High-altitude effects on respiratory gases, acid-base balance and pulmonary artery pressures in equids

Arterial and venous blood were analysed at rest and post exercise for pH, PCO2, and PO2, and bicarbonate ([HCO3-]), base excess (BE), and strong ion difference (SID) were calculated in response to a 10 day sojourn to 3800 m. Pulmonary artery pressures (PAP) were measured at rest. Post exercise samples were restricted to venous blood. The equids (n = 6) experienced a profound hypoxia-hypocapnia and a respiratory alkalosis. PaO2 decreased 42% and PaCO2 41%. PaCO2 increased to 80% of initial values after 8 days at altitude. Arterial [HCO3-] decreased by 34%; however, it returned to normal by Day 4. Base excess decreased initially, but increased at altitude with time. Strong ion difference was decreased during the altitude exposure and continued to be depressed even after return to low altitude. Pulmonary artery pressure increased 63% on Day 1 of exposure (from 27.9 +/- 2 to 45.4 +/- 3 mmHg); Days 2 and 6 averaged 36.3 +/- 3 and 37.5 +/- 3 mmHg. Thirty-six hours after return to 225 m, most variables (except [SID] and post exercise BE) returned to normal. The most profound changes in the indicators of gas exchange, at altitude, occurred during the first 3 days and only [HCO3-] returned to normal during the subsequent acclimatization to altitude.