Compliance of the respiratory system in newborn and adult rats after gestation in hypoxia

Abstract

We hypothesized that hypoxia during gestation modifies the compliance of the respiratory system of newborn and adult rats. Pregnant rats were placed in a hypobaric chamber at an inspired oxygen pressure of 86 mmHg (equivalent to 12% O₂ in normobaria) from day 4 of gestation until day 2 post-partum. Three- day-old rat pups were smaller than controls, with higher hematocrit; the lungs were also small, with less protein and DNA content. The pressure (x-axis)-volume (y-axis) curve of the respiratory system was displaced to the right of the control curve, and the compliance of the respiratory system, measured on the inflation or deflation limb of the pressure-volume curve, was decreased by ∼20–25%, depending upon the normalization procedure (per body mass or per dry lung weight). In 50-day-old rats exposed to hypoxia during gestation, body weight, hematocrit, lung mass and DNA content were normal; the compliance of the respiratory system, measured at ventilation frequencies between 20 cpm and 100 cpm, was higher than in controls by ∼20%. It is concluded that the effects of prenatal hypoxia on the compliance of the respiratory system can vary with age. In the rat the process of alveolar formation initiates postnatally. Hence, in the newborn the effects of the prenatal hypoxia on the compliance of the respiratory system are likely to be dominated by the hypoxic pulmonary hypoplasia and hypertension, which decrease the compliance of the respiratory system. In the adult, the effects of the decreased alveolar formation are the prevailing ones, increasing the compliance of the respiratory system.