Hemodilution with oxyhemoglobin. Mechanism of oxygen delivery and its superaugmentation with a nitric oxide donor...

Hemodilution (HD) with oxyhemoglobin colloid (oxyHb) provides a greater arterial oxygen content (CaO2) than HD with conventional colloids; however, oxygen delivery (DO2) is essentially the same, because, in contrast to conventional HD, cardiac output (CO) is not augmented. This study seeks to elucidate the mechanism that limits CO during oxyHb-HD and to test whether infusion of a nitric oxide (NO) donor would augment DO2, because oxyHb is known to inactivate in vitro endothelial-derived NO.

Anesthetized dogs were isovolemically hemodiluted with 10% oxyHb, 8% albumin, or 10% methemoglobin (weak NO inactivator) to 20% hematocrit. After HD, sodium nitroprusside (SNP) was titrated intravenously until decreases (> 10 mmHg) in mean aortic pressure (Pao) indicated the presence of exogenous NO. Systemic hemodynamics and regional blood flows (microsphere method) were measured.

Albumin-HD and metHb-HD produced typical HD-mediated responses: increased CO (63-65%), slight decreases (13-15%) in DO2, decreases in systemic vascular resistance (SVR) proportional to the decreases (49-52%) in blood viscosity of all three groups, and increased regional blood flows (RBF). Responses to oxyHb-HD were atypical: CO and its determinants were not changed, DO2 decreased (23%) proportional to CaO2, and SVR and most RBF were not changed except for a net redistribution of CO to myocardium and skeletal muscle. In albumin-HD or metHb-HD, SNP (2-5 micrograms.kg-1.min-1) induced comparable decreases in mean Pao (29-37%) and SVR (39-41%); however, CO, RBF, and DO2 were not affected. In oxyHb-HD, exceptionally large doses of SNP (54 +/- 5 micrograms.kg-1.min-1) decreased mean Pao only 19 +/- 1%; however, CO increased 78 +/- 5% and decreases (61 +/- 3%) in SVR were slightly greater than viscosity reductions. Other determinants of CO were not affected. Most RBF increased proportional to CO; there was, however, preferential distribution to myocardium and skeletal muscle. Consequently, the augmented CO, and CaO2 of oxyHb-HD, produced large increases in DO2, 77 +/- 5% from HD alone and 43 +/- 3% from prehemodilution values.

This study indicates that the limited CO and DO2 of oxyHb-HD resulted from opposing changes in two determinants of flow, i.e., reduced blood viscosity and increased arterial resistance (vasoconstriction). The vasoconstriction was not evident with metHb-HD and was reversed by the SNP infusion, indicating that oxyHb inactivated in vivo endothelial-derived NO. The ability of the NO donor (SNP) to facilitate large viscosity-mediated increases in DO2 during oxyHb-HD is an important finding that could potentially render oxyHb colloids more useful than conventional colloids, particularly for the individual with a compromised circulation who would benefit from an increased oxygen supply.