**2. Definition**

PH refers to increased pressure in the arterial site of the pulmonary circulation and is defined as persistent elevation of mean pulmonary arterial pressure (MPAP) above 25 mmHg.

The current hemodynamic definition of PH is described in **Table 1**. The first clinical classification of PH was proposed at the first international conference on primary pulmonary hypertension endorsed by the World Health Organization (Hatano & Strassert, 1975). The previous version of the European Society of Cardiology (ESC) PH guidelines adopted the Evian-Venice classification proposed at the second and third World meeting on PH in 1998 and 2003 respectively (Galiè et al., 2004). According to these classifications, clinical conditions associated with PH are divided in five groups according to pathological, pathophysiological and therapeutic characteristics. During the fourth World symposium on PH held in Dana Point, California, the consensus agreement of experts worldwide was to maintain the general philosophy and organization endorsed by the Evian-Venice classifications while at the same time amending some specific points so as to improve clarity and to take into account new information (Galiè et al., 2009). Accordingly, PH can be classified into six clinical groups with specific characteristics (**Table 2**).

Anesthesiologists encounter patients with PH in a variety of situations in the operating room (Tidswell & Higgins, 2007). The most common underlying pathology of cardiac surgery patients which leads to PH is due to left-sided valvular heart disease, left-sided ventricular heart disease and shear stress from increased pulmonary blood flow due to intracardiac shunts.


Table 1. Hemodynamic definition of pulmonary arterial hypertension. *CO, cardiac output; MPAP, mean pulmonary arterial pressure; PH, pulmonary hypertension; PCWP, pulmonary capillary wedge pressure; TPG, transpulmonary pressure gradient (MPAP-PCWP)* 

Data from the multinational database EuroSCORE have demonstrated that PH is an independent risk factor for increased morbidity and mortality in patients undergoing heart surgery (Roques et al., 1999).
