Whole-blood viscosity in the neonate: effects of gestational age, hematocrit, mean corpuscular volume and umbilical cord milking.

OBJECTIVE: The American College of Obstetrics and Gynecology Committee on Obstetric Practice recently endorsed delayed cord clamping at preterm delivery. However, the committee report expressed the concern by some practitioners that delayed clamping or cord milking might induce hyperviscosity in preterm neonates. To address this issue we: (1) established reference ranges for whole-blood viscosity among preterm neonates (viscosity reference ranges had previously been reported only in term neonates) and (2) determined the effect of umbilical cord milking at deliveries <32 weeks gestation on subsequent blood viscosity measurements.

STUDY DESIGN: This was a prospective study in two Neonatal Intensive Care Units. Blood viscosity was measured using a cone and plate viscometer. Associations were sought with gestation, hematocrit/hemoglobin and mean corpuscular volume. Reference ranges were determined for preterm infants <32 weeks gestation. Then, after umbilical cord milking at deliveries <32 weeks, viscosity was measured at birth and again during the 12 h after birth. In neonates with viscosities >95th % range, we sought signs of hyperviscosity (plethora, hypotonia, hypoglycemia, hyperbilirubinemia, thrombocytopenia).

RESULT: Viscosity at higher and lower sheer rates were linearly related (n=32, r=0.971). Within the range of hematocrits measured (29-63%) viscosity correlated with hematocrit (r=0.877) and hemoglobin (r=0.853) but not with erythrocyte size (r=0.179). Viscosity was related to gestational age (n=58), primarily due to the lower hematocrits at lower gestational ages. In the 12 h after cord milking viscosity ranged from 3.1 to 9.5 centipoise. Three of twenty preterm, neonates had viscosities >95th % reference range. However, all values were well below those where hyperviscosity is defined in term neonates and all lacked features of hyperviscosity.

CONCLUSION: Cord blood viscosity is directly proportional to hematocrit/hemoglobin, lower at early gestation and not associated with erythrocyte size. Cord milking at preterm delivery is associated with a low risk of clinical hyperviscosity. Practioners should not refrain from cord milking at preterm delivery because of a concern that it will commonly cause neonatal hyperviscosity.