Abstract
Rationale Dynamic hyperinflation in patients with COPD is usually assessed by inspiratory capacity (IC) manoeuvres with the assumption that maximal activation of the respiratory muscles can be reliably achieved.
Objectives We hypothesized that the neural output to the respiratory muscles during an IC manoeuvre will increase under the condition of exercise or hypercapnia, since these are known to increase neural output from the brainstem to the respiratory muscles.
Methods Nineteen patients with COPD (mean FEV1=29.7±12.5%) and 31 normal subjects were studied. Diaphragm electromyogram (EMGdi) and IC were measured during exhaustive exercise and during inhalation of 8%CO2 with or without hypoxia.
Measurements and Main Results IC changed from 1.78 ± 0.50 L and 1.41 ± 0.40 L at rest to 2.18 ± 0.58 L and 1.29 ± 0.46 L at the end of exercise for normal subjects and COPD patients, respectively. The mean EMGdi during an IC manoeuver increased during exercise from 0.158 ± 0.036 mV and 0.172 ± 0.048mV at rest to 0.203 ± 0.041 mV and 0.220 ± 0.056mV at the end of exercise for normal subjects and patients with COPD, respectively. Both the IC and EMGdi observed during an IC manoeuver during inhalation of 8%CO2 in normal subjects was significantly larger than those breathing room air (2.10±0.80L vs 1.87±0.60L for IC and 0.183±0.038 mV vs 0.154±0.040 mV for EMGdi, p<0.01).
Conclusions We conclude that the neural drive generated by an IC manoeuvre increases during exercise and hypercapnia, leading to potential underestimation of dynamic hyperinflation assessed by IC manoeuvres.
- Copyright ©the authors 2016