Antiarrhythmic Drugs

Blocking the rapid repolarizing potassium channel slows phase 3 repolarization, prolonging the action potential duration and QT interval. A sufficiently prolonged action potential can generate early afterdepolarizations during phase 3, which directly trigger torsades de pointes -- a polymorphic ventricular tachycardia characterized by its twisting QRS axis around the isoelectric baseline. Torsades de pointes typically occurs in runs that may self-terminate or degenerate to ventricular fibrillation.

QT prolongation is necessary but not sufficient for torsades de pointes. The following factors substantially increase risk and must be actively assessed and managed:

Female sex accounts for approximately 70 percent of drug-induced torsades de pointes. Women have a longer baseline corrected QT interval and less repolarization reserve than men.

Hypokalemia and hypomagnesemia directly promote early afterdepolarization formation. These are modifiable risk factors -- electrolytes must be corrected before initiating any QT-prolonging drug and maintained throughout therapy. Target potassium at or above 4.0 to 4.5 mEq/L.

Bradycardia and pauses amplify action potential duration prolongation through reverse use-dependence. Torsades de pointes characteristically occurs after a pause.

Corrected QT interval over 500 milliseconds is the single most important quantitative threshold. If the corrected QT interval exceeds 500 milliseconds during drug therapy, the offending agent must be dose-reduced or discontinued.

Combinations of QT-prolonging drugs produce additive or synergistic QT prolongation. Avoid co-prescribing Class Ia antiarrhythmics with Class III agents, or antiarrhythmics with fluoroquinolone antibiotics, azole antifungals, or antipsychotics.

Many commonly used non-antiarrhythmic drugs carry significant QT prolongation risk by the same mechanism -- blocking the rapid repolarizing potassium channel. Clinically important categories include fluoroquinolone antibiotics (ciprofloxacin, levofloxacin, moxifloxacin), macrolide antibiotics (azithromycin, clarithromycin), azole antifungals (fluconazole, ketoconazole, itraconazole), antipsychotics (haloperidol, quetiapine, ziprasidone), antiemetics (ondansetron, domperidone), and methadone. This list represents categories where the risk is well-established; a complete, continuously updated database is maintained by the CredibleMeds program, which is the clinical standard for QT risk stratification when prescribing decisions require it.

Step 1 -- Withdraw the offending drug immediately. Identify and stop all QT-prolonging agents.

Step 2 -- Intravenous magnesium sulfate is first-line regardless of the serum magnesium level. Magnesium suppresses early afterdepolarizations by blocking the calcium current that sustains them. A repeat dose may be given if torsades de pointes recurs.

Step 3 -- Correct electrolytes. Target potassium at or above 4.5 mEq/L and magnesium at or above 2.0 mg/dL. Hypokalemia is both a precipitant and a perpetuator of torsades de pointes.

Step 4 -- Increase heart rate to eliminate pauses and counteract reverse use-dependence. Intravenous isoproterenol infusion or temporary cardiac pacing at 90 to 110 beats per minute achieves this. Increasing rate shortens the action potential duration and eliminates the pause-dependent trigger for torsades de pointes recurrence.

Step 5 -- Defibrillate immediately if torsades de pointes degenerates to ventricular fibrillation. Do not delay for drug therapy.

Note: Do not give calcium in acquired drug-induced torsades de pointes. Calcium worsens early afterdepolarizations and is specifically indicated only for hyperkalemia and magnesium toxicity, not torsades de pointes.

Sotalol is entirely renally eliminated and requires mandatory dose interval extension as creatinine clearance falls. It is contraindicated when creatinine clearance is below 40 mL/min because accumulation produces life-threatening QT prolongation.

Dofetilide is approximately 80 percent renally eliminated. Strict four-tier dose adjustment by creatinine clearance is mandatory; any dose change requires re-initiation with monitored corrected QT interval surveillance. It is contraindicated when creatinine clearance is below 20 mL/min.

Digoxin is 70 to 80 percent renally eliminated with a narrow therapeutic index. Dose reduction proportional to renal function is required; levels are unpredictable in chronic kidney disease and must be monitored regularly.

Procainamide and its active metabolite are both renally eliminated. Accumulation in renal failure prolongs QT interval. Dose and frequency reduction are required, and procainamide should be used with caution in significant renal impairment.

Flecainide is approximately 30 percent renally eliminated; dose reduction is required when creatinine clearance falls below 35 mL/min to prevent QRS widening from accumulation.

Amiodarone is primarily hepatically cleared and requires no dose adjustment in renal impairment, making it the most pharmacokinetically straightforward antiarrhythmic choice in patients with severe chronic kidney disease.

Lidocaine is primarily hepatically metabolized; intravenous dosing requires no renal adjustment, though prolonged infusions can produce central nervous system toxicity from metabolite accumulation.

Mexiletine is approximately 90 percent hepatically eliminated; only minor dose consideration in severe chronic kidney disease. Primarily used for long QT syndrome type 3 and as a ventricular tachycardia adjunct.

Even without overt chronic kidney disease, kidney function declines with age. Renally-cleared antiarrhythmic drugs -- sotalol, dofetilide, digoxin, procainamide, and flecainide -- all accumulate in elderly patients at standard doses. Renal function should be formally calculated (not estimated from serum creatinine alone, which underestimates impairment in patients with low muscle mass) before initiating any renally-cleared antiarrhythmic.

The practical prescribing principles for elderly patients are: start at approximately half the standard adult dose and uptitrate cautiously; monitor the corrected QT interval at initiation, after any dose change, and after any new interacting drug is added; target digoxin levels in the lower therapeutic range (0.5 to 0.8 nanograms per milliliter); and review all co-medications systematically for QT-prolonging agents, because polypharmacy in elderly patients frequently includes multiple drugs with conditional QT risk (fluoroquinolones, azole antifungals, ondansetron, antidepressants).

For patients already on long-term amiodarone, annual monitoring -- thyroid function tests, liver function tests, chest radiograph, and ophthalmologic evaluation -- is more critical in elderly patients than in younger ones because of cumulative organ toxicity over years of exposure.

Significant hepatic impairment reduces the first-pass metabolism and ongoing clearance of drugs that are primarily hepatically eliminated. The most clinically important antiarrhythmics affected are lidocaine, propafenone, verapamil, and diltiazem. In cirrhosis, all four have substantially increased bioavailability and prolonged half-lives -- lidocaine maintenance infusion should be reduced by approximately half, and propafenone plasma levels can become unpredictable; dose reduction of 50 to 70 percent is required for verapamil and diltiazem in significant hepatic disease.

Amiodarone is primarily hepatically cleared and should be used with caution in hepatic impairment -- it is itself hepatotoxic and can worsen underlying liver disease. Liver function tests must be monitored closely. The renally-cleared drugs (sotalol, dofetilide, digoxin) require minimal adjustment for hepatic impairment alone.