Infants born with meconium aspiration syndrome are at increased risk of fetal hypoxia, evidenced by increased rates of abnormalities indicated by fetal monitoring in labor,1 low neonatal Apgar scores,2 and fetal deaths.3 One of the consequences of chronic fetal hypoxia is increased erythropoiesis caused by erythropoietin stimulation.4 Little information exists on hematologic status of infants with meconium aspiration. This study tested the hypothesis that the absolute nucleated red blood cell (RBC) count is elevated in infants with symptomatic meconium aspiration compared with infants with asymptomatic meconium aspiration or controls.
Materials and Methods
We prospectively studied three groups of term infants, 38–41 weeks' gestation by last menstrual period, confirmed by early (less than 20 weeks) ultrasonographic assessment, who were born vaginally at the Lis Maternity Hospital, Tel Aviv Sourasky Medical Center between May 1998 and July 31, 1999. The first group consisted of 11 infants who had meconium aspiration with respiratory symptoms, defined as oxygen requirement of at least 40% for more than 2 hours or mechanical ventilation for more than 2 hours, confirmed by a chest x-ray, compatible with meconium aspiration syndrome. The second group consisted of 45 infants who had asymptomatic meconium aspiration (no respiratory symptomatology, but presence of meconium below the cord during endotracheal suctioning). The third group consisted of 32 healthy control infants (no meconium-stained amniotic fluid [AF]).
Our delivery room policy is to suction the tracheas of infants with meconium-stained AF unless staining is thin and there is no neonatal respiratory depression. The resuscitation team included a neonatal nurse, a midwife, and a pediatrician or neonatologist fully trained and experienced in neonatal resuscitation. Appropriateness for gestational age was determined by using the fetal growth curves of Lubchenco et al.5 We excluded infants likely to have elevated nucleated RBC counts, such as small and large for gestational age infants; infants born to women with gestational diabetes diagnosed by abnormal glucose challenge screens at 24–28 weeks' gestation6 and confirmed by an abnormal 3-hour oral glucose challenge test7,8; infants born to women with pregnancy-induced hypertension9; placental abruption or placenta previa9; maternal heart, kidney, lung, or other chronic condition; maternal smoking,10 drug, or alcohol abuse; and perinatal infection (eg, fever, leukocytosis, or signs of chorioamnionitis). We also excluded infants with perinatal blood loss, hemolysis (ABO or other blood group incompatibility with positive Coombs test),9 and chromosomal anomalies. After each infant with meconium aspiration, the first suitable vaginally born infant was selected as a control. All eligible subjects were included.
Venous blood samples for complete blood count were collected from every infant within 12 hours of birth and analyzed according to laboratory routine using an STK-S counter (Coulter Counter Corporation, Hialeah, FL). Differential cell counts were done manually, and nucleated RBC counts were expressed per 100 white blood cells (WBC). We expressed the number of nucleated RBC as an absolute number rather than per 100 WBC, and the WBC count was expressed as corrected for presence of nucleated RBC. We showed that WBC count and absolute nucleated RBC count are not independent; thus, traditional expression of nucleated RBC as their number per 100 WBC might introduce a significant error.11
The study was approved by our local institutional review board. All infants were screened routinely for polycythemia with complete blood count at the age of 1–12 hours, so the requirement for informed consent was waived.
Data are reported as mean ± standard deviation (SD) or median and range. Statistical analysis (Minitab Inc., State College, PA) included Kruskal-Wallis test because of non-normal distribution of absolute nucleated RBC and Apgar scores (normality was tested by the Ryan Joiner test for normality12), stepwise backward regression analysis using two dummy variables for the group, and analysis of variance for comparisons of means followed by Tukey test for pairwise corrections. P < .05 was considered statistically significant.
Demographic data are depicted in Table 1. During the study there were 2864 deliveries in our hospital and infants were recruited consecutively. Two of 11 infants with meconium aspiration syndrome required mechanical ventilation and had evidence of right-to-left shunting at the foramen ovale or at the ductal level by echocardiography, allowing for diagnoses of persistent pulmonary hypertension. There were no significant differences among groups in gestational age; gravidity; parity; maternal analgesia; age at sampling time; lymphocyte, platelet, and granulocyte counts; and hematocrit. The median nucleated RBC count was significantly higher in the symptomatic meconium aspiration group than in the other two groups. The median 1-minute Apgar score was significantly lower in the symptomatic meconium aspiration group than in the other two groups. In stepwise backward regression analysis, taking into account gestational age, Apgar scores, and group as independent variables, and nucleated RBC count as the dependent variable, only group was significantly influential on the nucleated RBC count (R2 = 0.102, P < .05).
This study showed that, as hypothesized, infants with meconium aspiration syndrome with respiratory symptomatology had higher absolute nucleated RBC counts than infants with asymptomatic meconium aspiration and controls. Those findings support the theory that infants with significant meconium aspiration syndrome suffered from fetal hypoxia. A similar elevation of nucleated RBCs was documented in situations in which compensatory increases in erythropoiesis resulted from chronic fetal hypoxia, such as maternal diabetes,13 pregnancy-induced hypertension,14 or fetal growth restriction.15 The time it takes for nucleated RBC to increase after acute hypoxic stress is not known, but studies in lambs showed that within hours fetal hypoxia stimulates fetal erythropoietin production,13 which in turn stimulates erythropoiesis.
In the asymptomatic meconium aspiration group, nucleated RBC count was not significantly different from that of controls. We believe that infants without nonreassuring fetal status represent a subgroup of meconium aspiration in which there was no intrauterine hypoxic environment, unlike the symptomatic group. If we are correct, it might mean that respiratory symptoms are related more to pulmonary hypertension than to meconium. The easy assumption that the respiratory distress is caused by meconium in the airway is belied by the variable results reported with tracheal suctioning.16 It is also belied by the failure to establish a clear link between meconium in the airway and pulmonary hypertension, which accounts for most deaths among infants with meconium aspiration syndrome.17–19 Fetal hypoxia stimulates fetal evacuation of meconium,19 but most of the 5–20% of infants born with meconium-stained AF do not develop respiratory symptoms,20 although approximately half had meconium suctioned out of their trachea.20
It could be argued that in symptomatic infants with lower Apgar scores, it was the condition resulting in lower scores rather than meconium aspiration that led to increased nucleated RBC counts. We do not believe this because an acute hypoxic event is not likely to trigger erythropoietin production (a process that takes hours to develop21) and subsequent increase in erythropoiesis sufficient to elevate nucleated RBC counts, and in stepwise backward regression analysis taking into account gestational age, Apgar scores, and group as independent variables and nucleated RBC as dependent variable, only group remained significant. It is more likely that symptomatic meconium aspiration, accompanied by lower Apgar scores and elevated nucleated RBC counts, were all markers for intrauterine hypoxia that led to varied degrees of pulmonary hypertension.
It is possible theoretically that some infants with significant intrauterine hypoxia delivered by cesarean would have had very high nucleated RBC counts. Despite their exclusion, we still found significant differences among groups.
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