Final answer:
The M-mode echocardiographic sign for constrictive pericarditis is A. B notch. It's seen during the isovolumic ventricular relaxation phase of the cardiac cycle, where M-mode echocardiography can reveal characteristic indentations on the ventricular septum.
Step-by-step explanation:
An M-mode echocardiographic sign for constrictive pericarditis is A. B notch. Constrictive pericarditis is a condition where the pericardium becomes thickened and fibrotic, restricting the heart's ability to function properly. M-mode echocardiography, which stands for motion mode, allows physicians to see and measure cardiac structures in motion. The B notch refers to an indentation seen on the M-mode echocardiogram of the ventricular septum, which is characteristic of this condition.
This occurs during the early phase of ventricular diastole known as the isovolumic ventricular relaxation phase, where pressure in the ventricles falls below that in the aorta and pulmonary trunk. This phase is characterized by the closure of the semilunar valves to prevent backflow into the heart, and it's during this time the B notch can be detected on the echocardiogram.