Coronary vasodilator reserve persists despite tachycardia and myocardial ischemia

During myocardial ischemia, we tested whether coronary blood flow would increase in response to tachycardia, thereby employing known coronary flow reserve. We instrumented the left anterior descending (LAD) coronary circulation in anesthetized pigs and performed three sets of experiments while coronary pressure was controlled and several heart rate increases were produced. 1) Pacing-induced tachyardia at normal LAD presure was characterized by increased LAD flow and myocardial oxygen consumption, without production of lactate. 2) Tachycadia at a mean LAD pressure of 38 mmHg was associated with a lower, fixed coronary flow and oxygen consumption. At average heart rates of 90 and 150 beats/min, LAD flow was 19.6 and 19.4 ml/min and corresponding myocardial blood flows were 0.59 and 0.54 ml·g^-1·min^-1. Lactate was produced at all rates and local myocardial function declined progressively. 3) Coronary flow at low LAD pressure doubled during tachycardia when intracoronary adenosine was added. The increase to the subepicardium was >100%, whereas subendocardial flow changed little. There is persistent coronary flow reserve during moderately severe myocardial ischemia, even when metabolic demand is increased by tachycardia. This reserve, however, is predominantly subepicardial.