Quinidine

Quinidine is a medication (‘antiarrhythmic’) used to treat ventricular tachycardia (in the setting of a defibrillator), idiopathic ventricular fibrillation and Brugada Syndrome.

Mechanism

Quinidine blocks sodium and potassium channels in the heart, slowing electrical conduction, and reducing the ‘excitability’ of the heart. This may suppress some arrhythmias but, paradoxically, cause others.

Dosage

The dose is usually low to begin 100-200mg 2-4 times a day, and may be increased slowly up to 600mg 4 times a day. Side effects are very common at higher doses. T1/2 6-8 hours.

Side Effects

Cardiac:

• may cause QT prolongation on the ECG which predisposes to a specific ventricular arrhythmia, torsades de pointes
• heart block or slow rhythms, and low blood pressure (mainly if given IV) may also occur

Non Cardiac:

• Fatigue, headache
• Skin rash 5-6%
• Diarrhoea 25-35%, gastrointestinal upset 22%
• Fever 6%
• Blood abnormalities

Monitoring

Patients on quinidine are monitored for ECG abnormalities, especially QT prolongation. Hepatotoxicity (liver), thrombocytopaenia, and other blood abnormalities may occur.

On quinidine, and ECG will be performed every 3-6 months, as well as the blood tests: FBC/EUC/LFT, and a yearly echocardiogram.

Metabolism/Drug Interactions

Quinidine is metabolized by the liver enzyme system CYP3A4. Therefore, many prescribed drugs (as well as grapefruit juice, St John’s Wort, and Goldenseal (Hydrastis Canadensis)) which inhibit CYP3A4 may increase quinidine levels in the blood and the risk of toxicity. A list of important interactions may be found here.

Summary

Quinidine may be an effective medication to suppress troublesome and dangerous arrhythmia. However, side effects are common and the drug must be monitored closely. The risk of serious arrhythmia caused by quinidine is not insignificant. Therefore, quinidine is employed at low dose for conditions with lower risk (such as asymptomatic Brugada Syndrome), and monitored closely when higher doses are needed to suppress symptomatic arrhythmia - such as in the setting of ventricular tachycardia where a defibrillator is in place.