Expiratory muscle loading increases intercostal muscle blood flow during leg exercise in healthy humans

Abstract

We investigated whether expiratory muscle loading induced by the application of expiratory flow limitation (EFL) during exercise in healthy subjects causes a reduction in quadriceps muscle blood flow in favor of the blood flow to the intercostal muscles. We hypothesized that, during exercise with EFL quadriceps muscle blood flow would be reduced, whereas intercostal muscle blood flow would be increased compared with exercise without EFL. We initially performed an incremental exercise test on eight healthy male subjects with a Starling resistor in the expiratory line limiting expiratory flow to approximately 1 l/s to determine peak EFL exercise workload. On a different day, two constant-load exercise trials were performed in a balanced ordering sequence, during which subjects exercised with or without EFL at peak EFL exercise workload for 6 min. Intercostal (probe over the 7th intercostal space) and vastus lateralis muscle blood flow index (BFI) was calculated by near-infrared spectroscopy using indocyanine green, whereas cardiac output (CO) was measured by an impedance cardiography technique. At exercise termination, CO and stroke volume were not significantly different during exercise, with or without EFL (CO: 16.5 vs. 15.2 l/min, stroke volume: 104 vs. 107 ml/beat). Quadriceps muscle BFI during exercise with EFL (5.4 nM/s) was significantly (P = 0.043) lower compared with exercise without EFL (7.6 nM/s), whereas intercostal muscle BFI during exercise with EFL (3.5 nM/s) was significantly (P = 0.021) greater compared with that recorded during control exercise (0.4 nM/s). In conclusion, increased respiratory muscle loading during exercise in healthy humans causes an increase in blood flow to the intercostal muscles and a concomitant decrease in quadriceps muscle blood flow.

Fig. 1.

Effect of flow limitation on breathing pattern. Cardiac output (A), percentage of oxygen saturation (B), sensations of dyspnea (C), minute ventilation (D), end-tidal carbon dioxide (E), and sensations of leg discomfort (F) are shown during quiet breathing (QB), unloaded cycling (UNL), and exercise with expiratory flow limitation (EFL; ●) and without EFL (control; ○). Values are means ± SE for 8 subjects. Significant differences †between the two conditions, and *compared with 6th min of exercise: P < 0.05.

Fig. 2.

Intercostal and quadriceps muscle blood flow index. Individual changes are shown from baseline (i.e., QB) to the end of exercise in the blood flow index of the intercostal (A) and quadriceps (B) muscles with EFL and without EFL (control). ●, Mean values for 8 subjects. †Significant differences between the two conditions, P < 0.05.

Fig. 3.

Changes in intercostal and quadriceps muscle oxygenation. Changes are shown from baseline (QB) in percentage of oxygen saturation (St_O2%; A and C) and deoxygenated hemoglobin concentration (Deoxy Hb; B and D) [corrected for arterial arterial O₂ saturation from pulse oximetry (11)] for the intercostal muscles (A and B) and the quadriceps muscles (C and D) during UNL and exercise with EFL (●) and without EFL (control; ○). Values are means ± SE for 8 subjects. Significant differences †between the two conditions and *compared with QB: P < 0.05.