**4. Clinical impact of post bypass exacerbation of PH**

Independent of the exact cause, the exacerbation of preexisting PH can lead to a further increase in the right ventricular afterload and distension of an already dysfunctional right ventricle, resulting in increased right ventricular free wall tension and myocardial oxygen consumption.

Normally, the pulmonary circulation is a low pressure, high flow vascular bed accommodating the entire cardiac output with each heartbeat. Elevated pulmonary vascular resistance (PVR) may significantly contribute to right ventricular dysfunction, which may compromise the preload of the left ventricle inducing systemic hypotension. In patients with pathologically increased PVR, the right ventricle and left ventricle are interdependent and have similar vitally important functions. Right ventricular dilatation causes shifting of the intraventricular septum towards the left ventricle, leading to a smaller underfilled left ventricular cavity. The normal thin walls and crescent shape of the right ventricle result in a highly compliant right ventricular chamber, which is able to accommodate large increases in volume. However, the right ventricular adaptive mechanisms are not well suited to acute, large increases in pressure, (Fischer et al., 2003), as this may happen after CPB.

Furthermore systemic hypotension decreases right ventricular coronary perfusion pressure and oxygen delivery. Therefore, a vicious circle starts that can lead to exacerbation of right ventricular dysfunction.
