Effect of expiratory load on neural inspiratory drive.

Background: Neural respiratory drive is usually measured during inspiration, even in patients with chronic obstructive pulmonary disease (COPD) in whom the primary physiological deficit is expiratory flow limitation. The purpose of the study was to test the hypothesis that inspiratory muscle neural respiratory drive could be used to assess expiratory load.

Methods: Ten healthy young men, (26 ± 4) years old, were asked to expire through a tube immersed in water where an expiratory load was required. The load was judged by the depth of the tube in water and the different loads (0 cmH2O, 10 cmH2O, 20 cmH2O and 30 cmH2O) were randomly introduced. Each expiratory load lasted for 3 - 5 minutes and inspiration was unimpeded throughout. Diaphragm electromyogram (EMG) and transdiaphragmatic pressure were recorded by a catheter with 10 metal coils and two balloons. Incremental cycle exercise with and without an expiratory load at 30 cmH2O was also performed.

Results: Neural drive during expiratory loaded breathing was larger than during unloaded breathing but neural drive did not increase proportionally with increasing expiratory load; neural drive during expiratory loading at 0, 10, 20 and 30 cmH2O was (10.1 ± 3.1) µV, (16.7 ± 7.3) µV, (18.4 ± 10.7) µV and (22.9 ± 13.2) µV, respectively. Neural drive as a percentage of maximum at the end of exercise with or without load was similar ((57.4 ± 11.0)% max vs. (62.7 ± 16.4)% max, P > 0.05).

Conclusion: Neural respiratory drive measured at inspiration does not accurately quantify expiratory load either at rest or during exercise.