Within-subject electrocardiographic differences at equal heart rates: Roleof the autonomic nervous system

Various combinations of sympathetic and vagal tone can yield the same heart rate, while ventricular electrophysiology differs. To demonstrate this in humans, we studied healthy volunteers in the sitting position with horizont al legs. First, heart rate was increased by lowering the legs to 60 degrees and back. Thereafter, heart rate was increased by handgrip. In each subjec t, a leg-lowering angle was selected at which heart rate matched best with heart rate in the third handgrip minute. Thirteen subjects had a heart rate match better than 1%. Heart rate (control: 65.2+/-9.0 bpm) increased to 72 .1+/-8.7 (leg lowering) and to 72.1+/-8.8 (handgrip) bpm. QRS azimuth, QRS duration, maximal T vector, T azimuth, T elevation, ST duration, QRS-T angl e and QT interval differed significantly (P<0.05) between leg lowering and handgrip (QT interval 418+/-15 versus 435+/-21 ms). Also, septal dispersion of repolarization, assessed as the time difference between the apex and th e end of the T wave in the V2 and V3 leads, differed significantly (V2: 96. 7+/-19.3 versus 110.0+/-23.3 ms, P<0.01; V3: 88.7+/-19.3 versus 97.3+/-23.3 ms; P<0.01). Hence, leg lowering and handgrip cause different ventricular depolarization and repolarization. The hypertensive handgrip manoeuvre enta ils a longer QT interval and probably an increased septal dispersion of rep olarization.