3-DIMENSIONAL TRANSMURAL MECHANICAL INTERACTION BETWEEN THE CORONARY VASCULATURE AND PASSIVE MYOCARDIUM IN THE DOG

The ''garden hose'' effect of coronary perfusion on diastolic left ven tricular (LV) mechanics has been proposed to cause changes in systolic function by altering diastolic sarcomere length. We measured transmur al distributions of three-dimensional shape change using radiopaque ma rkers implanted in the LV free wall of eight isolated arrested canine hearts as functions of coronary arterial perfusion pressure (Pp) and L V pressure (P-LV) and related these deformations to the local muscle f iber architecture. Increased Pp from 0 to 110 mm Hg produced a 10% red uction in LV chamber volume (P<.01) and 25% to 40% decreases in local three-dimensional wall strain at matched P-LV, indicating myocardial s tiffening. Significant decreases in the magnitudes of local deformatio n occurred preferentially in the cross-fiber and radial directions (P< .02), with no change in fiber strain. This suggests that changing coro nary Pp does not alter diastolic fiber length; hence, the Frank-Starli ng law may not mediate the Gregg effect. Since the myocardial microves sels are primarily oriented parallel to the muscle fibers, the observe d myocardial stiffening occurs in the directions transverse to the mic rovessels rather than along their length. Local myocardial wall volume in the unloaded LV demonstrated a uniform 5% increase from the unperf used state to Pp of 50 mm Hg. With further increases in Pp up to 110 m m Hg, the change in regional wall volume from the unperfused state dev eloped a substantial transmural gradient increasing by 7% at the epica rdium and 15% at the subendocardium. This reflects a significant incre ase (P<.02) in intramyocardial coronary capacitance from epicardium to endocardium, which may be related to a transmural gradient in coronar y distensibility or vascularity.