1. The effect of a sudden acute change in blood pressure upon arrhythmia provocation has been studied in an isolated working heart model from the Wistar—Kyoto strain of rat. Twenty-four hearts were studied.

2. They were perfused with two different, modified, Krebs—Henseleit solutions at a fixed left atrial pressure.

3. Acute changes in pressure, both increases and decreases, were arrhythmogenic. Whilst ectopic activity was more predictably produced by pressure reductions, this consisted of simple ventricular ectopics only. Pressure increases, in contrast, were capable of provoking more complex and sustained arrhythmias.

4. The effect of pressure changes were highly dependent upon electrolyte concentrations in the perfusate. Low potassium and magnesium concentrations increased the amount of arrhythmia provoked by pressure increases but tended to reduce that provoked by pressure reductions.

5. We conclude that the direct effect of an acute change in pressure upon the myocardium is arrhythmogenic. However, the myocardial response to a pressure change is interdependent upon prevailing electrolyte concentrations.

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