Cerebral Autoregulation Monitoring During Cardiac Surgery

Brain injury during cardiac surgery results primarily from cerebral embolism and/or reduced
cerebral blood flow (CBF). The latter is of particular concern for the growing number of
surgical patients who are aged and/or who have cerebral vascular disease. Normally, CBF is
physiologically autoregulated (or kept constant) within a range of blood pressures allowing
for stable cerebral O2 supply commensurate with metabolic demands. Cerebral autoregulation is
impaired in patients undergoing cardiac surgery who have cerebral vascular disease and in
many others due to other conditions. This could lead to brain injury since current practices
of targeting low mean arterial blood pressure empirically (usually 50-70 mmHg) during
cardiopulmonary bypass may expose patients with impaired cerebral autoregulation to cerebral
hypoperfusion. The hypothesis of this proposal is that targeting mean arterial pressure
during cardiopulmonary bypass to a level above an individual's lower autoregulatory threshold
reduces the risk for brain injury in patients undergoing cardiac surgery. Monitoring of
cerebral autoregulation will be performed in real time using software that continuously
compares the relation between arterial blood pressure and CBF velocity of the middle cerebral
artery measured with transcranial Doppler and with cerebral oximetry measured with near
infrared spectroscopy. The primary end-point of the study will be a comprehensive composite
outcome of clinical stroke, cognitive decline, and/or new ischemic brain lesions detected
with diffusion weighted magnetic resonance (MR) imaging. Delirium assessed using a validated
procedure that includes validated tools is a secondary outcome measure. Autoregulation is
mediated by reactivity of cerebral resistance vessels. A secondary aim of this proposal is to
evaluate whether near infrared reflectance spectroscopy can be used to trend changes in
cerebral blood volume and provide a reliable monitor of vascular reactivity (the hemoglobin
volume index). Assessments for extra-cranial and intra-cranial arterial stenosis will be
performed using MR angiography to control for this potential confounding variable in the
analysis. Finally, an additional aim of the study will be to assess whether preoperative
transcranial Doppler examination of major cerebral arteries can identify patients who are
prone to the composite neurological end-point. Near infrared oximetry is non-invasive,
continuous, requires little care-giver intervention and, thus, could be widely used to
individualize patient blood pressure management during surgery. Brain injury from cardiac
surgery is an important source of operative mortality, prolonged hospitalization, increased
health care expenditure, and impaired quality of life. Developing strategies to reduce the
burden of this complication has wide public health implications.

Inclusion Criteria:

- Male or female patients undergoing primary or re-operative CABG and/or valvular
surgery or ascending aorta surgery that requires CPB who are at high risk for
neurologic complications (stroke or encephalopathy) as determined by a Johns Hopkins
risk score of >0.02

Exclusion Criteria:

- Contraindication to MRI imaging (e.g., permanent pacemaker, cerebral arterial vascular
clips)

- Liver function test before surgery more than twice the upper limit of institutional
normal

- Pre-existing renal dysfunction defined as an estimated glomerular filtration rate of
≤60 mL/min, or current renal dialysis

- Emergency surgery

- Inability to attend outpatient visits

- Visual impairment or inability to speak and read English. The patient will be excluded
from further study if an adequate temporal window for TCD monitoring can not be
identified before surgery.