Tissue oxidative metabolism after extreme hemodilution with peg-conjugated hemoglobin

NADH-localized fluorometry was used as a noninvasive technique to monitor changes in the energy state of intact tissue (muscle and connective tissue), without anesthesia, as a function of blood plasma O₂-carrying capacity in the hamster window chamber model. Acute moderate isovolemic hemodilution was induced by two isovolemic hemodilution steps: in the first step, 6% 70-kDa dextran (Dex70) was used to induce an acute anemic state (18% Hct); in the second step, exchange transfusion of polyethylene glycol (PEG) maleimide-conjugated Hb (4 g/dl, PEG-Hb) or Dex70 (6 g/dl) was used to reduce erythrocytes to 75% of baseline (11% Hct). PEG-Hb had six copies of PEG (5 kDa) conjugated to each human Hb (0.48 g PEG/g Hb) through extension arm-facilitated chemistry. Systemic parameters, microvascular perfusion, functional capillary density, intravascular and interstitial Po₂, and intracellular NADH fluorescence were monitored. Mean arterial blood pressure after extreme hemodilution was statistically significantly reduced for Dex70 compared with PEG-Hb. The presence of PEG-Hb in the circulation maintained positive acid-base balance. While microvascular blood flows were not different, functional capillary density was significantly higher for PEG-Hb than Dex70. Arteriolar Po₂ was higher in the presence of PEG-Hb than Dex70, but tissue and venular Po₂ were not different. Cellular energy metabolism (intracellular O₂) in the tissues was improved with PEG-Hb. Moderate hemodilution to 18% Hct (6.4 g Hb/dl) brings tissue O₂ delivery to the verge of inadequacy. Extreme hemodilution to 11% Hct (3.7 g Hb/dl) produces tissue anoxia, and high-O₂-affinity PEG-Hb (Po₂ at which blood is 50% saturated with O₂ = 4 Torr, 1.1 g Hb/dl) only partially decreases anaerobic metabolism without increasing tissue Po₂.