Bradycardia-Induced Polymorphic Ventricular Tachycardia

Electrophysiologic Mechanism

Slow heart rates prolong the action potential duration, which increases the QT interval and creates heterogeneity in ventricular repolarization across different myocardial regions 1, 3
Pauses in rhythm are particularly dangerous because they allow for maximal prolongation of repolarization and create the electrical substrate for triggered activity 1, 2, 4

The American Heart Association explicitly recognizes that polymorphic VT can be "precipitated by pauses in rhythm" when associated with acquired long QT syndrome 1, 2, 3, 4
The pattern is typically: bradycardia → pause → QT prolongation → premature ventricular contraction → torsades de pointes 1, 5
This is why pacing or isoproterenol (which increases heart rate) are recommended treatments - they eliminate the bradycardia and pauses that perpetuate the arrhythmia 1, 2, 6, 5

For bradycardia-associated polymorphic VT with long QT, temporary overdrive pacing at 90-110 bpm is recommended to prevent the pauses that trigger the arrhythmia 6, 5
IV isoproterenol may be considered when pacing is not immediately available, as it increases heart rate and shortens the QT interval 1, 2, 6
IV magnesium sulfate (1-2g) should be administered regardless of serum levels, as it stabilizes the myocardial membrane and reduces early afterdepolarizations 1, 3, 7, 5

Polymorphic VT without QT prolongation is most commonly caused by acute myocardial ischemia, not bradycardia, and requires completely different management (beta-blockers, revascularization) 1, 3
Isoproterenol and pacing (appropriate for bradycardia-induced torsades) would be inappropriate and potentially harmful in ischemic polymorphic VT 1

Drug-induced QT prolongation combined with bradycardia (from beta-blockers, calcium channel blockers, or high vagal tone) creates maximal risk 1, 2, 5
Electrolyte depletion (hypokalemia, hypomagnesemia) amplifies the effect of bradycardia on repolarization abnormalities 1, 3
Congenital long QT syndrome patients are particularly vulnerable when bradycardia occurs, though beta-blockers remain the mainstay of chronic therapy 1, 6

Look for the characteristic "short-long-short" sequence on telemetry: a premature beat, followed by a compensatory pause (bradycardia), followed by another premature beat that initiates torsades 1, 6, 5