Arterial Blood Gases

Arterial blood gases should be taken on any patients who appear to have respiratory depression or are unconscious, and on patients who have taken tricyclic antidepressants.

The presence of CO₂ retention, particularly if the pCO₂ is rising progressively, may be an indication for ventilation.

The presence of acidosis is usually secondary to some other effect of the poison:

• respiratory depression leading to respiratory acidosis
• seizures or shock leading to lactic acidosis

However, the acidosis will occasionally be due to a direct effect on metabolism from specific drugs leading to a high anion gap acidosis.

Acidosis can increase the toxicity of a number of drugs:

• acidosis significantly increases toxicity of tricyclic antidepressants
• acidosis significantly increases CNS salicylate concentrations
• acidosis directly increases the risk of arrhythmias with a wide range of cardiotoxic drugs

Thus, in the management of poisonings, it is important to prevent acidosis and correct it if it occurs with sodium bicarbonate and/or hyperventilation as appropriate.