Interrelation of heart rate and autonomic activity in asymptomatic men with unobstructed coronary arteries. Studies with atrial pacing, adrenaline infusion, and autonomic blockade.

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Abstract
Asymptomatic men were studied who had abnormal resting ECG changes indistinguishable from those of myocardial ischemia but with normal coronary arteriograms. The study aimed to separate the effects of heart rate alone from those of the sympathetic nervous system. Therefore the effects of atrial pacing alone, pacing and adrenaline [epinephrine] infusion combined, pacing after .beta.-blockade, and pacing after .beta.-blockade plus atropine were studied. Twenty asymptomatic men, aged 17-57, were investigated. All had unobstructed coronary arteries and normal left ventricular angiograms. Echocardiographic findings were normal. Sixteen had flat or inverted T waves in the lateral leads, 2 had ST depression and 2 had mixed patterns. T wave abnormalities and, to a lesser extent, ST changes returned to normal or regressed after an overnight rest in hospital. Subsequent atrial pacing to 160/min reproduced or increased the respective abnormalities. When adrenaline was infused in low doses just sufficient to produce discernible effects on the ST-T segment (between 0.024 and 0.091 .mu.g/kg per min) and atrial pacing was repeated, the effect of the latter was enhanced. Both adrenaline and pacing influenced the ST-T segment in the same direction. I.v. propranolol (0.2 mg/kg) blocked the effect of adrenaline and its synergistic effect with pacing but exerted little if any influence on the effect of pacing alone. Atropine given i.v. after propranolol (0.04 mg/kg) reduced the effect of atrial pacing on the ST-T configuration. Treadmill exercise tests were positive in 9 and borderline in 1. After .beta.-blockade (oral oxprenolol), all tests were negative. As .beta.-blockade did not prevent pacing-induced ST depression but normalized the false-positive exercise test, the latter does not appear to be rate related but more probably the result of the direct influence of catecholamines. Isolated T wave changes and ST depression in the resting ECG differ in that they are influenced both by heart rate and catecholamines acting synergistically.