Background: Measurement of blood volume is essential for understanding physiological processes underlying cardiovascular dysfunction in neonates. Published reports suggest that blood volume in term neonates is higher (80mL/kg) than in adults (65-75mL/kg) and higher again in preterm neonates (85-110mL/kg). However, many of these studies have used tracers that leak from the vasculature and thus may over-estimate blood volume. Alternatively, volume measurements that use labelled-RBCs are labour intensive. The aim was to develop a safe, accurate and economical method for measuring blood volume in preterm piglets.
Methods: Blood volume was measured in both term and preterm piglets (97/115d gestation) using fluorescently-labelled dextrans of two sizes (70kDa and 500kDa). Repeated blood samples (at 5 min intervals) following dextran administration allowed back calculation to time zero.
Results: In term piglets, blood volume measurements were 17% higher when using 70kDa dextrans compared to 500kDa (90±15 mL/kg vs 77±11mL/kg). In preterm piglets at 3-5h after birth, blood volume was found to be 22% higher when using 70kDa dextrans compared to 500kDa (74±7 mL/kg vs 61±8mL/kg). At 11h after birth, blood volume measured using 500kDa dextrans was 59±2 mL/kg in preterm piglets, 23% lower than in term animals.
Conclusion: The difference between blood volumes estimated using 70kDa and 500kDa was greater in preterm piglets than term piglets, possibly due to increased capillary permeability. Using a tracer that is too large to rapidly leak from the vasculature (500kDa) improves the accuracy of blood volume estimation in preterm neonates. We found that blood volume in preterm piglets was lower, not higher, than that in term piglets. It is possible that previous reports of blood volume in preterm neonates have over-estimated volume, and that many preterm neonates have a low blood volume which is likely to contribute to any cardiovascular instability.