Venous and Capillary Hematocrit in Newborn Infants and Placental Transfusion

Hematocrit values were measured on one hundred and eleven sets of capillary (from unwarmed and warmed heels) and venous blood samples obtained simultaneously during the first 5 days of life from 60 full term newborn infants, 40 of which the umbilical cords were clamped late, and 20 clamped early a t the time of birth. In the late clamped infants, the capillary hematocrits showed an initial rise during the first 6 hours of life seemingly due to fluid transudation in the capillary beds, followed by a fall a t 12 to 24 hours of age due to a subsequent fluid reabsorption into the vascular space in response to increasing circulatory demands in the visceral organs. In the early clamped infants, the capillary hematocrits remained stable during the first 6 hours, but a slight decline was observed a t 12 to 24 hours of age. The simultaneously measured venous hematocrits of both late and early clamped infants plotted against age revealed a strikingly similar pattern of alterations. A marked capillary venous hematocrit difference was observed in the late clamped infants and to a much lesser extent in the early clamped infants during the first 5 days of life, with the venous being lower than the capillary values. Warming the heels prior to capillary sampling improves the capillary venous hematocrit correlations in the late clamped infants and the improvement achieved by this procedure increases as the infant becomes older. In the early clamped infants heel warming produces relatively less effects because there was less capillary venous hematocrit discrepancy initially. In infants over 12 hours of age where venipuncture is difficult or inadvisable, capillary blood samples obtained from warmed heels could be used for hematocrit measurements. However, the hematoples do not exactly correspond with the venous values and the approximate venous reading could be estimated by using the regression lines derived from our samples, and their 95% confidence limit could be calculated from the appropriate formulas.