabolic alkalosis
• Severe asthma (late, ominous sign)

Causes of low values

• Hyperventilation (anxiety, pain, fever)
• Pulmonary embolism and early asthma
• Compensation for metabolic acidosis
• Excess mechanical ventilation
• Air bubble in sample

Method

Severinghaus CO₂ electrode on a blood gas analyser

Good to know

A pCO₂ above 50 mmHg with low pH indicates acute respiratory failure (type 2).

Related tests