Monitoring of Cerebral Blood Flow Autoregulation Using Near Infrared Spectroscopy

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.

Inclusion Criteria:

- Adult patients undergoing cardiac surgery