Electrocardiographic effects of myocardial ischemia induced by atrial pacing in dogs with coronary stenosis. II. Comparison of S-T segment and QRS changes in dogs with left circumflex arterial narrowing.

The effects of tachycardia on the QRS complex in the absence and in the presence of coronary arterial constriction are of possible clinical significance. To study the spatial and temporal distributions of these effects, a canine model simulating progressive coronary occlusion was studied. Tachycardia was induced by atrial pacing, coronary narrowing was produced by implantation of an ameroid constrictor about the left circumflex coronary artery and electrocardiographic effects were studied by isopotential mapping techniques at rates of up to 250 beats per minute, one to three weeks after constrictor implant. In five dogs. pacing without coronary constriction demonstrated the spatial and temporal dependence of QRS changes; tachycardia-induced changes varied from one torso site to another at any instant, and from one moment to another at any one torso location. Patterns in ten dogs with constrictors were dependent upon time after ameroid placement. Resting QRS and S-T segment distributions remained unchanged. One week after surgery, QRS and S-T segment patterns during pacing were as in controls. Two weeks after surgery, QRS patterns were as in controls but flat S-T segment depression resulted at high rates. After three weeks, QRS patterns during pacing were abnormal and S-T segment depression was observed. At each of the later times, the QRS response was similar at rates which did and which did not cause S-T depression. Thus, the effects of tachycardia on excitation and recovery forces in the presence of restricted coronary flow are independent of one another. This suggests that the two responses to ischemia may be due to different basic mechanisms and, hence, that their detection may reflect different myocardial abnormalities.