Itai Gueta1,2, Robert Klempfner2,3, Noa Markovits1,2, Hillel Halkin1,2, Shlomo Segev2,4, David Rott2,3, Yael Peled2,3, Ronen Loebstein1,2. 1. The Institute of Clinical Pharmacology and Toxicology, Sheba Medical Center, Tel Hashomer, Israel. 2. Sackler School of Medicine, Tel Aviv University, Tel Aviv, Israel. 3. The Olga and Lev Leviev Heart Center, Sheba Medical Center, Tel Hashomer, Israel. 4. The Institute for Medical Screening, Sheba Medical Center, Tel Hashomer, Israel.
Abstract
BACKGROUND: Prolonged QTc interval observed in daily practice is often deemed to be drug induced and might result in drug discontinuation, with possible therapeutic consequences. However, whether clinically significant prolonged QTc may be due to within-individual variability occurs has yet to be described. METHODS: A retrospective cohort study documenting within-individual QTc variability in subjects attending annual routine medical evaluation. At each visit, QT interval was measured and corrected for heart rate using Bazett and three other commonly used formulae. Outcome measures were rates of ΔQTc ≥60 msec, absolute QTc ≥500 msec and QTc ≥25% from baseline. RESULTS: A total of 188 subjects [54 (29%)] females were recruited. Mean age at first ECG was 54 ± 12.8 years with mean time interval of 12.2 ± 1.1 months between measurements. Mean Bazett QTc was higher compared to the other 3 formulae: 412 ± 20 vs. 400 ± 16 msec. Using Bazett formula, 18/188 (9.6%) and 5/188 (2.7%) subjects showed at least one measurement with ΔQTc ≥60 msec and QTc ≥500 msec, respectively. Of the former, 5/18 (27.8%) showed QTc ≥25% prolongation. In multivariate analysis, QTc ≥500 msec was significantly associated with number of measurements (HR: 5.01, 95%CI: 1.21-20.78, p = .026) with no effect of other known confounders. Lower rates were demonstrated with the other three formulae. CONCLUSION: In clinical practice, significant prolonged QTc may be attributed to within-individual variability, particularly when adjusting the QT interval with Bazett correction. This should be taken into consideration when decisions on changing current drug regimens are to be made.
BACKGROUND: Prolonged QTc interval observed in daily practice is often deemed to be drug induced and might result in drug discontinuation, with possible therapeutic consequences. However, whether clinically significant prolonged QTc may be due to within-individual variability occurs has yet to be described. METHODS: A retrospective cohort study documenting within-individual QTc variability in subjects attending annual routine medical evaluation. At each visit, QT interval was measured and corrected for heart rate using Bazett and three other commonly used formulae. Outcome measures were rates of ΔQTc ≥60 msec, absolute QTc ≥500 msec and QTc ≥25% from baseline. RESULTS: A total of 188 subjects [54 (29%)] females were recruited. Mean age at first ECG was 54 ± 12.8 years with mean time interval of 12.2 ± 1.1 months between measurements. Mean Bazett QTc was higher compared to the other 3 formulae: 412 ± 20 vs. 400 ± 16 msec. Using Bazett formula, 18/188 (9.6%) and 5/188 (2.7%) subjects showed at least one measurement with ΔQTc ≥60 msec and QTc ≥500 msec, respectively. Of the former, 5/18 (27.8%) showed QTc ≥25% prolongation. In multivariate analysis, QTc ≥500 msec was significantly associated with number of measurements (HR: 5.01, 95%CI: 1.21-20.78, p = .026) with no effect of other known confounders. Lower rates were demonstrated with the other three formulae. CONCLUSION: In clinical practice, significant prolonged QTc may be attributed to within-individual variability, particularly when adjusting the QT interval with Bazett correction. This should be taken into consideration when decisions on changing current drug regimens are to be made.
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