Arterial Blood Gas Analysis

Arterial Blood Gas Analysis (ABG) analysis should be performed when clinically indicated as per the BTS guidelines on oxygen therapy9:

  • All critically ill patients.
  • Unexpected or inappropriate fall in SpO2 below 94% in patients breathing air or oxygen or any patient requiring oxygen to achieve the above target range.
  • Deteriorating oxygen saturation (fall of ≥3%) or increasing breathlessness in a patient with previously stable chronic hypoxaemia (e.g., severe COPD)
  • Most previously stable patients who deteriorate clinically and require increased fraction of inspired oxygen (FiO2) to maintain a constant oxygen saturation
  • Any patient with risk factors for hypercapnic respiratory failure who develops acute breathlessness, deteriorating oxygen saturation, drowsiness or other features of carbon dioxide retention.
  • Patients with breathlessness who are thought to be at risk of metabolic conditions such as diabetic ketoacidosis or metabolic acidosis due to renal failure
  • Fall of O2 saturation of >3% or more.
  • Quantification of carboxyhaemoglobin/methaemoglobin is required.

Venous/ capillary gas sampling should be done in preference to arterial sampling for the following indications:-

  • An assessment of the acid/base (pH, bicarbonate, base excess) status is required
  • Quantification of lactate is required as part of assessment of tissue perfusion

The interpretation of ABG assays is outside the remit of this session and is discussed in the session Arterial Blood Gas Analysis.