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Pressure-volume characteristics of lungs of rats during pregnancy and postpartum
Measurements of lung volumes in pregnant women show that the functional residual capacity and residual volume are reduced by 17-25% at late pregnancy. The present study was conducted to test the hypothesis that a decrease in FRC at pregnancy may result from an increase in retractive forces of the lung. The air and saline pressure-volume characteristics of excised lungs were studied in rats daily during pregnancy and the postpartum period. In comparison to non-pregnancy rats, the air PV measurements indicated that: (1) the retractive forces of the lungs were increased late in pregnancy resulting in reduction in both MLV (lung air volume at 40 cm H2O Ptp) and in V%10 (volume at 10/volume at 40 cm H2O Ptp X 100); (2) MLV was greater in lactating than in non-lactating and in non-pregnant rats. Lung saline volume was also greater in lactating than in non-lactating rats at early postpartum period. Lung phospholipids content was increased at late pregnancy and in lactating rats; (3) there was an increase in MLV and in V% immediately after delivery and 2-3 days after an abrupt cessation of lactation ("dried" rats). The minimal surface tension of lung lavages also decreased in these rats. A shift to that left of the air PV curve in "dried" rats suggest that under normal circumstances lung surface forces are not at their lowest.
Measurements of lung volumes in pregnant women show that the functional residual capacity and residual volume are reduced by 17-25% at late pregnancy. The present study was conducted to test the hypothesis that a decrease in FRC at pregnancy may result from an increase in retractive forces of the lung. The air and saline pressure-volume characteristics of excised lungs were studied in rats daily during pregnancy and the postpartum period. In comparison to non-pregnancy rats, the air PV measurements indicated that: (1) the retractive forces of the lungs were increased late in pregnancy resulting in reduction in both MLV (lung air volume at 40 cm H2O Ptp) and in V%10 (volume at 10/volume at 40 cm H2O Ptp X 100); (2) MLV was greater in lactating than in non-lactating and in non-pregnant rats. Lung saline volume was also greater in lactating than in non-lactating rats at early postpartum period. Lung phospholipids content was increased at late pregnancy and in lactating rats; (3) there was an increase in MLV and in V% immediately after delivery and 2-3 days after an abrupt cessation of lactation ("dried" rats). The minimal surface tension of lung lavages also decreased in these rats. A shift to that left of the air PV curve in "dried" rats suggest that under normal circumstances lung surface forces are not at their lowest.