Transmural Changes in Myocardial Tissue Volume During the Cardiac Cycle: Implications for Microvascular Flow Patterns with Ventricular Pacing

Abstract

The transmural gradient in average myocardial blood flow is well known. However, few techniques have the temporal and transmural resolution to examine tissue volume and by implication vascular volume changes during the cardiac cycle during normal activation and with epicardial ventricular pacing. We studied the time course of transmural tissue volume change (TVC) in the anterior left ventricular wall of normal canine heart (n=14) using implanted markers and 125 Hz cineradiography. TVC was calculated from marker locations (as a relative fraction, 1.00=no change), and capillary volume was an assumed fraction of the compressible tissue. Capillary diameter (CD) was estimated from capillary volume and length changes (from myofiber strain). During normal activation there was a significant transmural gradient in TVC 72ms (Early) after QRS (.99±.04 (SD) epi, .95±.05 endo), and at end systole (ES) (.97±.06 epi, .89±.08 endo). This corresponded to an Early gradient in CD: 5.6±.6 (μm, epi) to 4.9±1.0 (endo), and a gradient at ES: 5.1±.8 (epi) to 4.2±1.4 (endo). During ventricular pacing the Early TVC gradients reversed: .97±.02 (epi) and .99±.05 (endo), with reduced gradients at ES. Early CDs were 5.3±.4 (epi) and 5.3±.6 (endo). Thus, there is a transmural gradient of reduction in tissue volume and CD during systole, which changes with ventricular pacing. Changes in transmural blood flow may trigger local tissue remodeling associated with areas of early ventricular activation. Supported by HL43617 and HL32583.