Respiratory and locomotor muscle blood flow regulation during physiological stress

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The respiratory muscles have been shown to develop fatigue during tasks that require high levels of muscular work (e.g., intense exercise). Respiratory muscle fatigue is thought to causes an important reflex response that reduces blood flow to the leg muscles. This reduction in blood flow has been termed the "steal phenomenon" whereby the respiratory muscles are hypothesized to steal blood flow from the leg muscles leading to an increased level of leg muscle fatigue, increased perceived leg discomfort and ultimately, reduced exercise performance. A major challenge in supporting this phenomenon has been the inability to measure respiratory muscle blood flow (RMBF) in humans, due in large part, to the complex anatomical arrangement and vascular network of the respiratory muscles. However, we recently overcame these challenges and became the first researchers to successfully measure RMBF in humans using near-infrared spectroscopy technology and the light absorbing tracer indocyanine green. The ability to measure RMBF will allow us to address several unanswered questions related to blood flow competition between the locomotor and respiratory muscles and will provide significant insight into the mechanisms of respiratory muscle fatigue in humans. Accordingly, the long-term goal of the proposed research program is to systematically examine the complex relationship between respiratory muscle function and blood flow distribution in humans under situations of increased physiological stress. Specifically, this research will experimentally manipulate the work of breathing to examine how respiratory muscle fatigue affects respiratory muscle recruitment patterns, neural respiratory drive, and blood flow distribution at rest and during exercise. The novel insight gained from this research will enhance our fundamental understanding of cardio-respiratory interactions and exercise limitations in healthy humans. This work will act as a foundation for future studies aimed at understanding the pathophysiology of several conditions where respiratory muscle function or blood flow may be compromised such as in patients with chronic cardio-respiratory disease or in patients that are being mechanically ventilated.