Phosphine - Chemical Poisons

Phosphine

Synonyms

Hydrogen phosphide; Phosphoretted hydrogen.

Physical Appearance

Colourless, flammable gas with an odour of garlic or decaying fish.

Uses

·              Fumigant.

·              Grain preservative in the form of aluminium phosphide.

·              Rat poison in the form of zinc phosphide.

Usual Fatal Dose

Inhalation of phosphine at a concentration of 400 to 600 ppm can be lethal in 30 minutes. Exposure to 50 ppm is considered dangerous to life and health.

Mode of Action

Phosphine produces widespread organ damage due to cellular hypoxia as a result of binding with cytochrome oxidase, an important respiratory enzyme. The organs with the greatest oxygen requirements appear to be especially sensitive to damage and include the brain, kidneys, heart, and liver.

Clinical Features

·      Inhalation produces vertigo, headache, restlessness, chest pain, vomiting, and diarrhoea.

·      In severe cases there may be onset of adult respiratory distress syndrome (ARDS), pulmonary oedema, tachy-cardia, hypotension, cardiac arrhythmias, ataxia, tremor, diplopia, paraesthesias, convulsions, coma, and hepatorenal damage.

·              ECG abnormalities may include sinus tachycardia, sinus arrhythmia with ST segment depression in lead II, III, AVF, and T wave inversion in V5-6, and ventricular premature complexes followed by ventricular tachycardia.

·      Ingestion of phosphine-releasing compounds such as aluminium or zinc phosphide produces predominantly gastrointesinal manifestations. But systemic toxicity can produce most of the symptoms mentioned earlier. Metabolic acidosis, hypokalaemia, hypo- or hypermagnesaemia may also be encountered.

·      Chronic poisoning, characterised by anaemia, bronchitis, gastrointestinal disturbances and visual, speech and motor disturbances, may result from prolonged exposure to low concentrations.

Diagnosis

·      Silver Nitrate Test: To 1 ml of gastric contents in a testtube, add 1 ml of water. Take two strips of filter paper impregnated with 0.1 N silver nitrate* and place one over the mouth of the test tube, while the other is placed over a clean open surface. Gently heat the tube at 50^o C for 15 to 20 minutes. Remove the filter paper strip and dry it. Darkening of filter paper (due to deposition of silver) indicates a posi-tive test. The other strip of filter paper acts as a control. If this shows darkening it means there is contamination of the atmosphere (usually by hydrogen sulfide).

_·               PH₃ + 8AgNO₃ + 4H₂O→8Ag⁺ + H₃PO₄ + 8HNO₃

·              This test can be done on the breath of the patient instead of gastric contents in the following manner. Use the impregnated filter paper as a mask and ask the patient to breathe through it for 15 to 20 minutes. Blackening indicates the presence of phosphine. However it is less reliable.

Treatment

·      Stomach wash with 1:5000 potassium permanganate is claimed by some physicians to be useful, by oxidising phosphine to non-toxic phosphate.

·      Activated charcoal as a slurry in the usual manner.

·      Magnesium sulfate is a disputed antidote claimed by some investigators to be very effective, while others are doubtful about its actual role. Magnesium sulfate has membrane stabilising effect and may help in controlling the cardiac arrhythmias produced by phosphine. The usual dose recom-mended is 3 grams as IV bolus followed by 6 grams infusion over 12–24 hours for 5 to 7 days.

·      For convulsions:

Diazepam—5 to 10 mg IV over 2 to 3 minutes (adult).

0.25 to 0.4 mg/kg IV over 2 to 3 minutes (child).

or

Phenytoin—10 to 15 mg/kg IV at 30 to 50 mg/min (adult& child).

or

Phenobarbitone—12 to 15 mg/kg IV in 60 ml of normalsaline at 25 to 50 mg/min (adult & child).

·      For shock:

Dopamine—4 to 6 mcg/kg/min IV.

IV fluids—4 to 6 litres over 6 hours.

·      For metabolic acidosis:

Sodium bicarbonate—50 mEq/15 min.

·      For pulmonary oedema: Furosemide—20 to 40 mg IV.

·      For local irritation of GI tract: Ranitidine—50 mg IV, 8th hourly.

·      For respiratory failure: Ventilatory support.

Autopsy Features

·      Garlicky or decayed fish odour.

·      Pulmonary oedema.

·      Centrilobular necrosis of liver.

·      Focal myocardial necrosis.

Forensic Issues

·              Many cases of poisoning result from occupational exposure in agriculture, or domestic exposure from rat pastes or powders.

·              But today it is suicidal ingestion of aluminium phosphide which has assumed alarming proportions, especially in the central and northern states of India.