Emerging Technologies May Lead to Advances in Patient Monitoring

Studies Focus on New Approaches to Monitoring Oxygenation of Brain and Heart

Emerging and experimental technologies for measuring oxygen levels in tissue may offer valuable new monitoring techniques for preventing injury to the brain and heart during surgery, according to a special feature in the April issue of Anesthesia & Analgesia, official journal of the International Anesthesia Research Society (IARS). "A year ago I asked the general question, 'What are important research questions in anesthesiology?'" comments Dr. Steven L. Shafer of Columbia University, Editor-in-Chief of Anesthesia & Analgesia. "Several individuals mentioned that monitoring tissue oxygen delivery continues to be a critically important research focus." This month's A&A presents three papers exploring new approaches to tissue oxygen monitoring. 

Near-Infrared Spectroscopy Can Monitor Oxygen Levels in Brain
Two papers evaluate the use of a technique called near-infrared spectroscopy (NIRS) to detect brain tissue endangered by inadequate oxygen supply, called hypoxic-ischemic injury. Changes in oxygenation of the brain can be assessed noninvasively, by means of light-transmitting and light-detecting devices (called "optodes") placed on the scalp. Based on responses to light in the near-infrared range of the spectrum, NIRS can identify brain areas of the brain with normal versus reduced oxygen supply. One study, led by Dr. C. Dean Kurt of Cincinnati Children's Hospital, used NIRS to evaluate responses to decreased blood flow to the brain in piglets. Used together with brain function monitoring, NIRS provided accurate data on how much blood deprivation the tissue could sustain before irreversible damage occurred. The results suggested that brain tissue remains viable for one to two hours during deprivation, as long as normal blood flow is restored. Based on the NIRS data, there appears to be "a several hour window of opportunity" in which the brain can survive ischemic hypoxic-ischemic injury, the researchers conclude. Dr. Barry D. Kussman of Harvard Medical School and colleagues used NIRS to monitor brain tissue oxygenation in 104 infants undergoing surgery for congenital heart defects. The results showed significant differences in brain oxygenation in infants with different types of severe heart defects. The findings supported the presence of adequate oxygenation to protect the brain during the relatively brief time the infants were on heart-lung bypass.
Data from NIRS might be useful in identifying infants at risk of brain damage related to hypoxic-ischemic injury, Dr. Kussman and colleagues believe. Fortunately, none of the infants in the study experienced such complications in the early period after surgery. The researchers plan further follow-up to see whether the NIRS results in any way predict the children's future developmental outcomes.  

Visible Light Spectroscopy May Help Protect the Heart
In the third study, Dr. Jonathan K. Ho of University of California, Los Angeles, evaluated a technique called visible light spectroscopy (VLS) to evaluate oxygenation in heart (myocardial) tissue. Visible light spectroscopy is an experimental technique, similar to NIRS but using light in the visible range of the spectrum. In experiments with pigs, VLS provided accurate and rapid data on how myocardial tissue responded to various degrees of blood and oxygen deprivation. The results suggested that VLS may one day provide a useful tool to monitor the viability of heart tissue in patients undergoing cardiac surgery.
Monitoring patient safety during surgery is an important role of anesthesiologists. New approaches like NIRS and VLS are potentially valuable new approaches to ensuring that vulnerable organs like the brain and heart remain adequately oxygenated during surgical procedures. The two techniques, especially VLS, are still in their infancy. Although they may seem high-tech now, NIRS and VLS may soon become an important part of routine patient monitoring. They work in much the same way as the now-familiar technique of pulse oximetry, which estimates oxygenation by interpreting skin responses to light. "Pulse oximetry obviously provided a quantum leap in terms of the 'big picture' for oxygen delivery, but the harder question, still in the works, is understanding and monitoring oxygen delivery to tissues," adds Dr. Shafer. "The featured articles are devoted to more work on cerebral oximetry, which has become a reasonably mature field; and myocardial tissue oximetry, which is still experimental but appears likely to be a significant advance in enhancing the safety of cardiac surgery."

About the IARS
The International Anesthesia Research Society (www.iars.org) is a nonpolitical not-for-profit medical society founded in 1922 to encourage, stimulate, and fund ongoing anesthesia-related research and projects that will enhance and advance the anesthesiology specialty.  The IARS has a worldwide membership of 15,000 physicians and others with doctoral degrees.  The mission of the IARS includes publication of a monthly scientific journal, Anesthesia & Analgesia in print and online (www.anesthesia-analgesia.org), the oldest publication for the specialty of anesthesiology.  The IARS is located in San Francisco, California.