Reference Ranges of Blood Flow in the Major Vessels of the Normal Human Fetal Circulation at Term by Phase Contrast Magnetic Resonance Imaging
Background—Phase contrast MRI with metric optimized gating is a promising new technique for studying the distribution of the fetal circulation. However, mean and reference ranges for blood flow measurements made in the major fetal vessels using this technique have yet to be established.
Methods and Results—We measured flow in the major vessels of the fetal circulation in 40 late gestation normal human fetuses using phase contrast MRI (mean gestational age 37 weeks, SD 1.1 weeks). Flows were indexed to the fetal weight, which was estimated from the fetal volume calculated by MRI segmentation. The following mean flows in ml/min/kg (± 2 SD) were obtained: combined ventricular output 465 (351,579), main pulmonary artery 261 (169,353), ascending aorta 191 (121,261), superior vena cava 137 (77,197), ductus arteriosus 187 (109,265), descending aorta 252 (160,344), pulmonary blood flow 77 (0,160), umbilical vein 134 (62,206), foramen ovale 135 (37,233). Expressed as percentages of the combined ventricular output, the mean flows ± 2 SD were as follows: MPA 56 (44,68), AAo 41 (29,53), SVC 29 (15,43), DA 41 (25,57), DAo 55 (35,75), PBF 16 (0,34), UV 29 (11,47), FO 29 (7,51). A strong inverse relationship between FO shunt and PBF was noted (r = -0.64, p = <0.0001).
Conclusions—Although too small a sample size to provide normal ranges, these results are in keeping with those predicted in humans based on measurements made in fetal lambs using radioactive microspheres and provide preliminary reference ranges for the late gestation human fetus. The wide range we found in FO shunting suggests a degree of variability in the way blood is streamed through the fetal circulation.
- Received October 16, 2013.
- Accepted May 23, 2014.