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Sodium Nitroprusside — Cyanide + Thiocyanate Toxicity Thresholds
TEXTPharmacology · 5 min read
Fast, potent vasodilator for hypertensive emergencies + controlled hypotension — and a metabolic time bomb above modest doses or in renal failure.
After this lesson you can
2 min read6 sections- Recall the cyanide toxicity dose threshold for SNP.
- Recognize cyanide toxicity at the bedside.
- Execute hydroxocobalamin treatment.
- Identify thiocyanate accumulation risk.
Mechanism + clinical use
Onset within seconds, half-life ~2 minutes — titratable beat-to-beat.
Used for hypertensive emergencies, controlled hypotension for surgery (aneurysm clipping, AVM, scoliosis), aortic dissection (with beta-block first), acute LV failure with hypertension.
Cyanide release + the dose threshold
Metabolism releases free cyanide, which is normally cleared by hepatic rhodanese (converts CN to thiocyanate using thiosulfate + B12).
- infusion rates >
2 mcg/kg/minfor prolonged periods - OR cumulative dose >
0.5 mg/kg - OR tachyphylaxis (rising dose requirement)
- hepatic dysfunction
- malnourished (low thiosulfate stores)
- B12 deficient
Maximum: 10 mcg/kg/min for ≤10 minutes only; never sustained.
Cyanide toxicity recognition
Tachyphylaxis to vasodilator effect (requires escalating doses), metabolic acidosis with normal-to-elevated mixed venous O₂ saturation (cells can't extract O₂), altered mental status, seizure, arrhythmia, cardiovascular collapse.
Lactate climbing despite adequate perfusion + oxygenation is a classic clue.
Whole-blood cyanide level confirmatory but rarely available in time — treat empirically on clinical suspicion.
Treatment of acute cyanide toxicity
Hydroxocobalamin (Cyanokit) 5 g IV over 15 minutes — binds CN to form cyanocobalamin, renally excreted; first-line + safe in hemodynamic compromise.
Alternative: sodium thiosulfate 150 mg/kg IV (provides substrate for rhodanese).
Avoid the older sodium nitrite step in surgical patients (induces methemoglobinemia — worsens O₂ delivery in already-compromised patient).
100% O₂.
- vasopressors
- mechanical ventilation
- fluid
Thiocyanate accumulation in renal failure
It's cleared renally with half-life 3-7 days.
In renal failure, accumulation produces a different toxicity syndrome: psychosis, hyperreflexia, hypothyroidism (competes with iodide uptake), nausea, vomiting.
Avoid SNP >24 hours in renal failure; consider alternatives (clevidipine, nicardipine).
Hemodialysis removes thiocyanate.
Modern alternatives
Many institutions have largely replaced SNP with clevidipine (selective arterial vasodilator, ultra-short half-life, lipid emulsion) or nicardipine (calcium-channel blocker, dihydropyridine class).
Both lack the cyanide concern.
SNP retains a niche for controlled hypotension when balanced venous + arterial dilation is desired + duration is short.
⚠ Common pitfalls
- Sustained infusion >2 mcg/kg/min — cyanide accumulation.
- Sodium nitrite step in surgical patient with marginal hemodynamics — methemoglobinemia worsens it.
- Trusting normal mixed-venous SvO₂ — cyanide poisoning leaves cells unable to extract O₂; SvO₂ stays high paradoxically.
- SNP > 24 hr in renal failure — thiocyanate (psychosis, hypothyroidism).
💎 Clinical pearls
- Hydroxocobalamin (Cyanokit) 5 g IV over 15 min — first-line + safe in shock.
- Tachyphylaxis to vasodilation + rising lactate + metabolic acidosis = cyanide.
- Clevidipine or nicardipine = modern alternatives without the cyanide concern.
- Each SNP molecule yields 5 CN⁻ ions — small dose, big metabolic load.
Recap
- Hydroxocobalamin (Cyanokit) 5 g IV over 15 min — first-line + safe in shock.
- Tachyphylaxis to vasodilation + rising lactate + metabolic acidosis = cyanide.
- Clevidipine or nicardipine = modern alternatives without the cyanide concern.
- Each SNP molecule yields 5 CN⁻ ions — small dose, big metabolic load.
Mark each section done to complete the module.
References
- · Stoelting & Hines Pharmacology 6e ch 11
- · Miller's Anesthesia 9e ch 21
- · Curry & LoVecchio, Crit Care Clin 2012 (cyanide toxicity)
- · FDA package insert (Nipride / Nitropress)