ABSTRACT
The fetus has a number of distinctive features that include a low arterial PO2 (30 Torr), parallel ventricular function with increased combined cardiac output, umbilical circulation, and the placenta, which is the organ of exchange for
respiratory gases, nutrients, and waste products. The pulmonary vascular resis-
tance is very high, which necessitates a direct conduit (i.e., ductus arteriosus)
from the right ventricular outflow tract to the aorta. Fetal breathing movements,
which start long before birth, contribute to normal lung growth and differenti-
ation, respiratory muscle development, and establishment of neural pathways
controlling respiration (1-4). Fetal breathing has unique features that include the
requirement for a physiological environment (intrauterine or exteriorized in a
warm saline bath) and the dominating control by sleep and behavioral states.
