Posttransfusion Increase of Hematocrit per se Does Not Improve Circulatory Oxygen Delivery due to Increased Blood Viscosity

Abstract

BACKGROUND Blood transfusion is used to treat acute anemia with the goal of increasing blood oxygen carrying capacity as determined by hematocrit (Hct), and oxygen delivery (DO2). However, increasing Hct also increases blood viscosity, which may thus lower DO2 if the arterial circulation is a rigid hydraulic system as the resistance to blood flow will increase. The net effect of transfusion on oxygen delivery in this system can be analyzed by using the relationship between Hct and systemic blood viscosity of circulating blood at the post transfusion Hct to calculate DO2 and comparing this value to pre-transfusion DO2. We hypothesized that increasing Hct would increase DO2, and tested our hypothesis by mathematically modelling DO2 in the circulation. METHODS Calculations were made assuming a normal cardiac output (CO) (5 l/min), with degrees of anemia ranging from 5% to 80% Hct deficit. We analyzed the effects of transfusing 0.5 or more units of 300 cc of packed red blood cells (PRBCs) at a Hct of 65% and calculated microcirculatory DO2 after accounting for increased blood viscosity and assuming no change in blood pressure. Our model accounts for O2 diffusion out of the circulation prior to blood arriving to the nutritional circulation, and for changes in blood flow velocity. The immediate post transfusion DO2 was also compared to DO2 after the transient increase in volume due to transfusion had subsided. RESULTS Blood transfusion of up to 3 units of pRBCs increased DO2 when Hct (or hemoglobin) was 60% lower than normal, but did not increase DO2 when administered prior to this threshold. CONCLUSIONS After accounting for the effect of increasing blood viscosity on blood flow due to increasing Hct, we found in a mathematical simulation of DO2 that transfusion of up to 3 units of pRBCs does not increase DO2, unless anemia is due to a Hct deficit greater than 60%. Observations that transfusions occasionally result in clinical improvement suggest that other mechanisms possibly related to increased blood viscosity may compensate for the absence of increase in DO2.

Anemia and acute coronary syndrome: current perspectives

“…Therefore, increasing the Hct in anemia may not correspondingly increase DO 2 . 33 RBC transfusion-related adverse events may also hamper clinical benefits, with special regard to the so-called not infectious serious hazards of transfusion, which include transfusion-related circulatory overload and, among immune-mediated adverse reactions, transfusion-related acute lung injury. 34 A role of RBC transfusion in increasing systemic inflammation and erythrocyte slugging in capillary vessels has also been hypothesized.…”

Transfusion of Red Blood Cells to Patients with Sepsis

“…Using a mathematical model, Zimmerman et al estimated the effect of an increase in blood viscosity on oxygen delivery. Their model predicted that with baseline hemoglobin concentrations of >5.8 g/dL, transfusion of 0.5–3.0 units of pRBCs would increase blood viscosity, which subsequently led to a decrease in oxygen delivery [ 71 ]. Therefore, pRBC transfusion may ultimately help maintain microcirculation by restoring blood viscosity.…”

An Established Early Rehabilitation Therapy Demonstrating Higher Efficacy and Safety for Care of Intensive Care Unit Patients

“…Previous studies [29–31] have never combined the above therapeutic approaches together for improving the care of ICU patients. Hyperbaric oxygen therapy has been widely employed to enhance blood oxygen levels, increase the blood oxygen diffusion velocity, and inhibit anaerobic glycolysis in damaged brain tissues [29, 30]. Hyperbaric oxygen therapy can also suppress inflammation and alleviate vascular endothelial injury in damaged brain tissues [31].…”