Objectives: (1) To calculate the feto-placental volume (FPV), using the haematocrit (Ht) values and the percentages of fetal haemoglobin (HbF), before and after red blood cell transfusion. (2) To estimate the transfusion-induced loss of plasma fluid.
Design: Retrospective analysis of data of 42 anaemic fetuses at the first transfusion [gestational age (GA) 19-36 weeks].
Setting: Department of Obstetrics, Leiden University Medical Centre, The Netherlands.
Sample: Fifteen hydropic and 27 non-hydropic fetuses.
Methods: Donor blood volume (V(donor)) and Ht (Ht(donor)), fetal pre- and post-transfusion Ht values (Ht(initial), Htfinal) and percentages of HbF (HbF(initial) and HbF(final)) were used to calculate the FPV. The total red cell volume after transfusion (RCV(final)) and Ht(final) were used to estimate the plasma fluid loss.
Main outcome measures: Feto-placental blood volume and loss of plasma fluid.
Results: The equations that use Htfinal over-estimate the FPV when the formula does not account for the difference between donor and post-transfusion Ht (FPV(Ht) = 21.36 * GA, - 390; r = 0.89). FPV is underestimated (FPV(Ht) = 9.90 * GA - 172; r = 0.84) when the blood volume increases with a volume less than the added donor blood volume. The calculation of FPV, using HbF percentages and the initial fetal RCV, is independent of volume changes (FPVHbF = 15.10 * GA - 279; r = 0.85). Comparing RCV(final) and Ht(final) values showed that 31.1 +/- 14.5% of the transfused volume was lost. Results of the hydropic fetuses did not differ from those of the non-hydropic fetuses.
Conclusions: FPV values based on Ht values are less reliable than those based on RCV and HbF findings. When, for practical reasons, Ht values have to be used, we propose an adapted equation for the calculation of the necessary volume of donor blood: V(donor) = FPV(HbF) * (Ht(final) - Ht(initial) / (Ht(donor) - 0.70 * Ht(final)).