BME PhD Dissertation Defense - Carlos Gomez

Title: "Non-Invasive Monitoring of the Respiratory Muscles Via Diffuse Optical Modalities"

Advisory Committee: David Boas, PhD – BU BME (Chair) Darren Roblyer, PhD – BU BME (Advisor) Cara Stepp, PhD – BU Professor of Speech, Language and Hearing Sciences Darlene Reid, PhD – University of Toronto Department of Physical Therapy Maria Angela Franceschini, PhD – HMS, Professor of Radiology

Abstract: Muscle metabolism is highly regulated by the body, and muscle function is highly dependent on appropriate oxygenation levels and blood supply. Muscle oxidative metabolism is affected by both the delivery of oxygenated blood to the muscle and the extraction of oxygen from the blood. Both of these processes can be negatively affected by a wide range of pathologies. For example, during severe respiratory distress, mechanical ventilation (MV) may be required in order to maintain oxygen supply to vital organs by unloading (i.e. reducing the effort) of the muscles of inspiration. MV is an essential tool in the intensive care unit, with patient care incorporating MV accounting for 12% of US hospitals’ expenses each year, equivalent to $27 million USD. US hospitals reported a 31.5% increase in MV usage during the COVID-19 pandemic, as severe cases of COVID-19 often require MV. While MV can be lifesaving, there are severe physiological and psychological drawbacks to MV use. It is therefore critical for clinicians to appropriately utilize MV to avoid inspiratory muscle disuse atrophy and/or muscle damage. Additionally, the process of weaning patients from MV is a major clinical challenge, often requiring multiple attempts before success. There are currently no adequate means to continuously monitor MV use and weaning from MV. A system consisting of a Near Infrared Spectroscopy (NIRS) and Diffuse Correlation Spectroscopy (DCS) can help to solve this unmet clinical need through non-invasive and label free continuous monitoring of inspiratory muscles during MV. NIRS is a non-invasive technique that can measure tissue hemoglobin and myoglobin concentrations, while DCS can measure regional blood flow rates. Prior work with DOS has shown that the sternocleidomastoid (SCM), an inspiratory muscle located in the neck, has potential to be used as an indicator of respiratory effort. Recently, there have been advances in combining NIRS and DCS techniques to work in unison, providing oxy-, deoxy-, and total hemoglobin and myoglobin concentrations (oxy [Hb+Mb], deoxy [Hb+Mb], and total [Hb+Mb]), oxygen saturation of tissue (StO2), blood flow index (BFi), and tissue regional oxygen metabolic rate (MRO2). To date, combined NIRS and DCS has only been used to measure SCM metabolism at baseline. A custom frequency domain NIRS system was combined with a custom DCS system for continuous SCM measurements during loading. The system was validated via a liquid titration and used in two healthy volunteer studies. Additionally, new computational models were developed that account for the tissue region’s multilayer composition of skin, subcutaneous lipid and muscle, as well as the wide variety of skin tones and lipid layer thicknesses likely to be encountered in a MV patient population.