Blood Gas Analysis

Introduction

Blood gas analysis is the gold standard for evaluating gas exchange in a patient. Analysers measure the pH, partial pressures of oxygen and carbon dioxide of the sample directly as well as calculating other parameters such as haemoglobin saturation and plasma bicarbonate. It provides details on oxygenation, ventilation and acid-base status of the patient. Samples used are usually heparinised and can be arterial or venous, but this should be considered when interpreting results.

pH

• Direct measurement
• Normal values - 7.35 -7.45
• Increases indicate an alkalaemia.
• Decreases indicate an acidemia.

Oxygen

Partial Pressure

• Direct measurement
• Assuming the inspired oxygen fraction is 21%, normal values are 80-100mmHg (10.7-13.3kPa)
• Increases may be due to increases in fractional inspired oxygen or atmospheric pressure.
• Decreases indicate hypoxaemia if below 60mmHg (8kPa)

Saturation

• Calculated value.
• Normal values - 95-100%
• Decreases indicate hypoxaemia if below 90%

Carbon Dioxide

Partial Pressure

• Direct measurement
• Normal values - 35-45mmHg
• Increases indicate a respiratory acidosis.
• Decreases indicate a respiratory alkalosis.

Plasma Bicarbonate

• Calculated value.
• Normal values - 22-26 mmol/l
• Increases indicate a metabolic alkalosis.
• Decreases indicate a metabolic acidosis.