Evaluation of a neurogenic rapid coronary dilatation during an excitatory response in conscious dogs

The present study was undertaken to evaluate the mechanisms of coronary adaptation to sudden changes in myocardial oxygen demand that occur during excitement. An excitatory response was evoked either by electrical stimulation of the hypothalamic defence area or by noise (discharge of a fire-arm). Continuous measurement of the oxygen saturation in coronary venous blood was used to judge, whether an increase in coronary flow was adequate to match an increased myocardial oxygen demand. During the excitatory response heart rate, cardiac output and coronary flow increased. However, the increase in coronary flow was not adequate to meet the increased metabolic requirement as indicated by a decrease in coronary venous oxygen saturation. In dogs with experimental atrioventricular block, and with heart rate controlled by external pacing, a rapid coronary dilation occurred during the excitatory response and was accompanied by an increase in coronary venous oxygen saturation. This rapid coronary dilation was abolished by beta-adrenergic blockade. The pattern of coronary flow and coronary venous oxygen saturation that occurred during the excitatory response in normal dogs could be mimicked in dogs with atrioventricular block by increasing the ventricular pacing rate. However, when identical increases in heart rate were induced either excitement or by external pacing, the drop in coronary venous oxygen saturation was significantly larger in the paced series. This demonstrates, that an increase in heart rate is responsible for the transient decrease in coronary venous oxygen saturation during the excitatory response. From these experiments it is concluded that a rapid neurogenic dilation of the coronary vessels occurs during the excitatory response. Under normal conditions this rapid neurogenic dilation is masked by the effect of the accompanying increase in heart rate on extravascular coronary resistance.