The Influence of Adrenaline, Nor‐adrenaline, and Acetylcholine on the Electrocardiogram of the Isolated Perfused Guinea‐pig Heart

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Abstract
Summary.: The effect of increasingly large doses of adrenaline, nor‐adrenaline, and acetylcholine on the electrocardiogram of the isolated, perfused guinea‐pig heart was studied before and after administration of dibenamine and atropine, respectively. The threshold value for nor‐adrenaline to cause electrocardiographic changes was lower than that for adrenaline. The chronotropic effect seemed to be the same for the two substances at low doses, while at higher doses nor‐adrenaline seemed to be the more effective positive chronotropic agent. Cardiac arrhythmias were observed in about one‐third of the experiments and nor‐adrenaline appeared to be as potent as adrenaline in developing conduction disturbances and evoking ectopic discharges. Adrenaline increased the amplitude of the P wave, while nor‐adrenaline had no effect on the amplitude. There was no difference between the two substances as to the changes in the P—R interval, Q—T duration, S—T segment, or on the T waves. At lower doses the P—R interval was shortened; a prolongation was seen at high doses. The Q—T duration was first prolonged, but 30 to 60 secs later a shortening appeared. The T waves showed reduction in amplitude, so that a previously positive or negative T wave became less positive or less negative, respectively. In some hearts the opposite results were seen. At high doses a previously positive T wave could become negative and vice versa. After dibenaminization, slightly higher threshold values of adrenaline and nor‐adrenaline were necessary in order to produce electrocardiographic changes. At higher doses of the two substances, dibenamine could not abolish or decrease the chronotropic effect or electrocardiographic contour changes. At lower doses acetylcholine produced decreased auricular rate, with increasing doses heart block occurred, partial or complete. A propensity to evoke auricular fibrillation was evident. Ectopic auricular or ventricular discharges occurred. The P wave often became notched, its amplitude and duration decreased. In some experiments the Q—T duration was shortened and was followed by a lengthening. The T wave amplitude increased so that a previously positive or negative T wave became more positive or more negative, respectively. The inhibitory effect of acetylcholine was abolished by atropine. High doses of acetylcholine caused acceleration of the heart rate in atropinized hearts.

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