Breathing by double-lung recipients during exercise: response to expiratory threshold loading

Ventilation during exercise is near-normal in double-lung transplant recipients despite lung denervation. We tested the hypothesis that denervation effects might be unmasked during exercise by exposing these patients to an expiratory load. Eight double-lung recipients and nine intact control subjects were exercised to exhaustion. Ergometer work increased 20 Watt every 2 min; expiratory threshold loading (4 cm H2O) was imposed for five to six breaths at each exercise level; ventilation and O2 consumption were measured. Transplant recipients and control subjects increased ventilation similarly for comparable fractions of maximal work. At maximal exercise, transplant recipients achieved lower work (62 versus 155 W; p < 0.001) and O2 consumption (0.88 versus 2.26 L/min; p < 0.001) than control subjects, with proportional reductions in tidal volume (1.6 versus 2.6 L; p < 0.05) and ventilation (38 versus 79 L/min; p < 0.01). Threshold loading decreased expiratory flow, breathing frequency, and minute ventilation in both groups (p < 0.05). Unlike control subjects, transplant recipients also slowed inspiratory flow (p < 0.05) and prolonged inspiration (p < 0.01), exaggerating the fall in breathing frequency and ventilation (p < 0.01). We conclude that afferent information from pulmonary receptors modulates inspiration during expiratory loading; bilateral denervation disrupts these pathways, causing double-lung recipients to inspire more slowly.