Respiratory muscle dynamics and control during exercise with externally imposed expiratory flow limitation

1 Centro di Bioingegneria, Fondazione Don Gnocchi e Politecnico I-20148 Milano; 2 Dipartimento di Bioingegneria, Politecnico di Milano, I-20133 Milan; 3 Fondazione Don Gnocchi, I-50020 Pozzolatico; 4 Clinica Medica III, Università di Firenze, I-50134 Florence, Italy; 5 Westmead Hospital, NSW-2145 Sydney, Australia; 6 University of Geneva, CH-1217 Geneva, Switzerland; 7 Meakins-Christie Laboratories, Montreal Chest Institute, McGill University Health Centre, Montreal, Quebec, Canada H2X 2P4; and 8 Department of Respiratory Medicine, Institute of Tuberculosis and Lung Disease, 01-138 Warsaw, Poland To determine how decreasing velocity of shortening (U) of expiratory muscles affects breathing during exercise, six normal men performed incremental exercise with externally imposed expiratory flow limitation (EFLe) at ~1 l/s. We measured volumes of chest wall, lung- and diaphragm-apposed rib cage (Vrc,p and Vrc,a, respectively), and abdomen (Vab) by optoelectronic plethysmography; esophageal, gastric, and transdiaphragmatic pressures (Pdi); and end-tidal CO 2 concentration. From these, we calculated velocity of shortening and power ( ) of diaphragm, rib cage, and abdominal muscles (di, rcm, ab, respectively). EFLe forced a decrease in Uab, which increased Pab and which lasted well into inspiration. This imposed a load, overcome by preinspiratory diaphragm contraction. Udi and inspiratory Urcm increased, reducing their ability to generate pressure. Pdi, Prcm, and ab increased, indicating an increased central drive to all muscle groups secondary to hypercapnia, which developed in all subjects. These results suggest a vicious cycle in which EFLe decreases Uab, increasing Pab and exacerbating the hypercapnia, which increases central drive increasing Pab even more, leading to further CO 2 retention, and so forth. muscle shortening velocity; respiratory failure; hypercapnia; ventilation; diaphragm; abdominal muscles; rib cage muscles; muscle power