Blood gas analysis (arterial blood gas, or ABG) is a blood test that measures the levels of oxygen, carbon dioxide, and acid-base balance in the bloodstream. A small sample of arterial blood (usually taken from the radial artery at the wrist) is analysed within minutes by a bedside machine in the critical care unit or emergency department.
The key values measured include: pH (blood acidity, normal 7.35 to 7.45; acidosis below 7.35 indicates poor tissue perfusion or metabolic disturbance, a critical finding after cardiac arrest); PaO2 (arterial oxygen tension, reflecting how well oxygen is crossing from the lungs into the bloodstream); PaCO2 (arterial carbon dioxide tension, reflecting how effectively the lungs are removing CO2; elevated levels indicate underventilation); bicarbonate (a marker of the metabolic component of acid-base balance; low bicarbonate alongside acidosis suggests metabolic acidosis as seen in shock or renal failure); and lactate (elevated levels indicate anaerobic metabolism due to inadequate oxygen delivery, a sign of shock or organ dysfunction).
In cardiac arrest management, ABG monitoring guides ventilation settings on the mechanical ventilator, informs vasopressor and fluid decisions, and helps detect and monitor hypoxic brain injury risk. Target oxygen and CO2 levels are tightly managed in the post-arrest period, as both extremes are harmful to the injured brain.
Serial ABG measurements allow the intensive care team to track trends in oxygenation and acid-base balance, adjusting ventilator settings and treatments in response. As a patient recovers, improving lactate clearance and normalising pH are positive prognostic signs indicating that tissue perfusion is being restored.
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