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1 School of Human Kinetics, University of British Columbia, Vancouver, Canada
2 Critical Care Medicine and Pulmonary Services, University of Athens, Athens, Greece; Department of Physical Education and Sport Sciences, University of Athens, Athens, Greece
3 Athens, Greece; Critical Care Medicine and Pulmonary Services, University of Athens, Athens, Greece
4 Department of Physical Education and Sport Sciences, University of Athens, Athens, Greece; Critical Care Medicine and Pulmonary Services, University of Athens, Athens, Greece
5 Department of Physical Education and Sport Sciences, University of Athens, Athens, Greece
6 Critical Care Medicine and Pulmonary Services, University of Athens, Athens, Greece; Athens, Greece
7 Critical Care Medicine and Pulmonary Services, University of Athens, Athens, Greece
8 Medicine, University of California, San Diego, La Jolla, California, United States
9 Departments of Medicine and Bioengineering, University of California-San Diego, La Jolla, California, United States
10 Exercise Science, Concordia University, 7141 Sherbrooke Street W., Montreal, H4B 1R6, Canada; Department of Biomedical Sciences, University of Copenhagen, Copenhagen, Denmark
* To whom correspondence should be addressed. E-mail: jordanal{at}interchange.ubc.ca.
Previous studies examining respiratory muscle blood flow (RMBF) in humans have required invasive methods on anesthetized subjects. To assess RMBF in awake subjects, we applied an indicator-dilution method using near-infrared spectroscopy (NIRS) and the light absorbing tracer indocyanine green dye (ICG). NIRS optodes were placed on the left 7th intercostal space at the apposition of the costal diaphragm and on an inactive control muscle (vastus lateralis). The primary respiratory muscles within view of the NIRS optodes include the internal and external intercostals. Intravenous bolus injection of ICG allowed for cardiac output, RMBF and vastus lateralis blood flow to be measured simultaneously. Esophageal and gastric pressures were also measured to calculate the work of breathing and trans-diaphragmatic pressure. Measurements were obtained in five conscious humans during both resting breathing and three separate 5-minute bouts of constant isocapnic hyperpnea at 27.1±3.2, 56.0±6.1 and 75.9±5.7% of maximum minute ventilation as determined on a previous maximal exercise test. RMBF progressively increased (9.9±0.6, 14.8±2.7, 29.9±5.8 and 50.±12.5 ml/100ml/min, respectively) with increasing levels of ventilation while blood flow to the inactive control muscle remained constant (10.39±1.4, 8.7±0.7, 12.9±1.7 and 12.2±1.8 ml/100ml/min, respectively). As ventilation rose, RMBF was closely and significantly correlated with a) cardiac output (r = 0.994, P = 0.006), b) the work of breathing (r = 0.995, P= 0.005) and c) trans-diaphragmatic pressure (r = 0.998, P= 0.002). These data suggest that the NIRS-ICG method provides a feasible and sensitive index of RMBF at different levels of ventilation in humans.
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