Respiratory and locomotor muscle blood flow during exercise in health and chronic obstructive pulmonary disease

NEW FINDINGS: Using an approach we developed to measure muscle blood flow by indocyanine green dye detected by near infrared spectroscopy, we have found that isocapnic hyperpnoea at rest leads to an increase in respiratory muscle blood flow that is proportional to the respiratory muscle work required. Surprisingly, cycling exercise interferes with respiratory muscle blood flow, especially in COPD, but even in health athletes. Intercostal muscle blood flow during exercise fails to reach flow rates observed at the same minute ventilation as during isocapnic hyperpnoea, and in COPD, intercostal muscle blood flow during exercise actually falls below flow during resting breathing. No evidence is found in intact subjects for redistribution of blood flow from the legs to the respiratory muscles during heavy exercise in health or COPD. Evidence of decrease in leg blood flow and increase in respiratory muscle flow was found only when imposing expiratory flow limitation (EFL) during exercise in healthy individuals. However, because EFL caused substantial physiological derangement, these results cannot be projected onto normal exercise. ABSTRACT: We have developed an indicator-dilution method to measure muscle blood flow at rest and during exercise using the light absorbing tracer indocyanine green dye (ICG) injected as an intravenous bolus, with surface optodes placed over muscles of interest to record the ICG signal by near-infrared spectroscopy. Here we review findings for both quadriceps and intercostal muscle blood flow (measured simultaneously) in trained cyclists and in patients with COPD. During resting hyperpnoea in both athletes and patients, intercostal muscle blood flow increased with ventilation, correlating closely and linearly with the work of breathing, with no change in quadriceps flow. During graded exercise in athletes, intercostal flow at first increased, but then began to fall approaching peak effort. Unexpectedly, in COPD, intercostal muscle blood flow during exercise fell progressively from resting values, contrasting sharply with the response to resting hyperpnoea. During exercise at peak intensity, we found no quadriceps blood flow reduction in favour of the respiratory muscles in either athletes or patients. In COPD at peak exercise, when patients breathed 21% oxygen in helium or 100% oxygen, there was no redistribution of blood flow observed between legs and respiratory muscles in either direction. Evidence of decrease in leg blood flow and increase in respiratory muscle flow was found only when imposing expiratory flow limitation (EFL) during exercise in healthy individuals. However, because EFL caused substantial physiological derangement, lowering arterial oxygen saturation and raising end-tidal PCO2 and heart rate, these results cannot be projected onto normal exercise. This article is protected by copyright. All rights reserved.