Effect of nitroglycerin on the electrical changes of early or subendocardial ischaemia evaluated by monophasic action potential recordings.

Intracavitary recording of monophasic action potentials (MAP) is a sensitive means of detecting the electrophysiological effects of early or subendocardial ischaemia. The effects of nitroglycerin (NTG) on the MAP was evaluated during pacing-induced angina in seven patients with localised, reversible ischaemia. Recordings from the ischaemic zone demonstrated a decrease in MAP amplitude and an abnormal rate-corrected shortening of MAP repolarisation. The "control" right ventricular MAP showed only the expected rate-dependent decrease in duration throughout the pacing stress test. The ischaemic MAP were unchanged following the intracoronary administration of NTG (100 micrograms). In contrast, intravenous NTG (200 to 300 micrograms) produced a normalisation of MAP amplitude and duration in spite of continuous pacing at the angina-provoking rate. These changes were preceded by a fall in aortic pressure (from mean 123/84 to 96/62) and subsequent lowering of the rate-pressure product. The major beneficial effects of NTG on the early electrical changes of pacing-induced ischaemia are thus related to decreased oxygen demand due to reduction in cardiac preload.