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Sodium Nitroprusside

Sodium Nitroprusside

SODIUM NITROPRUSSIDE

Use & Toxicity Overview

Use: Vasodilating agent used for hypertensive emergencies, controlled hypotension in anaesthesia and acute or chronic heart failure.

Toxicity

Toxic effects of sodium nitroprusside are due to two mechanisms:

1) Vasodilatation
via the production of nitric oxide
2) Cyanide Metabolism
in erythrocytes, which inhibits cytochrome oxidases and cellular respiration.
All patients who have ingested nitroprusside should be immediately referred to an ED observed for at least 4 hours after exposure. Asymptomatic patients can then be considered for discharge & advice to return if symptoms develop.
Clinical Features

Features due to direct effect of nitroprusside:
Nausea, vomiting, headache, dizziness, palpitations, muscle twitching and profound hypotension.

Features due to cyanide:
Headache, nausea, vomiting, sweating, tachycardia, hypotension, metabolic acidosis, convulsions, coma and cardio-respiratory arrest.

  • Lactic acidosis may occur up to 1 hour following discontinuation of nitroprusside.
  • Methaemoglobinaemia.

Features due to thiocyanate toxicity:
Anxiety, confusion, miosis, tinnitus, hallucinations, muscle cramps, hyperreflexia and convulsions.

Management: Intravenous Exposure
  1. Stop intravenous infusion of nitroprusside.
  2. Maintain a clear airway and ensure adequate ventilation.
  3. Monitor vital signs and cardiac rhythm; check the capillary blood glucose. Check and record pupil size.
    Please be aware that pulse oximetry is unreliable in the presence of methaemoglobinaemia.
  4. Perform a 12-lead ECG in all patients who require assessment. Check cardiac rhythm, QT interval and QRS duration.

    Consider repeating the ECG in ANY OF the following circumstances:

    • The initial ECG is abnormal
    • The patient is symptomatic
    • The recommended observation period is not yet complete
  5. In all patients, check arterial (or venous) blood gases for pH, lactate and methaemoglobin concentrations.

    Decrease lactate is expected once nitroprusside infusion is stopped, and dehydration resolves. Persistently high lactate concentration above 10 mmol/L may indicate a need for treatment of cyanide toxicity.

    Methaemoglobinaemia: refer to PCC protocol 34

Management: Ingestion
  1. Maintain a clear airway and ensure adequate ventilation.
  2. Consider activated charcoal (charcoal dose: 50 g for adults; 1 g/kg for children) if the patient presents within 1 hour of ingestion of any amount.

    If practical expertise exists, consider gastric aspiration/lavage in adults within 1 hour of ingestion of nitroprusside crystals, provided the airway can be protected.

  3. Monitor vital signs and cardiac rhythm; check the capillary blood glucose. Check and record pupil size.
    Please be aware that pulse oximetry is unreliable in the presence of methaemoglobinaemia.
  4. Perform a 12-lead ECG in all patients. Check cardiac rhythm, QT interval and QRS duration.

    Consider repeating the ECG if the initial ECG is abnormal, the patient is symptomatic, or the observation period is incomplete.

  5. Check arterial (or venous) blood gases for pH, lactate and methaemoglobin concentrations.

    Persistently high lactate concentration above 10 mmol/L may indicate a need for treatment of cyanide toxicity.

Risk Factors & Lethal Dose

Most adverse reactions occurred on day 5 of the IV infusion, and were associated with higher rates of infusion (more than 200 micrograms/min) and poor renal function.

Lethal Dose: The lowest published lethal parenteral dose is 10 mg/kg administered intravenously over 80 minutes (Davies et al, 1975).

High Risk Factors: A variety of factors—including poor nutrition, critical illness, surgery, diuretic use, and young age—place patients at risk for developing cyanide toxicity.