Electrocardiographic correlates of myocardial ischaemia induced by atrial and ventricular pacing in dogs with coronary stenosis

We determined the electrocardiographic response to pacing-induced tachycardia in 45 dogs. Pacing was performed using left atrial, left atrial-right ventricular sequential or left atrial-left ventricular sequential modes at rates of 90 to 250 beats X min-1. Body surface isopotential maps in 15 dogs with normal coronary circulations defined the normal response to rate; surface potential extrema during the S-T segment increased in strength with increasing rate but spatial features remained constant. In the other 30 dogs, an ameroid constrictor was placed around the left circumflex coronary artery. Two weeks after implantation, atrial pacing to rates of 190 beats X min-1 or greater resulted in flat, S-T segment depression, with new and abnormal negative voltages registered over the inferior and left posterior torso. However, with either form of ventricular pacing, tachycardia with coronary obstruction did not alter the S-T segment response seen in control animals, in either intensity or spatial parameters. We interpret these findings to suggest that: in normal dogs, tachycardia produced a common electrocardiographic effect regardless of activation pattern; and tachycardia in the presence of coronary constriction results in subendocardial myocardial ischaemia that, with atrial pacing, reverses the normal transmural S-T segment potential gradient and causes body surface S-T segment depression, but with primary ventricular stimulation, the subendocardial ischaemia does not alter the transventricular repolarisation gradients sufficiently to generate body surface S-T segment shifts.