Clinically significant incidental QTc prolongation is subject to within-individual variability

Area of Science:

• Cardiology
• Clinical Electrophysiology
• Pharmacology

Background:

• Prolonged QTc interval is often misattributed to drug effects, leading to unnecessary drug discontinuation.
• The role of within-individual variability in significant QTc prolongation remains under-described in clinical practice.

Purpose of the Study:

• To investigate the extent of within-individual variability in QTc interval measurements.
• To determine if this variability can account for clinically significant QTc prolongation.

Main Methods:

• Retrospective cohort study of 188 subjects with annual ECGs.
• QT interval measured and corrected using Bazett and three other formulae.
• Assessed rates of ΔQTc ≥60 msec, absolute QTc ≥500 msec, and QTc ≥25% prolongation.

Main Results:

• Using Bazett correction, 9.6% and 2.7% of subjects had at least one QTc ≥60 msec and QTc ≥500 msec, respectively.
• QTc ≥500 msec was significantly associated with the number of measurements (HR: 5.01).
• Lower rates of QTc prolongation were observed with alternative correction formulae.

Conclusions:

• Within-individual QTc variability, especially with Bazett correction, can mimic drug-induced prolongation.
• Clinicians should consider this variability when making decisions about drug regimens.