Cardiac Arrhythmias Associated with Bactrim DS

Mechanism of Arrhythmia Risk

The sulfamethoxazole component of trimethoprim-sulfamethoxazole can cause QT interval prolongation and torsades de pointes (TdP) by blocking potassium channels in cardiac tissue 1
Genetic polymorphisms can increase individual susceptibility - some patients have potassium channels that are several-fold more sensitive to block by sulfamethoxazole than normal, even though these genetic variants show normal function under normal conditions 1, 2

Patients with pre-existing heart disease, age over 80 years, female gender, concomitant use of other QT-prolonging medications, baseline prolonged QT interval, reduced drug elimination, bradycardia, electrolyte abnormalities, and genetic predisposition to QT prolongation are at increased risk of arrhythmias when taking Bactrim DS 3
Combination with inhibitors of the renin-angiotensin system can lead to hyperkalemia and increased risk of sudden death 4

The European Society of Cardiology recommends performing ECG screening before initiating Bactrim DS therapy to exclude prolonged QTc interval (>450 ms for men and >470 ms for women) 3
Patients with prolonged QTc interval should not receive Bactrim DS, and follow-up ECG should be performed one month after initiating therapy to monitor for QTc prolongation 3
The American Heart Association suggests obtaining a thorough medication history to identify other QT-prolonging agents that could interact with Bactrim DS, and discontinuing Bactrim DS if QTc prolongation develops during treatment 3
Use of Bactrim DS should be with extreme caution in patients with known cardiac disease or history of arrhythmias 4

The European Heart Rhythm Association recommends immediately discontinuing the drug if arrhythmias are suspected, maintaining normal serum potassium levels, and considering intravenous magnesium for management of TdP 4
Temporary cardiac pacing may be necessary in severe cases, and isoproterenol can be used to increase heart rate and shorten ventricular action potential duration in cases of TdP 4