Effects of Hemodilution and Oxygen Breathing on Gut Oxygenation in Anesthetized Dogs

Abstract

A heightened interest in perioperative strategies designed to reduce or avoid allogeneic blood transfusion has occurred as the list of potential transfusion-related complications has grown (Spahn, 1994). One such strategy, acute normovolemic hemodilution, involves the preoperative exchange of the patient’s blood with colloid or crystalloid to a predetermined hematocrit. As a result, blood lost during surgery contains less red blood cells and the patient may be transfused with his own normal hematocrit blood as needed towards the completion of the procedure (Messmer, 1986). Because hemodilution reduces arterial oxygen content, cardiac output must increase in order to maintain systemic oxygen delivery. This is facilitated by a decrease in blood viscosity with resultant lowering of vascular resistance and left ventricular afterload (Cain, 1994). Also, flow need not increase in direct proportion to the decrease in systemic hematocrit for microvascular oxygen delivery to be maintained. This is because at the capillary level, hematocrit is normally only half that of systemic hematocrit and it remains nearly constant until systemic hematocrit falls to levels below 20% (Inaglietta, 1989). Therefore, tissue oxygenation is unlikely to be affected by moderate degrees of hemodilution, a concept which has been supported by direct measurements of tissue Po₂ in various organs (Messmer, 1973). Still, the limits of hemodilution remain ill-defined. Recent data suggest that hemodilution to a hemoglobin of ≈3 g/dl may be “clinically acceptable in young, healthy patients” (Fontana, 1995). Yet such conclusions are based on global measures of oxygen transport alone. Some of the animal data do suggest that tissue oxygenation is impaired during severe hemodilution. For example, Noldge et al. (1991) examined the effects of hemodilution (hemoglobin 4 g/dl) on splanchnic oxygenation in pigs and found that hepatic and small intestine serosal surface Po₂ were significantly decreased. Because the gut mucosal layer appears to be especially vulnerable to hypoxia in other shock states (Vallet, 1994), we speculated that mucosal oxygenation may be even more affected by severe hemodilution than other tissues.