Coronary blood flow (CBF) is considered proportional to metabolic demand (MVO,). However, recent studies have reported inappropriate vasoconstrictor response to a-adrenergic stimulation in patients with coronary artery disease (CAD). To assess the interaction of vasodilatory reserve and adrenergic vasoconstriction, we compared changes in coronary vascular resistance (CVR) during the metabolic stress of rapid atrial pacing and during the a-adrenergic stimulus of cutaneous cold (cold pressor test, CPT) in 13 control patients and 14 patients with CAD. Similar heart rates were achieved with pacing in both control and CAD patients, and both groups had a similar hypertensive response to CPT; thus, both pacing and CPT increased major determinants of MVO2. In association with this increased MVO, CVR decreased with rapid pacing in control and CAD patients (-24% and -27%, respectively), but increased in CAD patients (+24%) during CPT. Seven of 13 CAD patients actually had a reduction in CBF, whereas CBF increased in all control patients in response to CPT. Compression of intramural coronary vessels by elevated left ventricular diastolic pressure was excluded as a pathogenic mechanism for increase in CVR in two CAD patients who showed marked reduction in CBF during CPT.
These data are further evidence that patients with CAD may have limited coronary vasodilatory mechanisms. Superimposed a-adrenergically mediated coronary vasoconstriction may contribute significantly to myocardial ischemia in patients with CAD.