Pulmonary hypertension induced in dogs by hypoxia at different high-altitude levels

Chronic natural hypoxia at 2300 m altitude induces mild pulmonary hypertension (PH) in healthy dogs. The influence of more severe hypoxia on the same group of dogs was evaluated by re-examining such dogs at 3500 m, after they had regularly exercised at this altitude level for half a year. Despite severe hypoxaemia at 3500 m (PaO2 52+/-5 mmHg), none of the dogs developed erythrocytosis, and their PCV at 3500 m (48% +/- 4%) did not differ from that at 2300 m (49% +/- 4%). There was a tendency towards an elevated systemic BP, with a significant increase in diastolic BP (105 +/- 13 mmHg at 3500 m versus 98 +/- 17 at 2300 m). Tricuspid regurgitation (TR) was detected in 7 dogs at 3500 m compared to 8 dogs at 2300 m. The mean TR Vmax was significantly higher at 3500 m, and all 7 dogs had systolic PH at 3500 m (33.6-54.8 mmHg), when PH was defined as TR Vmax > or = 2.8 m/s, i.e. a peak pressure gradient > 30 mmHg. Hence, in dogs, increasing altitude and the concomitant hypoxia result in a progressively more pronounced PH and an elevated systemic BP. Intermittent severe hypoxaemia of around 50 mmHg may not cause erythrocytosis in healthy dogs, even over a prolonged period.