Mapping of regional myocardial strain and work during ventricular pacing: Experimental study using magnetic resonance imaging tagging

OBJECTIVES The purpose of this study was to determine the spatial distribut ion of myocardial function (myofiber shortening and work) within the left v entricular (LV) wall during ventricular pacing. BACKGROUND Asynchronous electrical activation, as induced by ventricular pa cing, causes various abnormalities in LV function, perfusion and structure. These derangements may be caused by abnormalities in regional contraction patterns. However, insight into these patterns during pacing is as yet limi ted. METHODS In seven anesthetized dogs, high spatial and temporal resolution ma gnetic resonance-tagged images were acquired in three orthogonal planes. Th ree-dimensional deformation data and LV cavity pressure and volume were use d to determine midwall circumferential strain and external and total mechan ical work at 192 sites around the left ventricle. RESULTS During ventricular pacing, systolic fiber strain and external work were approximately zero in regions near the pacing site, and gradually incr eased to more than twice the normal value in the most remote regions. Total mechanical work, normalized to the value during right atrial pacing, was 3 8 +/- 13% (right ventricular apex [RVapex] pacing) and 61 +/- 23% (left ven tricular base [LVbase] pacing) close to the pacing site, and 125 +/- 48% an d 171 +/- 60% in remote regions, respectively (p < 0.05 between RVapex and LVbase pacing). The number of regions with reduced work was significantly l arger during RVapex than during LVbase pacing. This was associated with a r eduction of global LV pump function during RVapex pacing. CONCLUSIONS Ventricular pacing causes a threefold difference in myofiber wo rk within the LV wall. This difference appears large enough to regard local myocardial function as an important determinant for abnormalities in perfu sion, metabolism, structure and pump function during asynchronous electrica l activation. Pacing at sites that cause more synchronous activation may li mit the occurrence of such derangements. (C) 1999 by the American College o f Cardiology.