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CARDIOVASCULAR ANESTHESIA

Hematocrit, Volume Expander, Temperature, and Shear Rate Effects on Blood Viscosity

David M. Eckmann, PhD, MD^*,, Shelly Bowers, MD^*, Mark Stecker, MD, PhD, and Albert T. Cheung, MD^*

Departments of *Anesthesia and Neurology, and Institute for Medicine and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania

Address correspondence and reprint requests to David M. Eckmann, PhD, MD, Department of Anesthesia, University of Pennsylvania, Dulles 7/HUP, 3400 Spruce St., Philadelphia, PA 19096. Address e-mail to deckmann{at}mail.med.upenn.edu

Our goal was to determine and predict the effects of temperature, shear rate, hematocrit, and different volume expanders on blood viscosity in conditions mimicking deep hypothermia for cardiac operations. Blood was obtained from six healthy adults. Dilutions were prepared to hematocrits of 35%, 30%, 22.5%, and 15% using plasma, 0.9% NaCl, 5% human albumin, and 6% hydroxyethyl starch. Viscosity was measured over a range of shear rates (4.5–450 s^-1) and temperature (0°–37°C). A parametric expression for predicting blood viscosity based on the study variables was developed, and its agreement with measured values tested. Viscosity was higher at low shear rates and low temperatures, especially at temperatures less than 15°C (P < 0.016 for all conditions in comparison with 37°C). Decreasing hematocrit, especially to less than 22.5%, decreased viscosity. Hemodilution with albumin or 0.9% NaCl decreased blood viscosity more than hemodilution with plasma or 6% hydroxyethyl starch (P < 0.01 for all cases). The derived mathematical model for viscosity as a function of temperature, hematocrit, shear rate, and diluent predicted viscosity values that correlated well with the measured values in experimental samples (r² > 0.92, P < 0.001).

Implications: A theoretical model for blood viscosity predicted independent effects of temperature, shear rate, and hemodilution on viscosity over a wide range of physiologic conditions, including thermal extremes of deep hypothermia in an experimental setting. Moderate hemodilution to a hematocrit of 22% decreased blood viscosity by 30%–50% at a blood temperature of 15°C, suggesting the potential to improve microcirculatory perfusion during deep hypothermia.

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