MODULATION OF THE ARTERIAL CORONARY BLOOD-FLOW BY ASYNCHRONOUS ACTIVATION WITH VENTRICULAR PACING

This study aims to test the assumptions that: (1) coronary arterial fl ow is attenuated in an early activated region by ventricular pacing; ( 2) asynchronous mechanical activation caused by ventricular pacing und er controlled perfusion pressure and intact coronary tone is associate d with reduced coronary flow compared to atrial pacing; and (3) abolis hment of vascular tone under controlled perfusion pressure diminishes the expected difference in blood flow between atrial and ventricular p acing. Blood flow velocity (BFV) in the left anterior descending (LAD) and the left circumflex arteries (CFX) and a wall thickening index we re measured in 14 open-chest dogs under normal conditions, and constan t perfusion pressure. Four pacing sites were used: right atrium (RA(p) ), mid-right ventricle (RV(p)), mid-left ventricle (LV(p)), and left v entricular apex (Apex(p)). Pacing modes were either sequential ventric uloatrial (VA) (protocol A, n = 7), or sequential atrioventricular (AV ) (protocol B, n = 7), with a shorter AV difference (30 msec) than nor mal. Results: BFV was decreased in the LAD during RV(p) and Apex(p) pa cing by 9.7%-12.9% versus RA(p) and by 11.6%-14.6% versus LV(p) (P < 0 .05). No BFV variations were observed in the CFX. Flow velocity conduc tance (FVC = mean blood flow velocity divided by the mean aortic press ure) 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 LV(p) versus RA(p) pacing in the LAD (P < 0.05). Wall thickening index reduced during ventricular pacing in all three ventricular sites by 5 0%-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 d iastolic phase (16.6%-45.5%, P < 0.02). Normalized oscillatory flow am plitude (OFA(n)) was reduced in RV(p) pacing compared to RA(p) and LV( p) pacing (16.2 +/- 3.5 and 21.7% +/- 4.1%, respectively, P < 0.03). T he variations in blood flow and OFA(n) disappeared with adenosine-medi ated maximum vasodilatation. Summary: (1) Mean and phasic flows are re duced in the early activated LAD region by ventricular pacing (RV(p), Apex(p)). (2) Under controlled perfusion pressure and intact vascular tone, ventricular pacing compromises blood flow compared with atrial p acing. (3) This effect disappears when vascular tone is eliminated by intracoronary injection of adenosine, suggesting that the coronary aut oregulation is responsible for some of the effects.