Compensatory increases in cardiac output and SMA blood flows prevent splanchnic hypoperfusion during moderate isovolemic hemodilution in dogs

Introduction and objective

Intraoperative acute isovolemic hemodilution has been used to decrease the need for homologous blood transfusion. Decreased arterial oxygen content and total oxygen delivery to tissues promote a compensatory increase in cardiac output, related to the hemodilution-induced decrease in viscosity and/or vasodilatation, preserving tissue oxygen delivery. Regional blood flow distribution may vary widely between and within organs. Splanchnic hypoperfusion, particularly at the intestinal mucosal region, has been implicated in systemic inflammatory response and multiple organ dysfunction. We evaluated systemic and splanchnic oxygen-derived variables during a moderate acute isovolemic hemodilution to test the hypothesis that hemodilution may promote gastric mucosal acidosis, despite an apparent adequacy of global markers of oxygen delivery and consumption.

Methods

Eleven anesthetized mongrel dogs (16.7 ± 0.8 kg) were monitored with a Swan-Ganz catheter (cardiac output, cardiac filling pressures, mixed venous blood samples and lactate), an aortic catheter (mean arterial pressure and blood sampling), a portal vein catheter (portal lactate and blood gas), transit time ultrasonic flow probe (SMA blood flow) and a gas tonometer (PgCO₂ and PCO₂ gap). The animals were randomly assigned into two groups: controls (CT), no hemodilution; and acute isovolemic hemodilution (HD), induced by blood withdrawal (20 ml/min) with a simultaneous infusion of hydroxyethyl starch 6% in saline solution to a target hematocrit of 25 ± 3% for 30 min. The animals were then followed for 60 min.

Results

Hemodilution promoted significant decreases in hemoglobin, hematocrit, and pulmonary and systemic vascular resistances, and significant increases in cardiac output and in SMA blood flow. No significant differences between groups were detected on mean arterial and pulmonary artery pressures, oxygen delivery and extraction, PCO₂ gap, and systemic and portal vein PCO₂, pH and lactate.

Conclusion

Moderate isovolemic hemodilution induced decreases in hemoglobin and hematocrit; however, the associated compensatory increases in cardiac output and regional blood flows prevented splanchnic hypoperfusion in this experimental model.