Optics Technologies Could Advance Neuromonitoring During Heart Surgery

Gary Boas

A team of researchers at the MGH Martinos Center for Biomedical Imaging has reported an innovative light-based technique that could help reduce the incidence of neurological injury during aortic arch replacement and other cardiac surgeries.

Deep hypothermic circulatory arrest (DHCA) is a technique widely used to provide a bloodless field of view for aortic arch surgeries by cooling the patient’s body to temperatures as low as 68 degrees Fahrenheit and stopping blood circulation. However, because the brain needs oxygen for the lengths of time associated with the procedure – DHCA can be maintained for up to 20-30 minutes without the lack of oxygen causing brain injury, but most aortic arch surgeries take longer than this – surgeons use selective cerebral perfusion methods to continue oxygen delivery to the brain while the rest of the body is in DHCA.

In current practice, oximetry is often used to monitor and help guide cerebral blood perfusion during surgery. But cerebral oximetry only provides measures of oxygen content in the microvasculature; it cannot tell the surgeon how much blood the brain is really getting from the selective cerebral perfusion methods. As a result, by the time the method detects a significant change in oxygenation, injury to the brain may already have occurred. There is need for a device that can measure the brain’s blood flow. Here is where the Martinos researchers enter the picture.

Over the past two decades, members of the Optics @ Martinos group at the Center have developed several noninvasive near-infrared optical monitoring technologies and applied them for a range of uses. To help improve the monitoring of oxygen delivery and consumption during DHCA, the Martinos researchers employ a hybrid device including frequency-domain near-infrared spectroscopy (FDNIRS) and diffuse correlation spectroscopy (DCS) to characterize both oxygen consumption rates and blood flow in the brain during the procedure. FDNIRS measures modulated light intensity attenuation and phase shift for more quantitative assessment of blood oxygenation with respect to conventional, “continuous wave” NIRS techniques. DCS takes advantage of intensity fluctuations of laser light (speckles) to quantify cerebral blood flow, noninvasively, at the bedside.

In a recent abstract submitted to the Optical Society of America’s Biomedical Congress, “Using Diffuse Optics to Measure Cerebral Blood Flow and Oxygen Saturation during Hypothermic Circulatory Arrests,” the researchers describe a study seeking to validate this new approach for monitoring brain blood flow during DHCA. The work was performed in collaboration with Dr. Jason Qu, a cardiac anesthesiologist at Massachusetts General Hospital.  The results of the study suggest that, because the hybrid device directly measures blood flow – which changes much more quickly than oxygenation – the technology can provide “timely and accurate” insight into the efficacy of brain protection during the procedure, especially compared to the currently used methods.

Ultimately, says Alexander (Shurik) Zavriyev, a research assistant in the Optics @ Martinos group and first author of the abstract reporting the study, the hybrid technology could provide an important new tool for early detection of shortage of oxygen to the brain and potentially provide guidance for selective cerebral blood perfusion during DHCA for aortic arch surgeries. “Demonstrating the ability to accurately and simultaneously measure oxygenation and cerebral blood flow during surgery will lead to new approaches to reduce neurological injury and the overall morbidity and mortality associated with anesthesia in general, and aortic arch replacement surgeries in particular,” he says.

In the short-term, the researchers are seeking to validate the approach further by testing it on a larger group of patients as well as fine-tune the hybrid technology – for example, by focusing on the wavelengths of light most advantageous for this application.

In the photo above: The Center’s Alexander (Shurik) Zavriyev (right) and Kutlu Kaya, both research assistants in the Optics @ Martinos group, are working with surgeons and anesthesiologists at Massachusetts General Hospital to validate the hybrid optical imaging device. Photo by Allen Alfadhel.