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.