Fetal breathing movements are associated with changes in compliance of the left ventricle

We have tested the hypothesis that the limited fetal ventricular distensibility is not only an intrinsic cardiac characteristic but is also contributed to by the pressure exerted by the intrathoracic organs. To this purpose we have studied the diastolic cardiac function by Doppler velocimetry in 11 fetuses during fetal breathing and in 22 fetuses during apnea, controlling for gestational age at examination and heart rate. Inspiration was associated a significant increase in left ventricular compliance, as measured by deceleration time (inspiration 182.6 +/- 15.5 s vs. expiration 137.1 +/- 13.1 s vs. apnea 151.7 +/- 51.8 s), and in ventricular filling, as measured by velocity time integral (inspiration 0.086 +/- 0.020 m vs. expiration 0.064 +/- 0.014 m vs. apnea 0.065 +/- 0.011 m), compared with both the expiration and apnea groups. These increases most likely reflect changes in venous return and ventricular end-diastolic volume secondary to a decrease in intrathoracic pressure during fetal breathing.