Oxygen transport dynamics of acellular hemoglobin solutions in an isovolemic hemodilution model in swine

One of the perceived advantages of a hemoglobin-based blood substitute is the provision of oxygen-carrying capacity. Although the hemodynamic response to the infusion of acellular hemoglobin solutions has been extensively studied, less is known about the oxygen transport dynamics of such solutions. We hypothesized that acellular hemoglobin solutions are useful oxygen carriers in anemic states and that higher P⁵⁰ solutions transport O² more efficiently than low P⁵⁰ solutions. We sought to quantify O² transport dynamics of hemoglobin solutions in an isovolemic hemodilution model in swine. Eighteen swine were anesthetized, ventilated, and instrumented for hemodynamic measurements and for withdrawal of arterial and mixed venous blood. The swine were randomized into three groups and progressively bled from an initial hematocrit (Hct) of 30% to Hcts of 19%, 13%, and 8% using isovolemic exchange with pyridoxalated hemoglobin polyoxyethylene conjugate (PHP, n = 6); an identical hemoglobin solution without the pyridoxalation, resulting in a low P⁵⁰ solution (POE-Hb, n = 6); or an osmotically similar control solution of pentastarch (PS, n = 6). Hemodynamic measurements, arterial and mixed venous O² content, and O² extraction ratio (ER), were determined in whole blood (WB), the red blood cell (RBC) phase, and the plasma phase utilizing a compartmentalized approach. Mean pulmonary arterial pressure was higher with hemodilution in the PHP and POE-Hb groups than in the PS group (p < 0.05). Cardiac index increased with hemodilution in all groups, but was significantly less than the cardiac index in the PS group at Hcts = 19% and 13%. Oxygen delivery and consumption were maintained at all hematocrits at baseline levels in the PHP and POE-Hb groups, but O² delivery was significantly decreased in the PS group at Hct = 8% (p < 0.05 for PS vs. baseline and p < 0.05 for PHP and POE-Hb vs. PS). Oxygen extraction ratio increased with progressive hemodilution in both the RBC hemoglobin and plasma phases to a maximum of 39% for PHP and 36% for POE-Hb at Hct = 8%. The percent contribution from the plasma phase to total oxygen delivery and consumption likewise increased with hemodilution to maximum values of 52.7% (PHP) and 68.2% (POE-Hb) for delivery and 40.9% (PHP) and 39.3% (POE-Hb) for consumption. Acellular hemoglobin solutions provide a significant contribution to O² delivery and consumption, particularly in severe anemia, in this model of isovolemic exchange. The differences in the P⁵⁰ of the two hemoglobin solutions do not appear to significantly influence oxygen dynamics over the range of hematocrits studied.