1. The early electrophysiological patterns of regional subendocardial ischaemia were studied by using the paced endocardial evoked response and simultaneous endocardial monophasic action potential recordings in 16 experiments in open chested dogs. Ischaemia was produced by transient (1-3 min) coronary artery occlusion.

2. Regional subendocardial isochaemia caused asynchronous activation due to differential conduction delay and shortened repolarization as evaluated by the duration of the paced evoked response from 175±(sd) 18.7 ms to 167 ± 16 ms (P <0.001). These changes occurred within 60 s of occlusion and reversed rapidly after release of the occlusion.

3. In simultaneous endocardial monophasic action potentials there was a decrease in plateau amplitude and the duration of repolarization shortened from 180±(sd) 21.2 ms to 167 ±20.4 ms (P <0.001).

4. The delay in endocardial activation after 2 min ischaemia was 5.5 ms, which is considerably shorter than the conduction delay previously reported in the subepicardial layers.

5. The calcium-channel blocking drug verapamil (infused at 0.4 mg/kg) altered the rate at which shortening of repolarization and asynchronous activation occurred during ischaemia in six experiments.

6. These experiments suggest that intracavitary electrodes could provide earlier and more sensitive detection of regional subendocardial ischaemia and may permit the assessment of therapy on the early electrical changes in the intact heart.

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