**3. Classification of pulmonary hypertension based on pathophysiology and etiology**

The 2008 World Symposium on PH endorsed by The World Health Organization (WHO) proposed a classification system divided into 5 groups: 1) Pulmonary arterial hypertension, 2) PH owing to left heart disease, 3) PH owing to lung diseases and/or hypoxia, 4) Chronic thromboembolic PH, and 5) PH with unclear or multifactorial etiologies (Simonneau et al., 2009). In cardiac surgery, PH is more frequently classified as pre-capillary, capillary or postcapillary, depending on the site where the underlying cause of PH is found. In this context, PH during cardiac surgery is typically post-capillary since the cause is mainly of left ventricular (LV) origin, past the pulmonary capillary bed. To confirm this diagnosis, pulmonary artery catheterization can be used to demonstrate an equal value for diastolic pulmonary artery pressure (DPAP) and pulmonary artery occlusion pressure (PAOP). When the cause for PH is at the pre-capillary or capillary level, in absence of tachycardia, DPAP is significantly higher than PAOP (Gomez & Palazzo, 1998).

The causes underlying PH in cardiac surgery can be complex and may result from several mechanisms acting alone or in combination (Fig. **5**). These mechanisms may exist before the operation or appear during or after the procedure. Exacerbation of PH may happen at any time during cardiac surgery, before, during or after CPB. Indeed, patients are at risk of LV failure at all times, especially after CPB when the reperfusion of the ischemic lungs can cause pulmonary reperfusion syndrome. Finally, PH can persist postoperatively secondary to a patient-prosthesis-mismatch (PPM) after mitral or aortic valve replacement. The treatment of PH is based on the identification of its etiology, whence the importance of distinguishing between the different pathophysiologies.

Fig. 5. Major mechanisms of pulmonary hypertension in cardiac surgery. Other mechanisms may be operating at several levels: for instance, hypoxia (capillary) may lead to pulmonary hypertension, right ventricular systolic failure and, through interventricular interaction, left ventricular diastolic function (post-capillary).
