On standard electrocardiogram (ECG) PQ interval is known to be moderately heart rate dependent, but no physiologic details of this dependency have been established. At the same time, PQ dynamics is a clear candidate for non-invasive assessment of atrial abnormalities including the risk of atrial fibrillation. We studied PQ heart rate dependency in 599 healthy subjects (aged 33.5 ± 9.3 years, 288 females) in whom drug-free day-time 12-lead ECG Holters were available. Of these, 752,517 ECG samples were selected (1256 ± 244 per subject) to measure PQ and QT intervals and P wave durations. For each measured ECG sample, 5-minute history of preceding cardiac cycles was also obtained. Although less rate dependent than the QT intervals (36 ± 19% of linear slopes), PQ intervals were found to be dependent on underlying cycle length in a highly curvilinear fashion with the dependency significantly more curved in females compared to males. The PQ interval also responded to the heart rate changes with a delay which was highly sex dependent (95% adaptation in females and males after 114.9 ± 81.1 vs 65.4 ± 64.3 seconds, respectively, p < 0.00001). P wave duration was even less rate dependent than the PQ interval (9 ± 10% of linear QT/RR slopes). Rate corrected P wave duration was marginally but significantly shorter in females than in males (106.8 ± 8.4 vs 110.2 ± 7.9 ms, p < 0.00001). In addition to establishing physiologic standards, the study suggests that the curvatures and adaptation delay of the PQ/cycle-length dependency should be included in future non-invasive studies of atrial depolarizations.

Department of Internal Medicine and Cardiology University Hospital Brno Faculty of Medicine Masaryk University Jihlavská 20 625 00 Brno Czech Republic

Klinikum rechts der Isar Technische Universität München Ismaninger Straße 22 D 81675 Munich Germany

National Heart and Lung Institute Imperial College 72 Du Cane Rd Shepherd's Bush London W12 0NN England

Wilhelminenspital der Stadt Wien Montleartstraße 37 1160 Vienna Austria

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Spatial distribution of physiologic 12-lead QRS complex

Sex and Rate Change Differences in QT/RR Hysteresis in Healthy Subjects

Heart Rate Dependency and Inter-Lead Variability of the T Peak - T End Intervals