Maintaining Cerebral Blood Flow During Cardiac Surgery

In April 2020, a team of researchers at the MGH Martinos Center for Biomedical Imaging reported an innovative light-based technique that could help reduce the incidence of neurological injury during aortic arch replacement and other cardiac surgeries.

Hypothermic circulatory arrest (HCA) is 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—Deep HCA 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 HCA.

In current practice, surgeons often use oximetry 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 surgeons 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. Thus there is need for a device that can measure the brain’s blood flow. To help improve the monitoring of oxygen delivery and consumption during HCA, members of the Optics group at the Martinos Center devised an approach in which they employed 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.

To help improve the monitoring of oxygen delivery and consumption during HCA, members of the Optics group at the Martinos Center devised an approach in which they employed 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 presentation at the Optical Society of America’s 2020 Biomedical Congress (and in a paper later published in Journal of Thoracic and Cardiovascular Surgery Techniques, “The role of diffuse correlation spectroscopy and frequency-domain near infrared spectroscopy in monitoring cerebral hemodynamics during hypothermic circulatory arrests”), the researchers described a study seeking to validate this new approach for monitoring brain blood flow during HCA. 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 group and first author of the study, the hybrid technology could provide an important new tool for early detection of shortage of oxygen delivery to the brain and potentially provide guidance for selective cerebral blood perfusion during HCA 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 to fine-tune the hybrid technology—for example, by focusing on the wavelengths of light most advantageous for this application.

In the image 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.