Near infrared laser device can measure brain oxygen levels
ATLANTA – A new device that uses near-infrared light to non-invasively monitor the oxygenation of the brain during surgery appears to be a promising alternative to the more invasive techniques currently in use, according to a new study by Duke University Medical Center anesthesiologists.
The researchers said their findings offer the potential for accurate and reliable monitoring of brain oxygenation during cardiac surgeries, to more effectively protect the brain against reduced oxygen levels, or anoxia, which is known to cause cognitive impairment in some surgical patients.
During some surgeries anesthesiologists measure venous oxygenation by periodically removing blood samples from catheters inserted in major blood vessels in the neck and then analyze the samples by co-oximetry. Also, anesthesiologists frequently use a pulse oximeter, attached to the patient's finger, to measure arterial blood oxygenation. However, since these measurements are taken on blood outside the brain, physicians can only estimate the level of cerebral oxygenation.
Designed by CAS Medical Systems, Inc., the monitor, called a cerebral oximeter, uses one or more sensors attached to the forehead that emit non-harmful, low-level laser light through the skin and skull into the brain. Since the near-infrared light absorption characteristics of the hemoglobin in red blood cells are known, the system can calculate the brain tissue oxygen saturation by measuring the differences in intensity of light as it passes through the brain. When combined with pulse oximetry, the cerebral oximeter may be used to estimate the cerebral venous oxygen saturation.
The basic principle of cerebral oximetry is based on optical spectroscopy techniques. The discovery that near-infrared light can pass through the scalp and skull to examine levels of hemoglobin and other light absorbing compounds of the brain was made at Duke by Frans Jobsis, Ph.D., in 1977.
"It has always been a challenge to directly measure the oxygen levels in the brain," said Duke anesthesiologist David MacLeod, M.D., who presented the results of the Duke study Oct. 22, 2005, at the annual scientific sessions of the American Society of Anesthesiologists in Atlanta. "The main issues with the invasive approach are that it does not provide specific information in real time, and it is of course invasive, which can carry some risk to the patient.
"This new technology, which is non-invasive and provides real-time information, appears to be an accurate means for measuring cerebral oxygenation and indirectly cerebral perfusion," MacLeod said. "As anesthesiologists, protecting the brain from potential harm is one of the main functions we perform during a surgical procedure."
For their study, the researchers enrolled 12 healthy volunteers. The volunteers were monitored using the different blood oxygenation measurement systems – pulse oximetry, jugular and radial arterial co-oximetry, and the prototype cerebral oximeter. In a stepwise fashion, the researchers decreased and then increased the concentration of inhaled oxygen through a range of 70 to 100 percent arterial blood oxygen saturation. Frequent, concurrent measurements were made on all three systems throughout the process.
"We made a total of 171 readings and found a strong correlation between the reference co-oximetry measurements by the invasive methods to the non-invasive approaches," MacLeod said. "So it appears that we can use non-invasive approaches to estimate something we could in the past only measure with invasive sampling."
While pulse oximetry is used universally to measure arterial oxygen saturation for all patients undergoing surgery, interest in cerebral oxygenation levels have mainly been the domain of cardiac surgeons and anesthesiologists, according to MacLeod, given the rising concerns about potential cognitive impairments suffered by some patients undergoing open heart surgery.
Following this successful validation of the CAS cerebral oximeter, the Duke team is conducting a clinical trial to refine the optimal range of cerebral oxygenation in patients undergoing heart surgery. After surgery these patients will be periodically assessed to detect any correlation between cerebral oxygen levels during surgery and post-op changes in cognition.
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