Final answer:
The echocardiographic sign of constrictive pericarditis is the Square root sign, indicative of a distinctive diastolic pressure pattern in the ventricles.
Step-by-step explanation:
The M-mode echocardiographic sign of constrictive pericarditis is the Square root sign. This sign is reflective of the abrupt deceleration of ventricular filling in early diastole followed by a plateau phase, which is characteristic of constrictive pericarditis on echocardiography. During the isovolumic ventricular relaxation phase, pressure in the ventricles drops below the pressure in the pulmonary trunk and aorta leading to the closure of semilunar valves, which can be seen as the dicrotic notch in blood pressure tracings. However, the square root sign refers specifically to a pressure tracing finding that depicts a dip and plateau pattern of the ventricular pressure curve, which is different from the dicrotic notch but indicates pathology in the constriction of the pericardium affecting the cardiac cycle.