Theoretical treatise: arterial pressure during aortic surgery

The optimum arterial perfusion pressure during cardiopulmonary bypass (CPB) remains uncertain. A correlation in some form with the patients' resting pressure almost certainly exists. Temperature and hematocrit affect blood viscosity. The optimum perfusion pressure during aortic surgery will vary after the initiation of CPB resulting cooling, heating, and hematocrit changes. Poiseuille's Law was used in conjunction with the previously published effects of temperature and hematocrit on blood viscosity to determine the perfusion pressure that would result in the same organ blood flow. Two different scenarios were modeled, constant flow and flow as predicted by Q10 to reflect required oxygen delivery. Temperature, hematocrit, and flow all have a large effect on blood viscosity and, thus, through Poiseuille's Law, blood pressure. As patients are cooled, their blood viscosity goes up through the inherent viscoelastic properties of blood. As temperature drops from 37 degrees to 17 degrees, viscosity doubles. This increased viscosity is offset by a reduction in hematocrit, which is invariably associated with CPB. As the hematocrit drops from 30% to 10%, viscosity of blood halves. These two factors clinically can cancel each other out. The figure demonstrates the effect on blood pressure of a constant flow for various temperature and hematocrits. Reduced need for oxygen delivery, secondary to the principles of Q10, can result in a lower than expected theoretical perfusion pressure. As temperature drops from 37 degrees to 17 degrees, based on Q10, oxygen delivery reduces by 75%. This indicates that flow can be reduced by over 60% if the hematocrit falls from 30% to 20%. This theoretical treatise predicts that blood pressure management should be temperature- and hematocrit-dependent. The target optimal blood pressure will vary during the course of surgery as a result of heating, cooling, and hemodilution. Clinical correlation is needed.