BIRTH RELATED CHANGES IN THE INTERACTION BETWEEN THE RIGHT VENTRICLE AND THE PULMONARY VASCULATURE
D.A. Grant1, J-C. Fauchère1, J.J. Wang2, J.V. Tyberg2, A.M. Walker1, 1 Ritchie Centre for Baby Health Research, Monash Institute of Reproduction and Development, Monash University, 246 Clayton Road, Clayton, Melbourne, Victoria, 3168, 2 The Departments of Physiology & Biophysics and Medicine, University of Calgary, 3330 Hospital Drive, N.W., Calgary, Alberta, Canada, T2N 4N.
The mid-systolic plateau that differentiates fetal pulmonary trunk blood flow (Qpa) from that of the adult, arises when pressure-velocity waves (Forward-travelling Compression Waves, FCW) generated by right ventricle (RV) are reflected from high impedance sites in the pulmonary circulation1. These reflected waves travel back to the RV as Backward-travelling Compression Waves (BCW), increase pulmonary artery pressure (Ppa) and decrease Qpa. Intriguingly, BCW's are normally absent in the adult. Our study sought to determine if birth eliminates the BCW. Pregnant ewes (141 days gestation, n=6) were anaesthetised (induction 5 mg/kg ketamine and 100 mg/kg α-chloralose, maintenance 25 mg/kg/hr α-chloralose) and their lambs partially delivered by C-section. Qpa and Ppa were recorded before and after interrupting the umbilical circulation and ventilating the fetus for one hour. Ventilation decreased Ppa (61 ± 5 vs 39 ± 3 mmHg, Fetus vs Newborn, Mean ± SEM, P<0.05, paired t-test), increased PaO2 (23 ± 2 vs 116 ± 17 mmHg, P<0.05) and decreased PaCO2 (55 ± 4 vs 34 ± 2 mmHg, P<0.05). Wave intensity was calculated as the product of incremental differences in both pressure and velocity over 5 ms intervals for 6 cardiac cycles before and after ventilation. Two positive peaks of wave intensity were observed, a FCW early in systole and a forward-travelling expansion wave late in systole. Although a mid-systolic BCW was observed in both the fetus and the newborn, its magnitude (relative to the the FCW) was significantly reduced following ventilation (15.2% ± 2%, vs 6.0% ± 2%, P<0.05). We propose that the reduction in wave reflection that occurs within the pulmonary circulation at birth is an essential adaptation that facilitates the low-pressure high-flow characteristics of the adult pulmonary circulation and produces the characteristic postnatal profile in Qpa.
(1) Grant DA, Hollander E, Skuza E, Fauchère, JC. Applied Physiology. 1999;87:1637-1643.
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