**2. Haemodynamic classification of PH**

According to the European Society of Cardiology 2015 guidelines, PH is defined as an increase in mean pulmonary arterial pressure (mPAP) ≥25 mmHg at rest as assessed by right heart catheterization (RHC). Available data have shown that the normal mPAP at rest is 14 � 3 mmHg with an upper limit of normal of approximately 20 mmHg [5]. This definition was updated at the sixth world symposium of PH, held in 2018 in Nice: the mPAP threshold was lowered from ≥25 to >20 mmHg [6]. Whatever the mPAP cut-off value considered for defining PH, it is important to emphasize that this value used in isolation cannot characterize a clinical condition and does not define the pathological process per se. According to Poiseuille's law mPAP depends on cardiac output, left atrial pressure, and PVR (4).

$$\text{mPAP} = (\text{CO} \times \text{PVR}) + \text{PAWP} \tag{4}$$

Then, mPAP elevation may have several different causes with different prognoses and treatments, including high cardiac output syndromes (anemia, left�to�right shunts, AV fistula, and thyrotoxicosis.) or diseases characterized by high PWAP (left heart diseases) or high PVR because of pulmonary vascular disease [6]. Specifically, precapillary pulmonary hypertension due to pulmonary vascular disease is hemodynamically defined by a pulmonary artery wedge pressure (PAWP) ≤15 mmHg and an elevation in PVR of at least three wood units (WU).

Precapillary hypertension contrasts with postcapillary PH in which the PVR is less than 3 WU and the elevation in the mPAP is due to elevated filling pressures on the left side of the heart (PAWP > 15 mmHg) [7].

Postcapillary PH is further subclassified on the basis of the PVR, into isolated postcapillary PH (PAWP > 15 mm Hg and PVR < 3 WU) and combined pre- and post-capillary PH (PAWP > 15 mm Hg and PVR ≥ 3 WU). (See **Table 1**).
