Hemorrhagic tolerance of rats at sea level after acute exposure to high altitude.

Exposure of rats to a simulated altitude of 5,000 m for 1 and 3 d caused increases in hematocrit (Hct) and in hemoglobin (Hb) concentration and a decrease in total plasma volume (TPV) in comparison with sea level control animals. Total blood volume (TBV) was decreased after 1 d of exposure to altitude but returned to normal in 3 d of altitude exposure. The sea level hemorrhagic tolerance was measured in all animal groups by determining the bleeding volume which resulted in death following cannulation under anesthesia. This was recorded as a bleeding volume index (BVI), the total volume of blood lost per 100 g of body weight. The mean BVI decreased in 1-d altitude rats, but not in 3-d altitude rats. Changes in mean arterial blood pressure and Hct during bleeding were recorded. The decreased hemorrhagic tolerance was suggested to be due in part to a decreased TBV and to a loss in arterial blood pressure (BP) regulatory capability after severe hemorrhage. The results of the present study for acutely altitude-exposed rats are opposite to those of a previous study for chronically altitude-exposed rats in which BVI, TBV, and the ability for arterial BP regulation were increased.