Effect of hematocrit on regional oxygen delivery and extraction in an adult respiratory distress syndrome animal model

Background: The purpose of this prospective, randomized, controlled study was to investigate the effects of hematocrit (Hct) on regional oxygen delivery and extraction following induction of adult respiratory distress syndrome (ARDS) in an animal model. Methods: Animals were instrumented to monitor central venous pressure (CVP), systemic mean arterial pressure (MAP), pulmonary artery occlusion pressure (PAOP), and cardiac output (CO) and to measure blood flow in the renal, hepatic, and superior mesenteric arteries and portal vein. ARDS was induced, positive end expiratory pressure (PEEP) applied and CO was maximized with volume loading and epinephrine infusion. Data were acquired at baseline (BL) and at Hct levels ranging from 25% to 50%. Results: Systemic DO2 increased steadily and significantly with increased Hct. Systemic O2 extraction ratio (O2ER) decreased significantly with increasing Hct until a threshold value of 40%, after which further increases in Hct did not cause a statistically significant decrease in O2ER. Similarly, renal and hepatic DO2 increased and O2ER decreased in a statistical significant manner with transfusions up to a Hct of 35%. In the splanchnic circulation blood transfusions did not cause any statistically significant increase in DO2, and O2ER showed no decrease after an Hct of 35%. Systemic, renal, hepatic, and splanchnic VO2 were not affected by changes in Hct. Blood viscosity decreased from a baseline value of 2.9 ± 0.2 centipoise at a Hct of 38% to 2.3 ± 0.1 centipoise at a Hct of 25% (P <0.05). Viscosity increased progressively with increasing hematocrits and reached the value of 4.2 ± 0.2 centipoise at an Hct of 50% (P <0.05 versus Hct 30%, 35%, 40%, 45%). Conclusions: Based on the results of this non–supply-dependent animal model we conclude that a progressive increase in Hct up to 40% causes a corresponding increase in systemic DO2 associated with a decrease in O2ER. However, there is no improvement in renal, hepatic, and splanchnic DO2 and O2ER after a threshold Hct of 35%. All other factors being the same, an Hct greater than 35% may in fact cause a decrease in blood flow rate and change in blood flow characteristics as a consequence of increased blood kinematic viscosity, which may alter and compromise cellular oxygen transfer.