[Maximal heart rate and electrocardiographic changes due to maximal beta-adrenergic stimulation--studies with isoprenaline on cattle].

The average heart rate induced to cattle by a maximum isoprenaline dose of 250 microgram/kg was 166 beats/minute. Such drastic shortening of heart rate intervals, PP, was accompanied by highly significant shortening of all other electrocardiographic segments, such as PQ, ST, and TP, as well as of QRS-time (initial ventricular variation). Isoprenaline, a synthetic sympathicomimetic, was compared to well-known results reported for the action of adrenalin and noradrenalin on heart rate and ECG of cattle, and it was found to cause much stronger acceleration of cardiac activity with more strongly shortened ECG segments. Such difference between the three catecholamines nonradrenaline, adrenalin, and isoprenaline was found to be attributable to differences in the stimulation of alpha-adrenergic and beta-adrenergic receptors. Isoprenaline did not produce any alpha-adrenergic effect, and so purely beta-adrenergic vasodilatation and thus reduced secondary stress on the heart, accompanied by beta-adrenergic cardiac stimulation, eventually brought about an altered circulatory situation which was, basically, comparable to circulation under conditions of physical stress, in cases of physical stress, particular attention should be given to polyfactorially caused cardiovascular transformation, contrary to the beta-adrenergic stimulation by isoprenaline. ECG variations likely to occur along with change from maximum to supramaximum isoprenaline doses are described and discussed. The TQ segment was shortened first by approximation of the T and P waves and resulting disappearance of the TP segment. The PQ segment was further shortened by increase of dosage. Further ST reduction then was recordable within the constantly shortened QT time. The T wave was flattened and widened, and the R wave went up.