Derivation of myocardial fiber stiffness equation based on theory of laminated composite.

The constitutive equation with stress-dependent coefficients for laminated composite is derived and employed for iterative determination of myocardial fiber's stiffness equation E(f) = Ksigma(f) + C from myocardial strip's stiffness equation E(s) = K(s)sigma(s) + C(s). The strip's stiffness constants K(s) and C(s) are estimated by the least-square curve fitting of the stress-strain data experimentally obtained from uniaxially stretching of strips of left ventricular heart wall excised from seven canine hearts. The values of K and C computed at selected fiber orientations across the thickness of the strip and using three, five, and ten-layer approximations are reported.

The role of displacement of the mitral annulus in left atrial filling and emptying in the intact dog.

The variations in ventricular-atrial mitral annular position during the cardiac cycle and the simultaneous changes in left atrial silhouette area (obtained by angiography after injections of contrast material into the main pulmonary artery) were investigated in six experiments on intact dogs with chronically implanted intracardiac markers. Frame-by-frame measurements of the angiograms (120 frames/s) were used to determine, under various hemodynamic conditions, the duration, magnitude, and average rate of the mitral annular motion and of the simultaneous changes in left atrial area during atrial filling (ventricular systole) and atrial emptying (early in ventricular diastole). The mitral annulus was seen to move towards the ventricular apex during systole and towards the atrium early in diastole with the duration, average rate, and magnitude of displacement (although varying widely) showing good statistical correlations (P less than 0.0005-0.005) with the changes in projected left atrial area. These findings suggest that the duration, rate, and magnitude of atrial filling and emptying may be, in the intact heart, determined by the movements of the atrioventricular junction.

Relation of mitral valve opening and closure to left atrial and ventricular pressures in the intact dog.

The exact times of mitral valve opening and closure were determined in dogs under varying hemodynamic conditions in 143 cardiac cycles (five experiments). Radiopaque markers had been sutured to the cusps and the valve annulus 7-124 wk before the studies. Valve opening and closure times were correlated with simultaneously obtained high-fidelity intracardiac pressures. Closure of the mitral valve was completed 5-105 ms after the atrial-ventricular pressure crossover; the time interval between the onset of ventricular systole and the instance of complete valve closure varied less (10-40 ms). These observations suggest that in the intact heart alpha, rapid mitral cusp closure at the end of diastole is initiated and completed by ventricular systole alone, and beta, the ventricular isovolumic contraction period might be shorter than assumed. Opening of the valve during ventricular relaxation was characterized by 1) initial separation of the markers placed on the free edges of the cusps, of variable duration, apparently due to alterations in ventricular geometry, and 2) a rapid opening motion which clearly preceded the diastolic pressure crossover by 5-60 ms. This finding would suggest that ventricular isovolumic relaxation might be shorter than generally accepted, although the mechanism of early opening is not adequately explained by the data.

Left atrial size and geometry in the intact dog.

Multiple determination of size, shape, and diameter of the left atrium were made during the control state and under conditions of varied ventricular outflow resistance in intact anesthetized dogs with markers chronically attached to the mitral annulus and the valve cusps. Measurements were made from left atrial angiograms (120 frames/s) obtained after injections of contrast medium into the pulmonary artery. Changes in atrial size were characterized by symmetrical circumferential diameter changes and eccentric variations of anteroposterior axis due almost entirely to valve ring displacement. With normal aortic pressure, maximal atrial area averaged 11.0+/-3.1 cm2. When aortic pressure was increased maximal area increased to 12.9+/-3.7 cm2 and percent emptying decreased, whereas maximal and minimal atrial areas were similar to control values during decreased aortic pressure. These findings suggest the possibility that atrial emptying may be near maximal at the resting control state.

Sudden interruption of leaflet opening by ventricular contractions: a mechanism of mitral regurgitation.

The motion of both mitral cusps and the presence of valvular regurgitation during ventricular contractions were investigated in seven experiments on dogs in which radiopaque markers had been sutured to the cusps and the valve annulus 1-32 wk before the studies. Cineangiograms of the left ventricle were obtained during ventricular ectopic beats, interposed throughout the cardiac cycle (20-99% of cycle length) and during induced variations in the P-R interval (0-200 ms). Mitral regurgitation was observed only during a) weak, early ectopic beats (peak pressure below 34 mmHg) which were incapable of closing the cusps and b) when ventricular contractions suddenly interrupted normal leaflet motion toward the ventricle, during three well-defined periods of diastole (diastolic valve opening, diastolic rebound, and atrial opening). Valve closure following sudden reversal of cusp opening was slow and the leaflets often did not arrive simultaneously at their closed positions. These findings suggest that sudden interruption of leaflet opening by ventricular contractions is an important mechanism of transient mitral regurgitation in the normal heart.