Nitroprusside Sodium
**Nitroprusside sodium** is a highly potent, ultra-short-acting **mixed vasodilator** used exclusively in intensive care settings. Key clinical highlights: * **Primary Indications**: Management of hypertensive crises, acute heart failure secondary to severe mitral regurgitation, and refractory congestive heart failure (CHF). * **Hemodynamic Effects**: Rapidly reduces both preload and afterload by relaxing arterial and venous smooth muscle. * **Clinical Pearl**: Because of its profound hypotensive effects and ultra-short half-life, it must be administered as a Constant Rate Infusion (CRI) with **continuous direct arterial blood pressure monitoring**. * **Toxicity Risk**: Prolonged use (>48-72 hours) or high doses can lead to accumulation of toxic metabolites (cyanide and thiocyanate), especially in patients with renal or hepatic impairment.
Mechanism: Nitroprusside acts as a **nitric oxide (NO) donor**. * Once in the bloodstream, it interacts with tissue sulfhydryl groups to release NO. * NO activates the enzyme **soluble guanylate cyclase** in vascular smooth muscle cells. * This leads to an increase in intracellular **cyclic guanosine monophosphate (cGMP)**. * Elevated cGMP activates protein kinase G, which reduces intracellular calcium concentrations โ **vascular smooth muscle relaxation**. This mechanism causes direct peripheral vasodilation of both arterioles and venules (independent of autonomic innervation), resulting in: * Lowered blood pressure * Significant reduction in total peripheral resistance (afterload) * Reduced left ventricular end-diastolic pressure (preload) * Mild decrease in cardiac output (though in severe heart failure, CO may actually increase due to afterload reduction) * Reflex tachycardia (increase in heart rate)
Dosing by species
- Hypertensive crisis (systolic arterial BP >200 mm Hg) ยท Initiate dose at 0.5 micrograms/kg/minute; increase dosage incrementally every 3-5 minutes until a predetermined target BP is attained. Reduce BP 25% over 4-hour period to allow readaptation of cerebral blood vessels. ยท IV ยท CRI ยท Titrate to effect
- Adjunctive treatment of heart failure (cardiogenic shock; fulminant pulmonary edema) ยท Initiate dose at 0.5 micrograms/kg/minute; increase dosage incrementally every 3-5 minutes. Goal to decrease or maintain mean arterial pressure to support vital organ functions-approx. 70 mmHg. ยท IV ยท CRI ยท Titrate to effect ยท Concurrent use of dobutamine (1-5 micrograms/kg/min) often indicated.
- Adjunctive treatment of heart failure ยท Initiate dose at 0.5 micrograms/kg/minute constant rate infusion and increase by 0.5-1 microgram/minute every 5 minutes to desired systolic pressure (90-100 mmHg). ยท IV ยท CRI ยท Titrate to effect ยท Cats are more sensitive to oxidative damage; keep total dosages to a minimum. Use dedicated line; never flush.
- Catastrophic pulmonary edema ยท As a CRI initiated at 1 microgram/kg/min and carefully titrated to effect by increasing by 1 microgram/kg/min increments every 15 minutes as long as BP remains stable and until perfusion and pulmonary function improves (cats usually requires between 1-2 micrograms/kg/min with the upper limit being 2 micrograms/kg/min). ยท IV ยท CRI ยท Maintain most effective dose for 12-15 hours ยท Systolic BP must remain >90 mm Hg. Wean nitroprusside over 6 hours first, then dobutamine over 6 hours.
- Hypertensive crisis (systolic arterial BP >200 mm Hg) ยท Initiate dose at 1-2 micrograms/kg/minute; increase dosage incrementally every 3-5 minutes until a predetermined target BP is attained. Reduce BP 25% over 4-hour period to allow readaptation of cerebral blood vessels. ยท IV ยท CRI ยท Titrate to effect
Routes of administration
Contraindications
- Compensatory hypertension (e.g., AV shunts, coarctation of the aorta, Cushing's reflex)
- Inadequate cerebral circulation
- Emergency surgery in patients near death
Adverse effects
- Profound hypotension
- Nausea
- Retching
- Restlessness
- Apprehension
- Muscle twitching
- Dizziness
- Infusion site irritation (avoid extravasation)
- Thiocyanate and cyanide toxicity (with prolonged use)
Drug interactions
- General Anesthetics (e.g., halothane, enflurane) ยท May enhance the hypotensive effects of nitroprusside.
- Dobutamine ยท Synergistic effects (increased cardiac output and reduced wedge pressure) may result; often used together beneficially in severe heart failure.
- Other Hypotensive Agents (e.g., beta-blockers, ACE inhibitors) ยท Patients may be more sensitive to the hypotensive effects of nitroprusside.
Monitoring
- Blood pressure (constant, direct arterial monitoring preferred)
- Acid/base balance (to detect early metabolic acidosis/cyanide toxicity)
- Electrolytes (especially Na+)
- Respiratory effort and thoracic auscultation
- Serum thiocyanate levels (if on prolonged therapy or concurrent renal dysfunction)
Overdose
**Acute Overdosage**: Manifests as profound hypotension. * **Treatment**: Reduce or stop the infusion and administer IV fluids. Blood pressure will typically recover within 1-10 minutes. Monitor BP constantly. **Toxicity (Cyanide/Thiocyanate)**: Excessive doses, prolonged therapy (>3 days), depleted hepatic thiosulfate, or severe hepatic/renal insufficiency may lead to cyanogen or thiocyanate toxicity. * **Signs**: Metabolic acidosis (early sign of cyanogen toxicity), tolerance to therapy, delirium (thiocyanate toxicity in dogs). * **Monitoring**: Monitor acid/base status. Serum thiocyanate levels >100 mcg/mL are considered toxic. * **Treatment**: Hydroxocobalamin (Vitamin B12a) may prevent or treat cyanogen toxicity. Contact an animal poison control center for specific antidotal protocols if suspected.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.