QUINIDINE SULFATE
Clinical safety rating: safe
Animal studies have demonstrated safety
Quinidine is a class Ia antiarrhythmic agent that blocks sodium channels, prolonging the effective refractory period and slowing conduction. It also inhibits potassium channels, prolonging repolarization, and has vagolytic and negative inotropic effects.
| Metabolism | Primarily hepatic via CYP3A4. Major metabolite is 3-hydroxyquinidine (active). Also undergoes glucuronidation. Renal excretion of unchanged drug and metabolites. |
| Excretion | Renal excretion of unchanged drug accounts for 10-20% of elimination; hepatic metabolism (hydroxylation and N-oxidation) accounts for 70-80%; about 5% excreted in feces via biliary elimination. |
| Half-life | Terminal elimination half-life is 6-8 hours in healthy adults; prolonged to 12-18 hours in heart failure, hepatic cirrhosis, or severe renal impairment (CrCl < 10 mL/min). |
| Protein binding | 80-90% bound to albumin and alpha-1-acid glycoprotein; binding increases with higher alpha-1-acid glycoprotein levels (e.g., post-myocardial infarction). |
| Volume of Distribution | 2-4 L/kg; reflects extensive tissue distribution, particularly to myocardium (concentration 10-20 times plasma). |
| Bioavailability | Oral immediate-release: 70-80% due to first-pass hepatic metabolism; sustained-release: 60-75%. |
| Onset of Action | Oral: 1-3 hours for antiarrhythmic effect; IM: 0.5-1.5 hours; IV: immediate, within minutes. |
| Duration of Action | Oral: 6-8 hours for suppression of premature ventricular contractions; sustained-release forms: 8-12 hours. Duration increased with hepatic or renal dysfunction. |
300-600 mg orally every 6-8 hours; maximum 2-4 g/day. Extended-release: 300-600 mg every 8-12 hours.
| Dosage form | TABLET |
| Renal impairment | GFR 10-50 mL/min: administer 75% of normal dose every 8-12 hours. GFR <10 mL/min: administer 50% of normal dose every 12 hours or consider alternative antiarrhythmic. |
| Liver impairment | Child-Pugh Class A: no adjustment. Child-Pugh Class B: reduce dose by 25-50% and monitor serum levels. Child-Pugh Class C: reduce dose by 50% or avoid use. |
| Pediatric use | Oral: 15-60 mg/kg/day divided every 6 hours; maximum 3 g/day. Intravenous not recommended for children. |
| Geriatric use | Elderly patients: start at lower end of dosing range (e.g., 200-300 mg every 8 hours) due to increased risk of toxicity; monitor renal function and serum quinidine levels. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Inhibits CYP2D6 and CYP3A4 increasing levels of many drugs Can cause cinchonism and thrombocytopenia.
| Breastfeeding | Quinidine is excreted in breast milk with M/P ratio approximately 0.8-2.0. Infant dose <5% of maternal weight-adjusted dose. Consider risk of neonatal arrhythmias or thrombocytopenia; monitor infant for adverse effects. Use with caution. |
| Teratogenic Risk | First trimester: Limited human data; animal studies show no consistent teratogenicity; theoretical risk of arrhythmias. Second/third trimester: May cause fetal arrhythmias, thrombocytopenia, and eighth cranial nerve damage. Avoid use near term due to potential uterine irritability and premature labor. |
■ FDA Black Box Warning
WARNING: Quinidine may cause potentially fatal cardiotoxicity, including QT prolongation, torsades de pointes, and ventricular arrhythmias. It is contraindicated in patients with complete heart block or left bundle branch block. Due to its proarrhythmic potential, quinidine should be reserved for life-threatening arrhythmias when safer alternatives are ineffective.
| Common Effects | GI upset |
| Serious Effects |
["Complete heart block or left bundle branch block","History of QT prolongation or torsades de pointes","Myasthenia gravis (quinidine worsens symptoms)","Hypersensitivity to quinidine or cinchona alkaloids","Concurrent use with drugs that prolong QT (e.g., class III antiarrhythmics) or CYP3A4 inhibitors that increase quinidine levels"]
| Precautions | ["Proarrhythmic effects (QT prolongation, torsades de pointes) - monitor ECG","Cinchoism (tinnitus, headache, nausea) - dose-dependent","Hypotension and syncope due to vasodilation","Hemolysis in G6PD deficiency","Hepatotoxicity and immune-mediated reactions (e.g., thrombocytopenia)","Must be given with caution in hepatic or renal impairment; dose adjustment needed"] |
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| Fetal Monitoring | Maternal: ECG, serum quinidine levels, electrolytes (K+, Mg2+), liver and renal function. Fetal: Heart rate monitoring, ultrasound for fetal growth and arrhythmias. Neonatal: ECG, platelet count, and auditory screening if used near term. |
| Fertility Effects | No established effects on fertility in humans. Animal studies show no impairment. Theoretical risk due to antiarrhythmic properties but not clinically significant. |