Failure of Regional Cerebral Oximetry in a Polycythemic Patient
Michael S. Robinson D.O., Laurie M. Davies
M.D.
Department of Anesthesiology, University of Florida College of Medicine
Introduction: Regional cerebral oximetry is a relatively new
monitoring modality that is slowly gaining acceptance in
clinical medicine. At our institution it is primarily used for
observing trends in regional cerebral oxygen saturation (rSO2)
during cardiothoracic surgery. But the modality is also being
used in the critical care arena in the evaluation of cerebral
perfusion in patients with head injuries.
We present a case of an extremely polycythemic patient who
underwent surgery requiring cardiopulmonary bypass monitored by
regional cerebral oximetry.
Case Report: The 16-year-old patient had a Tetralogy of Fallot
and pulmonary atresia, which had not been corrected because of
very hypoplastic pulmonary arteries and numerous aorto-pulmonary
vascular anomalies. For palliation, he had undergone previous
right modified Blalock-Taussig shunt and aorto-pulmonary central
shunt. Over two-to-three months the patient's exercise tolerance
diminished to less than 5 minutes of exertion without dyspnea.
The patient presented to the operating room with generalized
cyanosis, digital clubbing, severe polycythemia (hematocrit 67%)
and oxygen saturation by pulse oximetry of 50-60% on room air.
The planned surgical procedure was a right
ventricular-to-pulmonary artery homograft and ligation of the
aorto-pulmonary central shunt.
Standard intraoperative monitoring was initiated with the
addition of regional cerebral oximetry (rSO2) (Somanetics Corp
INVOS 5100, Troy, MI), transcranial Doppler (Spencer
Technologies TCD 100M, Seattle, WA), electroencephalogram
(Aspect Corp A 1000, Framingham, MA) and transesophageal
echocardiography (Agilent Technologies Sonos 5328, Andover, MA).
The cerebral oximetry sensors were placed according to the
recommendations of the manufacturer. However, the INVOS monitor
reported poor signal quality and would not provide any output
data. The regional oxygen saturation sensors, connecting cables
and the INVOS cerebral oximeter were all replaced with no change
in signal output. The sensors were replaced a second time, again
without improvement in cerebral oximeter signal output.
The anesthesia team accepted the nonfunctional cerebral oximeter
and proceeded with the case using information from the other
monitoring modalities. A baseline hematocrit of 62.9%from an
arterial blood gas (ABG) was obtained and the operation
proceeded to cardiopulmonary bypass (CPB) without further
difficulty.
Upon initiation of CPB the cerebral oximeter immediately began
to provide signal output. A repeat ABG was obtained and
demonstrated a hematocrit of 44.6%. The cerebral oximeter
continued to provide data throughout the operation until the
regional oxygen saturation sensors were removed at the end of
the case. At no time after CPB was a hematocrit recorded greater
than 44.6%.
Discussion: Regional cerebral oximetry is based on the
Beer-Lambert law. The Beer-Lambert law uses absorption of
photons by a chromophore to determine the concentration of the
chromophore. Our initial difficulties in obtaining rSO2 signals,
may be due to the high concentrations of chromophores, i.e. oxy-
and deoxyhemoglobin in a polycythemic patient in relation to the
light output from the oximetry diode. Because regional cerebral
oximetry is not currently widely used the discussion will
explain the theory of its operation, potential short comings of
the modality and provide follow up data about the patient in the
case report.
