Abstract
During the past year two patients with cardiac complaints were given repeated injections of 1 mg. atropin sulphate and on each occasion atrioventricular rhythm was observed. This usually appeared in from eight to ten minutes after the drug was given, it persisted for only a few minutes, and disappeared before the maximum effect of the drug was reached.
In order to determine whether this tendency to A-V rhythm was peculiar to these patients or whether it exists normally, a series of experiments on normal individuals was carried out. Eighteen subjects, all under twenty-eight years of age and all with apparently normal hearts, were given hypodermic injections of 1 mg. of atropin. The effect of deep breathing and of ocular pressure upon the cardiac mechanism was studied both before and during the atropin action. Before the injection, A-V rhythm was not produced in any of the subjects tested. Between eight and fifteen minutes after the injection, however, A-V rhythm could be produced by ocular pressure or deep respiration in the majority of the subjects. After the atropin effect had reached its height A-V rhythm could no longer be produced.
Three types of A-V rhythm were observed. In the first, which occurred most frequently, the P-R interval was reduced and P was inverted; in the second, the P-R interval was zero; while in the third there was an R-P interval. The last was observed in only two subjects. These differences evidently depended upon the level of the pacemaker in the junctional tissues.
These observations may be explained on the assumption that atropin releases the A-V tissues from vagus control somewhat before it releases the sinus node. At this time stimulation of the vagus slows the sinus rhythm without a correspondingly great effect upon the inherent rhythms of the lower centers.
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