Rationale: It has been hypothesized that, because of the high work of breathing sustained by patients with chronic obstructive pulmonary disease (COPD) during exercise, blood flow may increase in favor of the respiratory muscles, thereby compromising locomotor muscle blood flow.
Objectives: To test this hypothesis by investigating whether, at the same work of breathing, intercostal muscle blood flow during exercise is as high as during resting isocapnic hyperpnea when respiratory and locomotor muscles do not compete for the available blood flow.
Methods: Intercostal and vastus lateralis muscle perfusion was measured simultaneously in 10 patients with COPD (FEV₁ = 50.5 ± 5.5% predicted) by near-infrared spectroscopy using indocyanine green dye.
Measurements and main results: Measurements were made at several exercise intensities up to peak work rate (WRpeak) and subsequently during resting hyperpnea at minute ventilation levels up to those at WRpeak. During resting hyperpnea, intercostal muscle blood flow increased with the power of breathing to 11.4 ± 1.6 ml/min per 100 g at the same ventilation recorded at WRpeak. Conversely, during graded exercise, intercostal muscle blood flow remained unchanged from rest up to 50% WRpeak (6.8 ± 1.3 ml/min per 100 g) and then fell to 4.5 ± 0.8 ml/min per 100 g at WRpeak (P = 0.003). Cardiac output plateaued above 50% WRpeak (8.4 ± 0.1 l/min), whereas vastus lateralis muscle blood flow increased progressively, reaching 39.8 ± 7.1 ml/min per 100 g at WRpeak.
Conclusions: During intense exercise in COPD, restriction of intercostal muscle perfusion but preservation of quadriceps muscle blood flow along with attainment of a plateau in cardiac output represents the inability of the circulatory system to satisfy the energy demands of locomotor and respiratory muscles.