Interventricular Dependance
It refers to the enhanced ventricular interaction whereby, during inspiration, right heart filling increases at the expense of left heart filling, and this reverses with expiration. During normal inspiration, the negative intrathoracic pressure draws more blood into the right heart, and also pools more blood in the pulmonary vasculature. The reduction in blood return to the left heart and consequent reduction in left heart stroke volume and cardiac output is usually minimal, causing a drop in BP that is less than 10 mmHg on inspiration. In cardiac tamponade, as negative intrathoracic pressure draws blood into the right heart (as usual), the intraventricular septum bows into the left ventricle, because the high pressure in the pericardium prevents the right ventricle from complying in any direction other than towards the IVS. Meanwhile, the high pericardial does not allow the left ventricle from expanding in any way to maintain an adequate end-diastolic volume, so LV filling is reduced and stroke volume and cardiac output are consequently reduced, to an exaggerated degree, with a drop in BP that is more than 10 mmHg on inspiration.
Respiratory Variation in Right or Left Heart Filling
Because of ventricular interdependence (described above) in tamponade, the Doppler velocities of the left and right side of the heart – measured at the mitral inflow tract and the tricuspid inflow tract – vary with respiration and with each other, exactly 180° offset from each other. During inspiration (denoted “I” in the figure), as right heart filling increases at the expense of left heart filling due to the high pericardial pressure, the tricuspid inflow E velocity increases by >60% (see bottom row of figure). The tricuspid inflow E velocity drops back down by >60% on expiration (denoted “E” in the figure). The sample volume should be at the leaflet tips, using pulse wave Doppler, in A4C view. You should acquire this using a slow sweep speed (25 mm/s) with the baseline shifted down and with the respirometer displayed on the screen, so that you can pick the first beat of expiration and first beat of inspiration to compare heights. The height will be in cm/s. Reciprocally, on the left side of the heart, during inspiration, the mitral inflow E velocity decreases with inspiration (because the interventricular septum is bowing into the LV and hindering mitral inflow), and increases with expiration, by >30% (see top row of figure).