Structure function interface with sequential shortening of basal and apical components of the myocardial band.

OBJECTIVE

To mechanically test the intact cardiac structure to determine the sequence of contraction within the myocardial mass to try to explain ejection and suction.

METHODS

In 24 pigs (30-85 kg), segment shortening at the site of sonomicrometer crystals was continuously recorded. The ECG evaluated rhythm, and Millar pressure transducers measured intraventricular pressure and dP/dt.

RESULTS

Study of segment shortening defined a sequence of contraction within the myocardial mass, starting at the free wall of the right ventricle and on the endocardial side of the antero-septal wall of the left. Crystal location defined underlying contractile trajectory; transverse in right ventricle followed by basal posterior left ventricle, and from the endocardial anterior wall to the posterior apical segment and finally to the epicardial side of the anterior wall. Mean shortening fraction averaged 18+/-3%, with endocardial exceeding epicardial shortening by 5+/-1%. Epicardial segment crystal displacement followed endocardial shortening by 82+/-23 ms in the anterior wall, and finished 92+/-33 ms after endocardial shortening stopped, time frame that matches the interval of fast drop of ventricular pressure and the start of suction.

CONCLUSIONS

Crystal shortening fraction sequence followed the rope-like myocardial band model to contradict traditional thinking, with two starting points of excitation-contraction, the right anterior free wall of the right ventricle, and the endocardial side of the anterior wall. Active suction may be due to active shortening of the epicardial fibers of the anterior wall, because relaxation was not detected when both mitral and aortic valves were closed during the interval previously termed 'isovolumetric relaxation'.