The adaptation of lung, chest wall and respiratory muscles during pregnancy: preparing for birth.

Abstract

AIM\nA plethora of physiologic and biochemical changes occur during normal pregnancy. The changes in the respiratory system have not been as well elucidated, since radio-imaging studies are usually not feasible. We aimed to use several non-invasive methods to characterize the adaptation of the respiratory system during the full course of pregnancy in preparation for childbirth.\n\n\nMETHODS\nEighteen otherwise healthy women (32.3±2.8 years) were recruited during early pregnancy. Spirometry, opto-electronic plethysmography and ultrasonography were used to study changes in chest wall geometry, breathing pattern, lung and thoraco-abdominal volume variations and diaphragmatic thickness in the first, second and third trimester. A group of non-pregnant women were used as controls.\n\n\nRESULTS\nDuring the course of pregnancy, we observed a reorganization of ribcage geometry, in shape but not in volume. In spite of the growing uterus, there was no lung restriction (forced vital capacity: 101±15 %predicted), but reduced ribcage expansion. Breathing frequency and diaphragmatic contribution to tidal volume and inspiratory capacity increased. In spite of the abdominal expansion, the thickness of the diaphragm was maintained (first trimester: 2.7±0.8 mm; third trimester: 2.5±0.9 mm, p=0.187), possibly indicating a length adaptive mechanism and/or a conditioning effect to compensate for the thinning effects of the growing uterus.\n\n\nCONCLUSIONS\nPregnancy preserved lung volumes, abdominal muscles and the diaphragm at the expense of rib cage muscles.