Toxicology and blood gases

A patient is brought into the resuscitation room with a reduced conscious level. He was   recognised as having been admitted a week earlier with deliberate self-poisoning. His blood gases were as follows: pH 7.01; PaO2 9.8 kPa; PaCO2 6.1 kPa; HCO3 – 12 mEq/L; base excess –18; anion gap 9 mEq/L.

Which one of the following is the patientMOST LIKELY to have been poisoned with?

a) Amitriptyline

b) Methadone

c) Paroxetine

d) Ethanol

e) Organophosphates

Answer: e

Explanation

Severe amitriptyline poisoning typically has amixed respiratory and metabolic acidosis. Methadone poisoning will manifest itself as a respiratory acidosis. Poisoning in which  paroxetine is the only agent taken generally produces mild symptoms such as vomiting and tremor. Paroxetine poisoning would not usually produce any derangement of arterial blood gases. Rarely, the patient may develop a ‘serotonin syndrome’ in which cognitive or behavioural changes (such as agitation or coma), autonomic changes (such as hyperthermia and tachycardia) and neuromuscular instability (such as hyperreflexia and trismus) are found. Ethanol would show a respiratory acidosis with a raised anion gap. Organophosphate overdose is a commonmethod of suicide around the world. It is particularly prevalent in rural settings where organophosphate pesticides are easy to access. Organophosphates inhibit both cholinesterase and pseudocholinesterase, leading to an accumulation of acetylcholine at synapses. This manifests itself as overstimulation of nicotinic andmuscarinic receptors. Typically, the patients get respiratory distress with tachypnoea following respiratory muscle weakness, pulmonary oedema and reduced respiratory drive. They present with a combined metabolic and respiratory acidosis. Treatment is with atropine and oximes.