Fetal anemia caused by red cell alloimmunization can be detected noninvasively by Doppler ultrasonography on the basis of an increase in the peak velocity of systolic blood flow in the middle cerebral artery. 1,2 Although there is not a strong correlation between these two parameters when the fetus is nonanemic or mildly anemic, the correlation becomes stronger as the hemoglobin decreases. 2 In a subsequent study, we prospectively evaluated the middle cerebral artery peak systolic velocity (MCA-PSV) to detect fetuses at risk for anemia because of maternal red cell alloimmunization and determined the need for invasive testing. Invasive procedures were avoided in 90/125 patients. 3
These two previous studies primarily assessed “significant” anemia, defined as moderate and/or severe. The objective of the current study was to assess the value of MCA-PSV in predicting the actual value of fetal hemoglobin concentration in pregnancies complicated by red cell alloimmunization.
MATERIALS AND METHODS
Doppler measurement of the middle cerebral artery was performed in 18 fetuses at risk for anemia because of red cell alloimmunization before the first cordocentesis with the equipment and methods previously described. 2 The subjects were 18 consecutive patients who were referred to the centers of the collaborative group for Doppler assessment of the blood velocity in anemic fetuses. In one patient, an amniocentesis was also performed before the cordocentesis. A second patient was referred to us for possible intrauterine transfusion because the results of an amniocentesis suggested severe anemia. The MCA-PSV was within the normal reference range and did not suggest the presence of fetal anemia. The patient was counseled and declined a cordocentesis at that time. Cordocentesis was performed 4 weeks later when the results of biweekly MCA-PSV determinations suggested anemia. The fetal hemoglobin was 7.4 g/dL (hematocrit 22%).
The length of the study enrollment period was 2 years. The types and distribution of antibodies found in our study population are shown in Table 1. All the patients gave oral consent for this study. Five patients have been previously reported.
We used three formulas obtained in our previous studies 1,2 to develop a methodology that could estimate the values of hemoglobin based on the results of gestational age in weeks and the value of MCA-PSV expressed in cm/s.
One formula 1 represented the correlation between the gestational age in weeks and the MCA-PSV (R2 = 0.78, P < .01):
A second formula 2 represented the correlation between fetal hemoglobin and gestational age (R2 = 0.34, P < .01):
The third formula 2 was the correlation between fetal hemoglobin and MCA-PSV (R2 = 0.74, P < .01):
Hemoglobin and MCA-PSV were expressed as MoM to adjust the changes that both parameters have with gestational age. The MoM were calculated by dividing the measured value by the expected value for gestational age.
Anemia was defined as mild (hemoglobin < 0.84 MoM for a given gestational age), moderate (hemoglobin < 0.65 MoM), and severe (hemoglobin < 0.55 MoM). 2
To calculate the expected hemoglobin, the values of both the MCA-PSV and the gestational age were entered in Microsoft Excel. Based on the three formulas reported above, the following information was then obtained:
- MCA-PSV MoM.
- Median value of hemoglobin for gestational age.
- Hemoglobin concentration expected for the fetus.
The study was designed to determine in each fetus the percentage difference between the expected and the observed hemoglobin values. Percentage difference is equal to absolute value of the difference between the expected and the observed hemoglobin values divided by the observed values. Percentage difference was selected because the fetal hemoglobin changes with advancing gestation 2 (eg, a difference of 1 g of hemoglobin has a different significance at different gestational ages).
The relationship between hemoglobin and the percentage difference between the expected and the observed hemoglobin values was assessed by regression analysis. The best regression line was selected based on the following factors: 1) t values of the coefficients statistically different from zero; 2) coefficient of correlation (R); and 3) residuals well distributed and around zero. For each patient, only the hemoglobin value obtained at the first cordocentesis was used for the analysis.
Statistical analyses were performed using the SPSS statistical package (Statistical Package for Social Sciences, SPSS, Inc., Chicago, IL). A P value of <.05 was selected to indicate statistical significance. Normality was evaluated by Kolgomorow-Smirnof test.
Gestational age ranged from 19 to 31 weeks. All the women were multigravidas. The fetuses were appropriately grown, and there were no maternal complications. The fetuses were nonanemic on three occasions and anemic on 15 occasions. Five fetuses were mildly anemic, three fetuses were moderately anemic, and seven fetuses were severely anemic. 2 The individual data are reported in Table 2. The values of the percentage of the error of estimation of hemoglobin were normally distributed (P >.05). In a previous study, we have shown that the hemoglobin MoM was normally distributed. 3
A quadratic relationship was found between the hemoglobin MoM (x) and percentage difference between the estimated and observed hemoglobin values (y) (Figure 1):
As the hemoglobin MoM decreased, the percentage difference between the expected and observed values significantly decreased (R2 = 0.48, P < .05). Therefore, the prediction became more accurate when the anemia was more severe. In fetuses with either moderate or severe anemia, the largest error between expected and observed values was in case 3 (percentage difference 36%). In nonanemic or mildly anemic fetuses, the largest error was in case 17 (percentage difference 49%).
In the United States, the most frequent cause of fetal hemolytic disease is red blood cell alloimmunization. 4–6 The primary cause of maternal alloimmunization is maternal sensitization to the D antigen of the rhesus blood group system. 5 However, more than 50 red blood cell antigens, including irregular antigens, have been implicated in hemolytic disease of the newborn. 6 Alloimmunization to irregular antigens is usually caused by incompatible red blood cell transfusions because blood is routinely cross-matched only for the rhesus and ABO groups. The incidence of maternal alloimmunization to potentially clinically significant antigens has been estimated to be 25 per 10,000 live births. 7 This value is similar to the incidence reported in other countries. 8,9 These antigens will continue to be a problem because prophylactic immune globulins are not available to prevent any type other than anti-D.
Amniocentesis has been used for the detection of fetal anemia secondary to red blood cell alloimmunization for the last 40 years. Cordocentesis has been used for diagnosis and management of fetal anemia for the last 16 years. These two procedures are invasive and associated with several risks for the fetus.
Liley's curve was designed to assess the amniotic fluid optical density (OD) change at 450 nm (Delta OD450) starting at 27 weeks' gestation. 10 There are no reliable data concerning the optimal frequency for repeated sampling. Attempts to extrapolate the results before 27 weeks' gestation have been less successful. 11 Therefore, other curves have been developed. The most known is the Queenan's curve. 12 Scott and Chan used the Delta OD450 before 27 weeks and found it to be quite effective. 13 Rahman et al used single determinations of Delta OD450 before 27 weeks with less good results. 14 One limitation of the amniocentesis is that its usefulness in Kell alloimmunization has been questioned. 15,16 However, neither the MCA nor the Delta OD450 curve differentiates unaffected fetuses from mild disease. Thus, there still may be utility in amniotic fluid rhesus DNA typing.
Although cordocentesis accurately reflects the fetal hematologic status, it can be associated with complications such as infection, bleeding, fetal bradycardia, premature rupture of the membranes, and death. 17,18 If an initial sample does not demonstrate anemia, the timing of repeat cordocentesis is arbitrarily determined. Furthermore, cordocentesis and amniocentesis may be associated with a worsening of the maternal alloimmunization. 19,20
In an earlier study, we reported that based on traditional criteria, 70% of the fetuses which undergo a cordocentesis are either nonanemic or mildly anemic and, therefore, the procedure could have been avoided or delayed. 2 In a subsequent prospective study, the need for cordocentesis was based on the results of the MCA-PSV. An invasive procedure was avoided in most of the patients. 3 With appropriate training, Doppler assessment of the MCA-PSV may represent an alternative to amniocentesis and cordocentesis for timing the need of an intrauterine transfusion either in fetuses never transfused or previously transfused. 21 This would avoid unnecessary procedures and spare the fetus from many potential complications.
We have previously defined anemia as mild, moderate, and severe. 2 In the current study, we estimated the actual value of hemoglobin concentration using a cubic mathematical function between hemoglobin MoM and MCA-PSV MoM. Our results show that when the fetus is nonanemic or mildly anemic, the error between expected and observed hemoglobin concentrations can be large. When anemia becomes more severe, the error between expected and observed values becomes smaller.
In anemic fetuses, change in hematocrit lead to a corresponding alteration in blood viscosity and to an impaired release of oxygen to the tissues. Increased cardiac output and vasodilatation are the main mechanisms by which the fetus attempts to maintain the oxygen and metabolic equilibrium in various organs. 22,23 It is likely that when the fetus is nonanemic or mildly anemic, there are only minor or insignificant hemodynamic changes. Therefore, the blood velocity does not change. When the fetus becomes more anemic, various mechanisms compensate to maintain the oxygen and metabolic equilibrium in the various organs. The MCA-PSV changes proportionally to the hemoglobin deficiency.
In conclusion, Doppler measurements appear to be valuable for estimating hemoglobin concentration in fetuses at risk for anemia. The correlation between the hemoglobin and the blood velocity becomes more accurate as the severity of anemia increases. This furthers our knowledge of the correlation between fetal hemoglobin and velocity of blood flow. This technique may allow for the noninvasive determination of fetal hemoglobin and timing of transfusion. This would decrease the need for cordocentesis and its potential risks.
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© 2002 The American College of Obstetricians and Gynecologists
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