• Respiratory mechanics (i.e. lung compliance, resistances and work of breathing) [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
  

• exercise capacity (i.e. 6 mwd), dyspnea, oxygen saturation and cardiac function (i.e. hemodynamic evaluation) [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
  

Endothelins are powerful vasoconstrictor peptides that also play numerous other functions in many different organs. Endothelin-1 (ET-1) is the most abundant and important of this family of peptides in blood vessels. Production of ET-1 is increased in the endothelium and the kidney in salt-dependent models of hypertension ET-1 elicits an inflammatory response by increasing oxidant stress in the vascular wall, which induces vascular remodeling and endothelial dysfunction found in the hypertensive models that exhibit an endothelin-mediated component. Endothelin receptor antagonists lower blood pressure in hypertensive patients. They could become therapeutic agents for prevention of target organ damage in hypertension and in type 2 diabetes, chronic renal failure and congestive heart failure. Side effects of endothelin receptor blockers have prevented up to the present their development for these indications. Endothelin antagonists have been approved only for the treatment of pulmonary hypertension, a rapidly fatal condition in which the endothelin system plays an important role and endothelin antagonists exert favorable effects.The exact mechanism of action of ERAs on the pulmonary vascular bed remains unclear. Vasodilatation is just a part of the mechanism, since usually 70%-80% of Idiopathic PAH patients do not respond acutely to vasodilators. Endothelin is likely to be involved in pulmonary vasoconstriction, inflammation, cellular proliferation and fibrosis ie. remodelling Recent research illustrates that bosentan is capable of blunting the vascular remodelling normally associated with PAH If ERAs could prevent remodelling, they might substantially improve the long-term survival in patients with mild symptoms (WHO class II or I).

Bosentan, the most popular endothelin receptor antagonist, has been largely used in the treatment of pulmonary hypertension (PH). It can improve exercise capacity, lower Borg dyspnoea score nad these effects are usually associated with the concomitant improvement in cardiopulmonary haemodynamics.

No physiological study has so far verified the hypothesis that Bosentan may laso have an effect on the "respiratory side" of the cadio-pulmonary system (i.e. on pulmonary mechanics and work of breathing)