Cerebral Oximetry
Adaptation of the technology used for pulse oximetry and intravascular
oximetry has been used to develop a monitor that can measure the oxygen saturation
in the vascular bed (primarily venous component) of the cerebral cortex.[246]
The rationale for monitoring this information is that as CBF falls, oxygen saturation
decreases because of increased extraction by tissue. Proponents of this device,
supported by some laboratory data, claim that cerebral oximetry can detect inadequate
CBF before the more established and accepted EEG.[247]
This finding is expected, because brain is able to increase oxygen extraction in
the face of decreased flow and maintain function until the flow falls below a level
at which increased extraction cannot compensate. Cerebral oximetry should therefore
be a sensitive and specific indicator of the adequacy of regional blood flow in the
tissue beneath the sensor.
Cerebral oximetry has been used with various degrees of success
to monitor the adequacy of cerebral perfusion in head trauma patients,[248]
[249]
[250]
[251]
[252]
during carotid vascular surgery,[253]
[254]
[255]
and
during
CPB.[256]
[257]
[258]
[259]
[260]
The monitoring device is easy to use and requires only the application of a probe
similar to a pulse oximetry probe to the forehead (two for bilateral monitoring).
This simplicity also gives rise to one of the limitations of this device. Frontal
application of sensors allows monitoring only of a small portion of the cortex, and
the well-being of the remainder of the cortex is inferred by the information derived
from the frontopolar cerebral cortex. This assumption is frequently inaccurate.
Vascular insufficiency in the distribution of the middle cerebral or posterior cerebral
arteries cannot be detected with frontal sensors.
Samra and colleagues evaluated cerebral oximetry during 94 carotid
endarterectomies in awake patients.[253]
They found
that the cerebral oximetry was able to identify 97.4% of patients with adequate CBF
as indicated by the absence of clinical symptoms. However, the investigators found
that there was a false-positive rate of 66.7%. The monitor frequently indicated
inadequate CBF although the patient had no symptoms of inadequate CBF. This finding
should not be surprising because oxygen extraction increases before any loss of function.
The real problem is that the lower limit for acceptable regional oxygen saturation
is not known in a large population of patients. This value may be different from
patient to patient, and addition of anesthetic drugs that influence cerebral metabolism
may further confuse the picture.
There has been much concern regarding the exact source of the
signal monitored by cerebral oximetry.[261]
[262]
Contamination of the oximetry signal by extracranial blood sources may be a source
of serious error for this device. Adjustment of the algorithm is reported to have
largely eliminated this problem,[263]
but other
serious concerns remain. No normative data exist across a large patient population
for normal values or for expected changes during surgery. Trend patterns are emerging
from various clinical reports and trials, but large-population data do not exist.
The meaning of isolated values is even less clear. Technology associated with this
monitoring is developing very fast, and cerebral oximetry will probably find a place
in the clinical care of patients after more data become available and further technologic
advances occur.
