1. The effect of different levels of ventricular pressure upon the prevalence of ventricular arrhythmias has been studied in 42 rat hearts by using the isolated working heart model.

2. The results have shown that there is an increased prevalence of arrhythmia at the highest levels of pressure tested in hearts from both normal and hypertensive rats. In hearts from normal rats total ectopic counts were 304 at 140 mmHg and 150 at 60 mmHg (P < 0.05). In hearts from hypertensive rats total ectopic counts were 4217 at 180 mmHg and 2179 at 100 mmHg (P < 0.05).

3. Hearts from hypertensive rats demonstrated significantly more ectopic activity than hearts from normal rats at all test pressures. Median values for ectopic counts for each study period were 68 in hearts from hypertensive rats and seven in hearts from normotensive rats (P < 0.001).

4. There was evidence that the increased arrhythmia observed was not due to ischaemia or fatigue, and it seems likely that these effects of raised pressure are due to its effect on ventricular wall stress. The differences between various pressures were relatively small, but do suggest that a sustained increase in ventricular wall stress is arrhythmogenic.

5. This small arrhythmogenic effect over the relatively short period of these studies may be of much greater significance when considered in the context of the prolonged periods for which the diseased heart may be subjected to raised wall stress.

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