Heart rate response during vasodilator stress myocardial perfusion imaging: Mechanisms and implications

Myocardial perfusion imaging (MPI) using adenosine, dipyridamole, or regadenoson (a selective A2A receptor agonist) is an established method for detecting coronary artery disease (CAD) and risk stratification. In high-risk populations, as in those with diabetes mellitus (DM) or chronic kidney disease, MPI has been shown to be a powerful predictor of risk, but nevertheless, patients with normal myocardial perfusion are at higher risk than those without DM or chronic kidney disease. Thus, there continues to be a need to extract more useful prognostic data from stress MPI especially in high-risk populations. A blunted heart rate (HR) response to exercise stress has been known to be an independent predictor of poor outcome and is used clinically in conjunction with other prognostic variables such as perfusion defects, left ventricular (LV) ejection fraction (EF) and volumes, exercise time, and symptoms during exercise to derive an overall assessment of risk in a particular patient. In patients undergoing stress testing, for one reason or another, with adenosine or regadenoson (and dipyridamole, which acts indirectly by increasing interstitial levels of endogenous adenosine), there is a modest increase in HR and a modest decrease in blood pressure (BP). The increase in HR has been traditionally attributed to a reflex response to the vasodilatory effect on the systemic circulation and the resultant increase in sympathetic discharge. The true mechanism of HR increase, however, is more complicated and involves direct stimulation of the sympathetic nervous system. The administration of adenosine as a bolus, as done for the interruption of supra-ventricular tachycardias, has a negative chronotropic effect on the atrio-ventricular node via stimulation of the A1 receptor. This is in contradistinction to its effect on the A2A receptor when given as an infusion in stress MPI studies where it induces an increase in HR. The development of selective A2A receptor agonists (such as regadenoson) has allowed for the dissection of the effects of adenosine on the multiple receptors. A well-done pivotal study in rats by Dhalla et al that used regadenoson in combination with a selective A2A receptor antagonist, B-blocker, a ganglionic blocker (to block the sympathetic nervous system), and a direct vasodilator (nitroprusside) demonstrated the dissociation of tachycardia and hypotension (secondary to peripheral vasodilation) responses to regadenoson. This and other data (reviewed in Ref. ) confirm that A2A receptor agonists cause a direct stimulation of the autonomic nervous system, which results in sinus tachycardia independent of the baroreflex mechanism. Thus, the change in HR in response to A2A receptor agonists can be used to evaluate autonomic function. In order to evaluate the HR response to adenosine and regadenoson in relation to DM (since there is a high prevalence of autonomic dysfunction in patients with DM), we used data from the ADenoscan Versus RegAdenosoN Comparative Evaluation for Myocardial Perfusion Imaging (ADVANCE MPI 1 and 2) Trials. The ADVANCE MPI Trials are randomized multicenter phase 3 trials that demonstrated the noninferiority of regadenoson to adenosine in detecting reversible defects by comparing the strength of agreement between sequential adenosine-regadenoson and adenosine-adenosine images. The HR and BP were measured at baseline and at predetermined intervals extending to 45 minutes after the administration of adenosine or regadenoson. In the 2,000 patients with known DM status (643 with DM, 1357 non-DM, 15 unknown DM status) there was a blunted HR response (maximal percent change of HR from baseline) in those From the Division of Cardiovascular Diseases, University of Alabama at Birmingham, Birmingham, AL; Section of Cardiology, Birmingham Veteran’s Administration Medical Center, Birmingham, AL. Reprint requests: Fadi G. Hage, MD, Division of Cardiovascular Diseases, University of Alabama at Birmingham, Zeigler Research Building 1024, 1530 3rd AVE S, Birmingham, AL 35294-0006; fadihage@uab.edu. J Nucl Cardiol 2010;17:536–9. 1071-3581/$34.00 Copyright 2010 by the American Society of Nuclear Cardiology. doi:10.1007/s12350-010-9248-9