Zinc Phosphide (Zn₃P₂) Poisoning - Management

Overview

Zinc phosphide is a steel-grey crystalline powder with a garlic/fish-like odour. It is a widely used, cheap, and effective rodenticide and pesticide. Its primary danger is the generation of phosphine gas (PH₃) on contact with gastric acid or atmospheric moisture:

Zn₃P₂ + 6HCl → 3ZnCl₂ + 2PH₃↑

Fatal dose: ~5 grams Fatal period: A few hours to 24 hours LD₅₀: ~40 mg/kg

Mechanism of Toxicity

Phosphine inhibits Cytochrome C Oxidase (Complex IV) of the mitochondrial electron transport chain, disrupting cellular respiration - similar to cyanide. This results in:

Free radical generation and oxidative stress
Multi-organ dysfunction (heart, liver, kidney, lungs, CNS)
Cellular hypoxia despite adequate oxygen delivery

Key difference from aluminium phosphide (AlP): ZnP has a delayed onset of systemic symptoms due to slower phosphine release kinetics. It also has a higher propensity for acute liver failure early in the course.

Clinical Features

GI (early, within minutes to hours)

Metallic taste, burning in throat, oesophagus, stomach
Nausea, vomiting, diarrhoea, abdominal pain
Garlic/fish odour on breath and vomitus
Symptom-free interval possible before systemic effects

Cardiovascular

Bradycardia, sinus tachycardia, ECG abnormalities
Arrhythmias, heart block
Hypotension, cardiovascular collapse
Myocardial damage, pericarditis, congestive cardiac failure (rare)

Respiratory

Dyspnoea, pulmonary oedema, respiratory distress
Cyanosis (terminal)

Neurological

Agitation, excitement
Hypocalcaemia, tetany, convulsions, coma

Other

Oliguria, renal failure
Metabolic acidosis
Hepatotoxicity / acute liver failure
Haemorrhages, bleeding diathesis
Shock

Diagnosis

Clinical: Garlic/fishy odour on breath + history of exposure + multi-organ failure.

Silver nitrate paper test: Patient breathes on filter paper impregnated with 0.1 N silver nitrate for 5-10 minutes. Blackening = positive (phosphine reduces silver nitrate to metallic silver).

Post-mortem findings: Petechial haemorrhages in skin, garlic odour on opening stomach, greyish-black residue sticking to gastric mucosa, congestion of liver/spleen/kidney/brain/lungs, necrobiosis of liver.

Management

1. Decontamination (Priority - Before Absorption)

Skin/eyes: Remove contaminated clothing immediately; wash thoroughly with soap and water.

GI decontamination (critical window - do early):

Induced vomiting: 1 tablespoon of salt in warm water, repeated until vomiting fluid is clear
Gastric lavage with 3-5% sodium bicarbonate solution - alkaline pH minimises the conversion of zinc phosphide to phosphine gas. Also, 1% copper sulphate or 1% potassium permanganate, or mineral oil (coconut/liquid paraffin) can be used - these convert the compound to phosphate within ~30 minutes and reduce phosphine absorption
- Note: Some recent studies caution against gastric lavage because added moisture accelerates phosphine generation; this remains debated
Activated charcoal: May be considered, though evidence is limited

Important: Zinc phosphide adheres firmly to the crypts of the gastric mucosa. Even small residual amounts after lavage can cause death by slow absorption. Extended observation is mandatory even after apparent initial recovery. - Parikh's Textbook of Medical Jurisprudence

2. Supportive Care (Cornerstone of Management)

There is no specific antidote for zinc phosphide poisoning.

Complication	Intervention
Respiratory distress / pulmonary oedema	Oxygen inhalation, mechanical ventilation (IPPV), steroids
Cardiovascular collapse / arrhythmias	Magnesium sulphate IV (3 g bolus, then 6 g over 24 hours for 5-7 days); low-dose dopamine infusion
Hypocalcaemia / tetany	IV calcium salts
Metabolic acidosis	Sodium bicarbonate 50 mEq IV every 15 min until arterial bicarbonate >15 mmol/L
Infection	Broad-spectrum antibiotics
Hepatic injury	Liver-targeted supportive therapy (N-acetylcysteine; see below)
Haemorrhage / coagulopathy	Vitamin K
Seizures	Benzodiazepines, sedatives
Refractory shock	ECMO (evidence from AlP studies; applicable to ZnP)

3. Antioxidant / Adjunct Therapies (Emerging Evidence)

Based on evidence from aluminium phosphide poisoning (mechanism largely shared) and direct ZnP studies:

N-Acetylcysteine (NAC): Meta-analysis of RCTs (OR 3.97, 95% CI 1.69-9.30 for recovery; OR 0.25 for mortality reduction) - significant benefit. Replenishes glutathione, reduces oxidative injury, directly supports hepatic recovery (especially relevant in ZnP's propensity for acute liver failure). [PMID: 34455951]
Vitamin E: Antioxidant; shown to reduce mortality in AlP clinical studies
Glucose-Insulin-Potassium (GIK) infusion: Myocardial protective
Fresh packed RBCs: Used in some centres with reported benefit
Magnesium sulphate: Anti-arrhythmic and membrane-stabilising; standard adjunct

4. Proposed ZnP-Specific Protocol (Faress et al., 2025)

A 2025 review (PMID: 41768810) proposes a protocol specifically for zinc phosphide - emphasising:

Aggressive GI decontamination first (due to delayed symptom onset giving a longer therapeutic window vs. AlP)
Early targeted treatment for acute hepatic failure (ZnP has higher liver failure risk than AlP)
Application of existing AlP management protocols for systemic phosphine toxicity once absorption occurs

5. Advanced Life Support

ECMO (Extracorporeal Membrane Oxygenation): For refractory cardiovascular failure, supported by evidence from AlP poisoning cases; should be considered early in severely poisoned patients
ICU admission for all symptomatic patients
Continuous ECG monitoring

Special Considerations

Observe for delayed symptoms even after initial recovery - ZnP binds to gastric mucosa and can be absorbed slowly
Healthcare workers must avoid inhaling vomitus or gastric contents (phosphine off-gassing risk)
Rooms where patients are managed should be ventilated
No specific antidote exists - management remains supportive; evidence-based standardised protocols are still lacking per 2023 systematic review [PMID: 37387512]

Key Evidence

Source	Findings
Parikh's Textbook of Medical Jurisprudence	Gastric lavage with NaHCO₃, emesis, Vit K, corticosteroids, supportive care
Dikshit's Textbook of Forensic Medicine	NaHCO₃/KMnO₄/CuSO₄ lavage; MgSO₄ IV, calcium, steroids, dopamine, O₂
Sobh ZK et al. (2023) - Systematic Review [PMID: 37387512]	79 treatment modalities reviewed; ECMO, NAC, Vit E, GIK, RBCs, GI decontamination with oils shown to reduce mortality; no antidote or standardised protocol yet
Rashid M et al. (2022) - Meta-Analysis [PMID: 34455951]	NAC significantly reduces mortality (OR 0.25) and improves recovery in rodenticide poisoning
Faress F et al. (2025) - Review [PMID: 41768810]	First ZnP-specific protocol proposed; aggressive decontamination + hepatic treatment + AlP protocols for systemic phosphine

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