**8. High altitude PH**

High altitude PH (HAPH) prevalence is between 5 and 18% in those living at ≥3000 metres and may be more common in children than adults [9,11,92]. As mentioned previously, the roles of the endothelin-1 and prostaglandin I2 pathways in the pathophysiology in high altitude associated PH have not been clearly defined [9]. Alteration in trans-membrane transport of K+ and Ca2+ has been implicated in the process. Recent work by Beall et al. has suggested a role of free radical-mediated reduction in NO bioavailability [93, 94].

Migration to a lower altitude reverses HAPH. However, due to family, social and economic reasons, migration is not an option for some patients. As an alternative, sildenafil for 3 months has been shown to reduce PAP, improve 6MWD and cardiac index in patients with HAPH [95]. Reduction in mean PAP of up to -6.9 mmHg and improvement in walking distance of up to 45 m was observed and sildenafil was well tolerated [95].

The role of endothelin receptor antagonists in HAPH is yet to be determined. A small rando‐ mised cross-over study of 8 patients on bosentan did not improve pulmonary pressures or functional capacity when initiated prior to ascent during high intensity exercise [96]. Aceta‐ zolamide was successful in reducing pulmonary pressures and improving cardiac output at 6 months of therapy in patients with excessive erythrocytosis and HAPH [97]. Other drugs under evaluation include angiotensin inhibitors and results of the ongoing studies are pending.
