Fetal aortic isthmus blood flow and the fraction of cardiac output distributed to the upper body and brain at 11-20 weeks of gestation

Objective To measure serial changes in fetal aortic isthmus (AI) blood flow and estimate the fraction of fetal cardiac output distributed to the upper body, including the brain, at 11-20 weeks of gestation. Methods Using pulsed-wave Doppler and two-dimensional ultrasound, blood flow velocities and inner diameter of the AI, aortic valve (AV) and pulmonary valve (PV) were measured longitudinally in 143 fetuses and volume blood flows (Q) were calculated for each site using the formula: Q (mL/min) = pi x (diameter/2)(2) x velocity time integral x heart rate x 60. The sum of Q(av) and Q(pv) constituted the combined cardiac output (CCO) and the fraction (%) of the upper body (including brain) blood flow was calculated as: (Q(av) - Q(ai)) x 100/CCO. Results AI blood velocities as well as the vessel diameter increased with advancing gestation, resulting in a significant increase in Q(ai) from 1.9 to 40.5 mL/min during weeks 11 to 20. The AI peak systolic velocity increased from 29 to 63 cm/s, end-diastolic velocity from 1.2 to 5.2 cm/s, and the time-averaged maximum velocity from 11 to 22 cm/s, resulting in a fairly stable pulsatility index (PI) of 2.4-2.6 and resistance index (RI) of 0.91-0.94. On average, 75% of blood ejected by the left ventricle (which represented about 35% of the CCO) passed through the AI to the descending aorta. The fraction of CCO distributed to the upper body, including the brain, was estimated as approximately 13%. Conclusion We have established longitudinal reference ranges for fetal AI diameter, blood flow velocities, PI, RI and volume blood flow at 11-20 weeks of gestation. The human fetus appears to direct a relatively small fraction (13 %) of its CCO to the upper body, including the brain, during this period of pregnancy. Copyright (C) 2009 ISUOG. Published by John Wiley & Sons, Ltd.