FETAL BRANCH PULMONARY ARTERIAL VASCULAR IMPEDANCE DURING THE SECOND HALF OF PREGNANCY

OBJECTIVE: Our purpose was to establish normal physiologic parameters in the fetal proximal and distal branch pulmonary arterial vascular im pedance during the second half of pregnancy and to analyze relationshi ps between proximal and distal pulmonary arterial blood velocity wavef orms. STUDY DESIGN: In this cross-sectional study 100 uncomplicated si ngleton pregnancies were studied by pulsed color Doppler techniques be tween 18 and 41 weeks of gestation (median 30 weeks). Both right and l eft proximal (immediately after the bifurcation of the main pulmonary artery) and distal (beyond the first bifurcation of the branch pulmona ry artery) pulmonary artery blood velocity waveforms were recorded and pulsatility index values were calculated. Peak systolic velocities an d time-to-peak-velocity intervals were measured. Time-to-peak-velocity intervals were also analyzed at the level of aortic and pulmonary val ves and at the ductus arteriosus. Right and left pulmonary artery; dia meters and right lung length were measured. RESULTS: In both right and left proximal and distal pulmonary arteries pulsatility index values decreased (p < 0.0001) and the peak systolic velocities (p < 0.003) an d time-to-peak-velocity intervals (p < 0.0001) increased during the se cond half of pregnancy. In the proximal pulmonary arteries the pulsati lity index values decreased linearly until 34 to 35 weeks of gestation and in the distal pulmonary arteries until 31 weeks of gestation. The reafter they remained unchanged. In pulmonary arteries time-to-peak-ve locity intervals were shorter (p < 0.01) than at the pulmonary valve l evel. There were no significant differences between the right or left pulmonary arteries in the pulsatility index values, peak systolic velo cities, time-to-peak-velocity intervals, or pulmonary artery diameters . In the proximal pulmonary arteries the pulsatility index values (p < 0.02) and peak systolic velocities (p < 0.0001) were higher and time- to-peak-velocity intervals (p < 0.0001) were longer than in the distal pulmonary arteries. There was a 2.5-fold increase in pulmonary artery diameters and right lung length. CONCLUSIONS: Fetal branch pulmonary arterial vascular impedance decreases significantly during the second half of pregnancy. The linear decrease in vascular impedance during th e second trimester and in the beginning of the third trimester may be related to the growth of the lung and the increase in the number of re sistance vessels. During the latter part of the third trimester pulmon ary vascular impedance does not decrease further.