Quinidine

Quinidine is a class Ia antiarrhythmic with an established historical role in equine atrial fibrillation conversion. It is also used less commonly for selected supraventricular or ventricular arrhythmias in other veterinary patients. Sulfate and gluconate formulations provide different delivery options within specialist cardiac treatment plans.

Group

Class Ia antiarrhythmic

Mechanism of Action

Quinidine blocks fast cardiac sodium channels, slowing phase-0 depolarization and impulse conduction in responsive tissue. Additional potassium-channel effects prolong repolarization and increase refractoriness. These combined actions can interrupt re-entry circuits and suppress selected ectopic rhythms. Its vagolytic effects also influence atrioventricular conduction, so the overall response depends on the arrhythmia and cardiac substrate.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Arrhythmia with gluconate	6–20 mg/kg	IM	Every 6 h	Expressed as quinidine base.
Arrhythmia with gluconate	6–20 mg/kg	PO	Every 6–8 h	Quinidine base.
Arrhythmia with polygalacturonate	6–20 mg/kg	PO	Every 6 h	Quinidine base.
Arrhythmia with sulfate	Reference only	PO		Reported oral interval reads q68h, which is ambiguous; amount 6-20 mg/kg as base is retained without guessing an interval.
Arrhythmia with sulfate	5–10 mg/kg	IV	Every 6 h	Reported states dose as quinidine base.

Cattle

Use / stage	Dose	Route	Schedule	Instructions
Atrial fibrillation	Reference only	IV		Reported gives 49 mg/kg IV over 4 hours then 42 mg/kg maintenance without timing, or 40 mg/kg in 4 L fluid at 1 L/hour until conversion. Do not generalize these linked protocols to bolus dosing.

Horses

Use / stage	Dose	Route	Schedule	Instructions
Atrial fibrillation IV conversion	Reference only	IV		1-1.5 mg/kg IV every 10-15 minutes until sinus rhythm or cumulative 10 mg/kg. Cumulative course cap is not a per-dose cap.
Atrial fibrillation oral conversion — Initial	11.1111111111 mg/kg	PO	Once	5 g per 450 kg via stomach tube; ECG and blood-pressure monitoring required.
Atrial fibrillation oral conversion — Maintenance	22.2222222222 mg/kg	PO	Every 2 h	10 g per 450 kg every 2 hours until sinus rhythm; stop on conversion or adverse effects.

Indications

Pharmacologic conversion of selected atrial fibrillation in horses.
Conversion or control of selected atrial arrhythmias in cattle or dogs under a specialist cardiac plan.
Treatment of selected refractory ventricular or supraventricular tachyarrhythmias in veterinary patients.

Contraindications

Underlying cardiac disease requires caution.

Interactions

Inhibits P-glycoprotein and can increase digoxin and other substrate concentrations.

Adverse effects

Gastrointestinal effects, hypotension, tachyarrhythmias and occasional sudden cardiac death.

Toxicity

Overdose causes conduction abnormalities, hypotension, tachyarrhythmias and delayed CNS effects including seizures. Maintain continuous ECG and hemodynamic monitoring; advanced support may be required.

Preparations

Preparation	Form	Strength	Route
Quinidine gluconate 324 mg tablet 324 mg salt contains the stated 202 mg quinidine base.	Tablet	202 mg / tablet	PO
Quinidine polygalacturonate 275 mg tablet 275 mg salt contains the stated 167 mg quinidine base.	Tablet	167 mg / tablet	PO
Quinidine sulfate 300 mg tablet 300 mg salt contains the stated 250 mg quinidine base.	Tablet	250 mg / tablet	PO

Monitoring and Administration

Continuous ECG and blood-pressure assessment are essential during conversion. Dose basis is quinidine base; commercial salt amounts differ.

Protect from light and discard darkened preparations; reported oral syrup stability is 60 days.

New quinidine gluconate/sulfate tables include canine 6.6-22 mg/kg and feline 6-16 mg/kg regimens without consistently saying salt or base; they are not merged into base-normalized entries. One sulfate oral interval is clearly every 6-8 hours, resolving the earlier printed q68h schedule, but the amount basis still needs qualification.

Equine cardioversion protocols require ECG-guided cumulative limits: gluconate IV 0.5-2.2 mg/kg every 5-10 minutes to totals 8.8-12 mg/kg; sulfate 22 mg/kg by nasogastric tube every 2 hours for 4-6 doses, then every 6 hours if needed. Salt basis differs from base-normalized prior regimens. Stop for significant tachycardia, QRS widening above 125% baseline or other toxicity; hypokalemia increases torsades risk.

Only clear, colorless quinidine gluconate injection is suitable; discard darkened material and use a slow, monitored infusion.