RESUMO
INTRODUCTION: The effect of standing on heart rate and QT is well known but its impact on QTc is less clear. METHODS: Serial supine and standing 12-lead ECGs (seven pairs each day) were recorded from 54 healthy volunteers each day of a three-day period. ECGs were captured digitally and over-read by a cardiologist. RESULTS: A statistically significant shortening of RR (216 ms), QT (40 ms), and decreases in QTc-F (Fridericia) and QTc-LR (Framingham) were demonstrated on standing (8.3 and 6.9 ms, respectively). In contrast, QTc-B (Bazett's) significantly increased by 9.6 ms. Two subject-individualized correction methods were derived using each subject's supine measurements. Both showed significant decreases in QTc of approximately 13-14 ms upon standing. Using the bin analysis method, comparisons between positions using 25 ms interval RR bins revealed significant QT shortening of up to 15 ms upon standing. CONCLUSION: We have demonstrated a postural effect on cardiac repolarization independent of heart rate using two individualized correction methods, as well as QTc-F and QTc-LR, and the bin method. Characterization of postural differences in QT/QTc (other than QTc-B) may provide a safe and inexpensive physiological control to validate the ECG methodology used in clinical trials to assess potential drug-induced QT interval changes.
