Increased ventricular ectopy precedes Torsades de Pointes in patients with prolonged QT

Torsades de Pointes (TdP) is a potentially lethal ventricular tachydysrhythmia. Prolonged heartrate corrected QT interval (QTc) predicts TdP; however, with poor specificity. We performed this study to identify other predictors of TdP among patients with prolonged QTc.

We performed a retrospective case control study with 2:1 matching at an urban academic hospital. We searched our hospital electrocardiogram (ECG) database for tracings with heartrate ≤ 60, QTc ≥ 500, and QRS < 120, followed by a natural language search for electronic records with "Torsades," "polymorphic VT," or similar to identify TdP cases from 2005 to 19. We identified controls from a similar ECG database search matching for QTc, heartrate, age, and sex. We compared cardiologic and historical factors, medications, laboratory values, and ECG measurements including ectopy using univariate statistics. For those cases with saved telemetry strips that included preceding beats or TdP onset, we compared ectopy and TdP onset characteristics between the ECG and telemetry strips using mixed linear modeling.

Seventy-five cases including 50 with telemetry strips and 150 controls were included. Historical, pharmacologic, laboratory, and cardiologic testing results were similar between cases and controls. The proportion of telemetry tracings with premature ventricular contractions (PVC's) preceding TdP was 0.78 compared to 0.16 for case ECG's (difference 0.62(95%CI 0.44-0.75)) and 0.10 for control ECGs (difference 0.68(95%CI 0.56-0.80)). Average telemetry heartrate was 72 and QTc 549 immediately preceding TdP, similar to the ECG values.

Clinical factors don't differentiate patients with long QTc who develop TdP, however, an increase in PVC's in patients with prolonged QTc may usefully predict imminent TdP.