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Data Interpretation
Blood gases
ECGs
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Blood Gas Interpretation

This is a subject which is almost guaranteed to come up in one or more aspects of your exams. It a skill that, although not hard, needs to be practised because mistakes are easily made. Blood gases are obtained from an arterial blood sample usually taken from the radial artery. The normal values are given below:

Normal Values

Source: Oxford Handbook of Clinical Medicine 7th Ed.

Abnormalities

The essential question you want to answer are: is there acidosis or alkalosis? Is this caused by a change in the PaCO2 or the HCO3? Is there compensation?

The routine that you should go through to answer these questions is straightforward:

  1. Look at the pH: is it acidotic (<7.35) or alkalotic (>7.35) or within the normal range.
  2. Look at the PaCO2. Raised levels cause acidosis and low levels cause alkalosis. Does the CO2 level explain the pH (respiratory acidosis or alkalosis)?
  3. Look at the HCO3. 

This assumes that the pH is abnormal. If it is normal, check whether this the PaCO2 and HCO3 are within the normal ranges or whether one is fully compensating the other. There is never over-compensation: the pH will never go from acidosis to alkalosis or vice versa as a result of compensation.

Example

The pH in this example is less than 7.35 and therefore acidotic. Next look to see whether this can be explained by the PaCO2. The PaCO2 is higher than the normal range. Think of CO2 as an acid and this explains the blood acidity. Finally is there any compensation? the base excess is within the normal range so there is non.

Causes

Common causes of blood gas results should be memorised:

Gas Deviation Characteristics Common Causes
Respiratory Acidosis Acidosis with increased PaCO2 Type II respiratory failure: Asthma/COPD, muscle weakness (e.g. Guillain Barre syndrome), respiratory depression (e.g. sedatives)
Respiratory Alkalosis Alkalosis with low PaCO2 Commonly hyperventilation
Metabolic Acidosis Acidosis with low HCO3
  • Normal anion gap: HCO3 loss (e.g. diarrhoea), H retention (e.g. renal tubular acidosis)
  • High anion gap: Lactic acidosis (often post-MI), DKA, salicylate overdose
  • Metabolic Alkalosis Alkalosis with high HCO3 Potassium depletion, vomiting
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