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

Abstract

We have extended our observations on asymptomatic men with abnormal resting electrocardiograms showing changes indistinguishable from those of myocardial ischaemia but with normal coronary arteriograms. In the present investigation we aimed to separate the effects of heart rate alone from those of the sympathetic nervous system. We therefore studied the effects of atrial pacing alone, pacing and adrenaline infusion combined, pacing after beta-blockade, and pacing after beta-blockade plus atropine. Twenty asymptomatic men, aged 17 to 57, were investigated. All were shown to have unobstructed coronary arteries and normal left ventricular angiograms. Echocardiographic findings were normal. Sixteen had flat or inverted T waves in the lateral leads (designated 'T'), two had ST depression (designated 'ST'), and two had mixed patterns. T wave abnormalities had, to a lesser extent, ST changes returned to normal or regressed after an overnight rest in hospital. Subsequent atrial pacing to 160/minute reproduced or increased the respective abnormalities. When adrenaline was infused in low doses just sufficient to produce discernible effects on the ST-T segment 9between 0.024 and 0.091 μg/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. Intravenous 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 intravenously after propranolol (0.04 mg/kg) reduced the effect of atrial pacing on the ST-T configuration. Treadmill exercise tests were positive in nine and borderline in one. After beta-blockade (oral oxprenolol), all tests were negative. As beta-blockade did not prevent pacing-induced ST depression but normalised 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 electrocardiogram differ in that they are influenced both by heart rate and catecholamines acting synergistically.