Bradycardia in Long QT Syndrome: A Critical Risk Factor
Bradycardia is NOT acceptable in patients with Long QT Syndrome (LQTS) and represents a dangerous trigger for life-threatening arrhythmias that must be actively prevented and treated. 1
Why Bradycardia is Dangerous in LQTS
Bradycardia is a recognized cause of acquired LQTS and directly triggers torsades de pointes in patients with congenital LQTS. 1 The mechanism is clear:
- Bradycardia prolongs cardiac repolarization, further extending the already-prolonged QT interval in LQTS patients 1
- Slow heart rates create pause-dependent malignant arrhythmias, particularly torsades de pointes 1
- Bradycardia-dependent torsades de pointes is a well-documented phenomenon in LQTS, where the slow rate itself becomes the arrhythmia trigger 2
Clinical Management Algorithm
Step 1: Recognize Bradycardia as an Arrhythmia Trigger
- Cardiac pacing is indicated in LQTS patients whenever there is evidence of bradycardia-dependent or pause-dependent malignant arrhythmias 1
- Acute pacing is reasonable for patients who present with recurrent pause-dependent torsades de pointes 1
- Beta blockade combined with pacing is reasonable acute therapy for patients who present with torsades de pointes and sinus bradycardia 1
Step 2: Address Beta-Blocker-Induced Bradycardia
This is a critical clinical dilemma, as beta-blockers are the primary prevention therapy for LQTS but cause bradycardia:
- Pacemakers can be considered in combination with beta-blocking therapy to prevent the occurrence of excessive bradycardia in selected patients, such as those with the LQT3 variant 1
- Either atrial or ventricular pacing combined with beta-blocker therapy appears effective for preventing episodes of torsade de pointes or alleviating symptoms due to bradycardia from beta-blocker therapy 3
- Permanent pacing at rates of 70-85 beats/min significantly decreases the mean QT interval (from 534±51 to 426±19 ms) while maintaining beta-blocker protection 3
Step 3: Implement Long-Term Pacing Strategy
- Acute and long-term pacing is recommended for patients presenting with torsades de pointes due to heart block and symptomatic bradycardia 1
- Permanent dual-chamber pacing is preferred, as one patient with AAI pacing developed AV block requiring conversion to DDD pacing 3
- Pacing rates should be set to 70-85 beats/min to prevent pause-dependent arrhythmias 3
Special Considerations by LQTS Genotype
The relationship between heart rate and QT interval varies by genetic subtype:
- LQT1 patients: QTc prolongs with exercise, making bradycardia at rest less problematic but still requiring prevention 1
- LQT2 patients: QTc remains unchanged with exercise, making bradycardia a consistent risk 1
- LQT3 patients: QTc shortens excessively with exercise but bradycardia at rest is particularly dangerous, making pacing especially important in this subtype 1
Common Pitfalls to Avoid
Never accept bradycardia as "physiologic" in LQTS patients, even in athletes 1 While athletes commonly have bradycardia from training, bradycardia is a cause of acquired LQTS and can trigger torsades in those with congenital LQTS 1
Do not discontinue beta-blockers due to bradycardia without implementing pacing first 1, 3 Approximately 25% of LQTS patients who died had been off beta-blockers for a significant time 1
Avoid using isoproterenol to increase heart rate in congenital LQTS patients 1 While isoproterenol is reasonable for pause-dependent torsades in acquired LQTS, it should NOT be used in patients with congenital LQTS 1
Evidence for Pacing Efficacy
In a series of 8 LQTS patients with permanent pacemakers followed for a mean of 35 months, all patients remained alive and free of syncope, with only one experiencing two episodes believed due to hyperventilation 3 This demonstrates that pacing combined with beta-blocker therapy effectively prevents bradycardia-triggered events while maintaining the protective effects of beta-blockade 3