Cardiopulmonary bypass (CPB) is a method of extracorporeal circulation used to preserve circulation and oxygenation of the body during a variety of cardiovascular and pulmonary surgeries. Risk of temporary and permanent cognitive impairment has been associated with CPB, possibly due to alterations in blood flow. CPB involves hemodilution and induction of hypothermia, two interventions that have opposite effects on blood viscosity.
Hemodilution improves cerebral blood flow by decreasing blood viscosity. Increasing cerebral blood flow too much, however, may be detrimental during CPB as it may enhance travel of emboli to the cerebral microvasculature, increasing risk for microembolic injury. Adequate cerebral blood flow can be maintained at modest hemodilution of hematocrit levels between 21-25%. Despite this low hematocrit range, oxygenation can be maintained utilizing supplemental oxygen and pulse oximetry monitoring.
On the other hand, reducing core body temperature during CPB reduces metabolic demand while causing an increase in blood viscosity. In theory, a perfusionist can optimize tissue perfusion by altering pump output settings, in spite of hyperviscous blood. However, hypoperfusion and hypoxia can still occur as a result of microvascular shunting within the brain. Such a condition would allow for blocked capillaries to be bypassed, resulting in a failure of reperfusion and a subsequent local ischemia.
Established links between cognitive impairment and CPB may be caused by hyperviscosity of blood due to reduced body temperature during CPB. More generally, increased blood viscosity and impaired cerebral blood flow has been associated with cognitive decline (see Blood Viscosity: The Role of Blood Flow in Cognitive Function). Monitoring blood viscosity prior to or during CPB may prove useful for managing perfusion during CPB and preventing neurological impairment related to the procedure (see Treatment Monitoring: Preventing Cognitive Decline).