Exercise testing
The normal physiologic response of the pulmonary vasculature to exercise consists of distension of pulmonary arteries and arterioles as well as recruitment of previously unused vascular bed. Thus, in health, pulmonary artery pressure rises minimally in response to increased blood flow and pulmonary vascular resistance decreases because of the remodeled vasculature. These mechanisms are impaired in the course of PH. Cardiopulmonary exercise testing (CPET) has been shown to be useful in assessing the severity and prognosis of PAH [15,185]. Several mechanisms are associated: (1) failure to perfuse the ventilated lung, leading to an increase of physiologic dead space and ventilatory requirement; (2) failure to increase cardiac output appropriately in response to exercise, causing an early lactic acidosis, thereby increasing acid ventilatory drive; and (3) exercise-induced hypoxemia increasing the hypoxic ventilatory drive. The ventilatory expired gas abnormalities precipitated by PH are multifactorial and associated with disease severity. CPET assesses and measure the ventilation–perfusion mismatch (i.e. acceptable ventilation/diminished perfusion), reflected by an elevated VD/VT or VE/VCO2 ratio or slope and diminished partial pressure of end-tidal carbon dioxide (PETCO2), and the decreased peak VO2 and VO2 at the ventilatory threshold (VT). Peak VO2, VE/VCO2 ratio or slope or PETCO2, measure during CPET, all demonstrated independent and strong prognostic value as univariate markers [186].