Blood flow to the brain is normally regulated to ensure a constant supply of blood with oxygen and nutrients. During heart surgery using cardiopulmonary bypass, blood pressure is kept at a level that may or may not be below an individual's lower level of brain blood flow autoregulation. If lower, the brain may be exposed to an inadequate blood flow that could result in brain damage. The purpose of this study is to examine whether monitoring with a non-invasive FDA approved device that measures oxygen saturation of the superficial layers of the brain (near infrared spectroscopy) can, when combined with blood pressure measurements, provide information on the blood pressure level where brain blood flow is not autoregulated. The goal of this research is to develop a method to individualize blood pressure during surgery to a level that is within a patient's brain blood flow autoregulation range as a means for improving outcomes for patient undergoing heart surgery.
Cerebral blood flow (CBF) is normally autoregulated within a range of blood pressures, thus, allowing for a constant cerebral O2 supply commensurate with metabolic demands. The lower limit of blood pressure during general anesthesia deemed tolerable is usually empirically chosen without regard to an individual's lower autoregulatory threshold. This practice may lead to cerebral hypoperfusion in some patients placing them at risk for cerebral ischemic injury. This is particularly true during cardiac surgery using cardiopulmonary bypass where mean blood pressure is maintained at some level greater than 50 mm Hg or even lower during surgical mandated situations. This practice is concern for the rising number of aged surgical patients with cerebral vascular disease whom are prone to cerebral hypoperfusion. Individualizing blood pressure to be within the patient's autoregulatory range would more likely ensure adequate CBF during surgery. In this study we will evaluate whether real-time monitoring using a near infra-red spectroscopy (NIRS) based method accurately detects the lower CBF autoregulation threshold in patients compared with a validated, but technically more challenging method of CBF autoregulation measurement using transcranial Doppler. This approach involves continuous calculation of a moving linear regression correlation coefficient between blood pressure and cerebral O2 saturation measured with NIRS as a surrogate for CBF. A correlation coefficient, termed cerebral oximetery index, is generated and displayed versus blood pressure. When CBF is dysregulated, this correlation coefficient becomes markedly positive indicating CBF is pressure passive. We will further assess the added value of this type of monitoring to detect the lower CBF autoregulatory threshold compared with predictions based on preoperative blood pressure. These aims will be addressed in the study of 178 patients greater than 60 years of age undergoing coronary artery bypass graft surgery and/or valve surgery using cardiopulmonary bypass. Due to the many hemodynamic fluctuations during cardiac surgery, autoregulatory thresholds can be determined without administration of vasoactive drugs. Neurological complications are an important source of patient morbidity, mortality, hospital costs, and impaired quality of life after cardiac and other types of surgery. NIRS monitoring is non-invasive, continuous, requires little caregiver intervention and, thus, would have wide applicability for providing information about CBF autoregulation in a variety of clinical settings including cardiac surgery. This adoption could be enhanced with the likely development of "plug-and-play" monitors.