Interstitial pressure and lung oedema in chronic hypoxia

We evaluated how the increase in lung interstitial pressure correlates with the pulmonary vascular response to chronic hypoxia. In control and hypoxic (30 days; 10% O₂) Wistar male rats, we measured: pulmonary interstitial pressure (P(ip)), cardiac and haemodynamic parameters by echocardiography, and performed lung morphometry on tissue specimens fixed in situ. In control animals, mean ± sd P(ip), air/tissue volume ratio and capillary vascularity index in the air-blood barrier were -12 ± 2.03 cmH₂O, 3.9 and 0.43, respectively. After hypoxia exposure, the corresponding values of these indices in apparently normal lung regions were 2.6 ± 1.7 cmH₂O, 3.6, and 0.5, respectively. In oedematous regions, the corresponding values were 12 ± 4 cmH₂O, 0.4 and 0.3, respectively. Furthermore, in normal regions, the density of pre-capillary vessels (diameter ~50-200 μm) increased and their thickness/internal diameter ratio decreased, while opposite results were found in oedematous regions. Pulmonary artery pressure increased in chronic hypoxia relative to the control (39.8 ± 5.9 versus 26.2 ± 2.2 mmHg). Heterogeneity in local lung vascular response contributes to developing pulmonary hypertension in chronic hypoxia. In oedematous regions, the decrease in capillary vascularity correlated with the remarkable increase in interstitial pressure and morphometry of the pre-capillary vessels suggested an increase in vascular resistance; the opposite was true in apparently normal regions.