Common Causes

• Taking more tablets than prescribed (e.g., double‑dosing)
• Accidental ingestion of crushed tablets (especially in households with children)
• Interaction with drugs that inhibit quinidine metabolism (e.g., macrolide antibiotics, antifungals, protease inhibitors) leading to elevated levels even at normal doses
• Renal or hepatic impairment delaying drug clearance

Risk Factors

• Age > 65 years
• Chronic kidney disease (eGFR < 30 mL/min/1.73 m²) or hepatic dysfunction
• Concomitant use of QT‑prolonging medications (e.g., sotalol, certain antipsychotics)
• Electrolyte abnormalities (hypokalemia, hypomagnesemia) that potentiate arrhythmias
• History of cardiac conduction disease

Diagnosis

Diagnosis is a combination of clinical suspicion, a careful medication history, and targeted investigations.

Initial Assessment

• Obtain a detailed history: dose taken, time of ingestion, co‑administered drugs, and presence of symptoms.
• Physical exam focusing on cardiovascular, neurological, and gastrointestinal findings.

Laboratory Tests

• Serum quinidine level – measured by high‑performance liquid chromatography; levels > 10 µg/mL strongly suggest overdose.
• Electrolytes (K⁺, Mg²⁺, Ca²⁺) – correct deficits promptly.
• Renal and liver function tests – assess clearance capacity.
• Cardiac biomarkers (troponin) if chest discomfort is present.

Electrocardiogram (ECG)

ECG is essential; look for:

• Prolonged PR interval (first‑degree AV block)
• Widened QRS complexes
• QTc > 500 ms (risk for torsades)
• Any ventricular tachyarrhythmia

Additional Tests (if indicated)

• Continuous cardiac monitoring (telemetry) in an ICU setting.
• Serum digoxin level (if patient is also on digoxin, as quinidine can increase digoxin concentrations).

Treatment Options

Treatment aims to stabilize the heart rhythm, enhance drug elimination, and correct metabolic derangements. Management is usually performed in an emergency department or intensive care unit.

Immediate Measures

• Airway, Breathing, Circulation (ABCs) – ensure oxygenation; administer supplemental O₂ if SpO₂ < 94%.
• Place patient on continuous cardiac monitoring.
• Obtain large‑bore IV access.

Decontamination (if presentation < 1 hour)

• Activated charcoal 1 g/kg (max 50 g) oral; may bind quinidine and reduce absorption.
• Gastric lavage is rarely indicated and only considered if a massive overdose is known and the patient presents within 30 minutes.

Pharmacologic Interventions

• Sodium bicarbonate IV: 1–2 mEq/kg bolus, then infusion (150 mEq in 1 L D5W) to treat QRS widening and prevent ventricular arrhythmias. It raises serum pH, reducing quinidine’s binding to sodium channels.
• Magnesium sulfate 2 g IV over 5 minutes for torsades de pointes, regardless of serum Mg²⁺ level.
• Isoproterenol infusion or temporary overdrive pacing (60–80 bpm) can shorten QT interval in refractory torsades.
• Diazepam or other benzodiazepines for seizures or severe agitation.

Enhanced Elimination

Quinidine has a moderate half‑life (6–8 hours) and is extensively protein‑bound, making dialytic removal inefficient. However, **hemodialysis** may be considered in severe cases with renal failure, though evidence is limited.

Supportive Care

• IV fluids to treat hypotension (avoid rapid overload in patients with heart failure).
• Vasopressors (norepinephrine) if blood pressure remains low despite fluids.
• Correction of electrolytes, especially K⁺ > 4 mmol/L and Mg²⁺ > 2 mg/dL.

Disposition

Patients with any cardiac conduction abnormality, persistent QTc > 500 ms, or ventricular arrhythmias should be observed in an ICU for at least 24 hours and cleared only after ECG normalization and stable serum quinidine level (< 6 µg/mL).

Living with Quinidine Overdose

Surviving an overdose does not automatically mean the medication can be continued as before. Follow‑up care focuses on preventing recurrence and managing underlying heart disease.

Medication Management

• Review the prescribing regimen with a cardiologist or clinical pharmacist.
• Consider switching to a safer anti‑arrhythmic (e.g., sotalol, amiodarone) if risk of overdose is high.
• Use pill organizers or pre‑filled blister packs to avoid dosing errors.

Monitoring

• Regular ECGs (baseline, after dose changes, and annually).
• Serum quinidine levels every 2–4 weeks during dose titration.
• Routine labs for electrolytes, renal and hepatic function.

Lifestyle Tips

• Maintain adequate hydration and a balanced diet rich in potassium (bananas, oranges) and magnesium (nuts, leafy greens).
• Avoid over‑the‑counter medications or supplements that can prolong the QT interval (e.g., certain antihistamines, antacids containing aluminum).
• Keep an up‑to‑date medication list and share it with every healthcare provider.
• Store quinidine in a locked cabinet, out of reach of children and visitors.

Prevention

Preventing quinidine overdose focuses on education, medication safety, and addressing risk factors.

• Patient education: Explain the exact dose, timing, and possible side effects. Provide written instructions.
• Pharmacy counseling: Pharmacists should verify the dosage and discuss potential drug interactions.
• Use of technology: Set alarms on smartphones or use electronic pill dispensers.
• Screen for mental health issues: Routine depression screening for patients on chronic quinidine, especially those with prior suicide attempts.
• Regular follow‑up: Ensure periodic labs and ECGs to detect early toxicity.

Complications

If not promptly recognized and treated, quinidine overdose can lead to serious, potentially fatal complications.

• Life‑threatening arrhythmias – torsades de pointes, ventricular fibrillation, asystole.
• Cardiogenic shock due to severe bradycardia or hypotension.
• Neurological injury from prolonged hypoperfusion or seizures.
• Renal failure secondary to hypotension or rhabdomyolysis (rare).
• Persistent conduction disease requiring permanent pacemaker implantation.

When to Seek Emergency Care

Even if symptoms seem mild, it is prudent to seek medical evaluation because cardiac toxicity can develop or worsen several hours after ingestion.

Sources: Mayo Clinic. “Quinidine: Uses, Side Effects, Dosage.”; CDC. “Poison Center Data (2022).”; NIH. “Quinidine Toxicity.”; WHO. “Pharmacovigilance Manual.”; Cleveland Clinic. “Management of Drug‑Induced QT Prolongation.”; JAMA Cardiology. “Outcomes of Anti‑arrhythmic Overdose.”