Reference values for an index of fetal aortic isthmus blood flow during the second half of pregnancy

Abstract

Objective: During fetal life, the parallel position of the two cardiac ventricles confers a special status to the aortic isthmus. Flow through the isthmus reflects the balance between the performances of the two ventricles and their respective peripheral impedances. This study proposes a fetal aortic isthmus flow velocity index and its reference values defined on the basis of gestational age (GA). Methods: Video recordings of 111 normal fetuses from 18 to 39 weeks of gestation were retrospectively reviewed. An isthmus flow velocity index (IFI) was calculated as follows: IFI = (systolic + diastolic)/systolic velocity integrals. GA-specific reference ranges of IFI were constructed. Results: An IFI of 1.33 ± 0.03 was found at 18 weeks. This value decreased slightly but steadily with GA to reach 1.23 ± 0.16 at 39 weeks. This change is mainly related to a decrease in diastolic velocity integrals. Conclusion: The proposed IFI provides information on the direction and, indirectly, on the volume of blood flow through the fetal aortic isthmus. Copyright © 2003 ISUOG. Published by John Wiley & Sons, Ltd.