Exposure to Hypoxia Results in Uneven Pulmonary Blood Flow Distribution Prior to Pulmonary Edema

The effects of hypoxia on pulmonary arterial pressure (PAP) and on development of pulmonary edema, ascertained by changes in lung water and pulmonary vascular permeability were studied in rats using bronchoalveolar lavage (BAL). Rats were exposed to hypobaric hypoxia (PB=290 Torr) for 24h followed by 4h of normobaric hypoxia (FiO2 0.07) (Hx). Controls were rats maintained in a normoxia (Nx). Mean PAP was 28.3 ± 0.8 mmHg in Hx, and 18.8 ± 1.7 mmHg in Nx (mean ± SD). The wet-to-dry lung weight ratio was significantly higher in Hx. The ratio of fluorescence activity between BAL fluid and plasma 4h after i.v. injection of FITC-albumin was higher in Hx, suggesting an increased pulmonary microvascular permeability in Hx. In a separate study, pulmonary blood flow distribution, measured after 10 min of hypoxia (FiO2 0.07) using non-radioactive microspheres, was significantly more heterogeneous than Nx, suggesting a non-homogeneous hypoxic pulmonary vasoconstriction. The combined data of both studies suggest that hypoxia induces heterogeneous pulmonary blood flow distribution which is followed by increased vascular permeability and the development of pulmonary edema.