Respiratory Muscle Blood Flow Measured by Near-Infrared Spectroscopy (NIRS) and Indocyanine Green Dye (ICG)

Abstract

The ability to measure intercostal muscle blood flow in humans has implications for understanding blood flow distribution patterns and circulatory regulation during exercise. Accordingly, the advantage of performing actual muscle blood flow measurements is that it provides an insight into the “blood flow competition” theory, based on which it is proposed that the respiratory muscles compete with the locomotor muscles for the available blood flow. Until very recently it was impossible to measure respiratory muscle blood flow in humans during exercise owing to several difficulties. Near-infrared spectroscopy (NIRS) combined with indocyanine green dye (ICG) is a new valid and minimally invasive technique for measuring respiratory muscle blood flow during exercise in humans allowing for continuous and variable frequency data collection. On the basis of tracer principles of mass conservation, the NIRS–ICG technique makes it possible to quantify blood flow because the rate of accumulation of ICG in a given tissue reflects perfusion in the specified tissue of interest. Respiratory muscle blood flow measurements by NIRS–ICG technique primarily reflect the internal and external intercostal muscles and only to a lesser extent the costal part of the diaphragm. However, while the diaphragm acts as the primary flow generator during exercise, the role of the intercostal muscles is also important as they develop the necessary pressure to move the rib cage. This chapter focuses on the intercostal muscle perfusion measured by the NIRS–ICG technique in healthy individuals and patients with chronic obstructive pulmonary disease (COPD) during exercise who often experience greater work of breathing than healthy subjects both at rest and during exercise.