Modulation of the Arterial Coronary Blood Flow by Asynchronous Activation with Ventricular Pacing

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Abstract
This study aims to test the assumptions that: (1) coronary arterial flow is attenuated in an early activated region by ventricular pacing; (2) asynchronous mechanical activation caused by ventricular pacing under controlled perfusion pressure and intact coronary tone is associated with reduced coronary flow compared to atrial pacing; and (3) abolishment of vascular tone under controlled perfusion pressure diminishes the expected difference in blood flow between atrial and ventricular pacing. Blood flow velocity (BFV) in the left anterior descending (LAD) and the left circumflex arteries (CFX) and a wall thickening index were measured in 14 open-chest dogs under normal conditions, and constant perfusion pressure. Four pacing sites were used: right atrium (RAp), mid-right ventricle (RVp), mid-left ventricle (LVp), and left ventricular apex (Apexp). Pacing modes were either sequential ventriculoatrial (VA) (protocol A, n = 7), or sequential atrioventricular (AV) (protocol B, n = 7), with a shorter AV difference (30 msec) than normal. BFV was decreased in the LAD during RVp and Apexp pacing by 9.7%-12.9% versus RAp and by 11.6%-14.6% versus LVp (P < 0.05). No BFV variations were observed in the CFX. Flow velocity conductance (FVC = mean blood flow velocity divided by the mean aortic pressure) was higher by 16%-28% in the CFX for the three ventricular pacing sites versus the atrial pacing, and higher by 14.1% +/- 6.1% only in LVp versus RAp pacing in the LAD (P < 0.05). Wall thickening index reduced during ventricular pacing in all three ventricular sites by 50%-64% (P < 0.05) compared to atrial pacing. Under constant perfusion pressure, LAD blood flow decreased with ventricular pacing as compared to right atrial pacing; this was particularly pronounced during the diastolic phase (16.6%-45.5%, P < 0.02). Normalized oscillatory flow amplitude (OFAn) was reduced in RVp pacing compared to RAp and LVp pacing (16.2 +/- 3.5 and 21.7% +/- 4.1%, respectively, P < 0.03). The variations in blood flow and OFAn disappeared with adenosine-mediated maximum vasodilatation. (1) Mean and phasic flows are reduced in the early activated LAD region by ventricular pacing (RVp, Apexp). (2) Under controlled perfusion pressure and intact vascular tone, ventricular pacing compromises blood flow compared with atrial pacing. (3) This effect disappears when vascular tone is eliminated by intracoronary injection of adenosine, suggesting that the coronary autoregulation is responsible for some of the effects.