Elektrophysiologie der Herzmuskelfaser

Nifedipine antagonizes ouabain-induced ST-segment changes and derangement of epicardial activation pattern in isolated rabbit hearts

Isolated rabbit hearts, perfused according to the Langendorff technique were treated with 0.2 mumol/l ouabain or with a combination of 0.2 mumol/l ouabain and 3 nmol/l nifedipine. In a second series of experiments, a combination of 0.2 mumol/l ouabain with 0.2 mumol/l nitroglycerin was examined. Under this treatment, coronary flow and left ventricular pressure were continuously measured and, furthermore, the epicardial potential distribution was mapped with high temporal (4 kHz/channel) and spatial resolution (256 electrodes, arranged in 4 plates with 64 electrodes each, 8 x 8 matrix with 1 mm mesh size). Ouabain led to the expected positive inotropy of 10%, but also to a decrease in coronary flow of 15%. Besides these changes a nearly generalized ST-segment elevation could be observed. Moreover, the epicardial activation pattern was disturbed by changes in the location of epicardial breakthrough-points. Concomitantly, the epicardial activation vector field was deranged. Under the additional influence of nifedipine, coronary flow was reduced by only 5%, whereas the positive inotropic effect remained unchanged. The epicardial ST-elevation was diminished significantly and there was no derangement in the process of epicardial activation. Nitroglycerin led to an increase in relative coronary flow comparable to that observed under nifedipine but did not antagonize the disturbances of the process of activation by ouabain and only partially inhibited ST-elevation. Hence, it is concluded that ouabain already provokes coronary vasoconstriction in therapeutic concentrations leading to ST-elevation and prearrhythmic changes in the process of excitation. These changes could be diminished by additional treatment with nifedipine, but not with nitroglycerin.

Theoretical analysis of T-wave polarity based on a model of cardiac electrical activity

The effect of the gradient of transmembrane action potential duration through the ventricular wall on T-wave polarity and QRS-T angle was investigated using a mathematical model of the electrical activity of the heart which incorporates the characteristic electrophysiological properties of the left ventricular wall. Two models, a rectangular solid model and a concave model, were constructed to simulate a part of the left ventricular wall. The ventricular gradient was defined as a linear decrease (beta msec/cm) of the action potential duration from the endocardium to epicardium. The theoretically-obtained relationship between the QRS-T angle and the ventricular gradient revealed that the transmural gradient (beta) was 10--40 msec/cm when the QRS-T angle was within the normal range. The positive T wave was obtained at the observation point which would correspond to the precordial lead when the transmural gradient was more than 30 msec/cm. The amplitude of the simulated T-wave increased with the ventricular gradient. Thus, our mathematical models can provide the quantitative relationship between the transmural ventricular gradient and T-wave polarity and are compatible with further simulation study for various pathological conditions.

Electrophysiological characteristics of canine atrial plateau fibers

Right atria from dog hearts were immersed in Tyrode's solution with the endocardial surface of the anterior wall exposed. Glass microelectrodes were used to impale fibers in different anatomical areas of this preparation. Electrical characteristics of atrial plateau fibers located along the caval border of the crista terminalis were compared with those of regular atrial fibers from other sites. The former conducted at a rate (0.803±0.110 M/sec) two to three times more rapidly than pathways composed of regular atrial fibers. A period of supernormal excitability was demonstrated for plateau fibers but not for regular fibers. Graded premature responses could be elicited in plateau fibers by both electrical stimulation and propagated action potentials. Duration of the effective refractory period of plateau fibers (196.1±2.5 msec) was longer than that of regular fibers (147.8±5.4 msec). Plateau fibers required a lower level of membrane potential (-62.1 ± 0.8 mv) to generate a propagated action potential than did regular fibers (-65.5 ± 1.3 mv). Regular atrial fibers could respond at faster rates to high frequency stimulation than plateau fibers.