Journal of Applied Physiology, Vol 59, Issue 6 1947-1954, Copyright © 1985 by American Physiological Society
Respiratory load compensation in chronic airway obstruction
M. Lopata, E. Onal and G. Cromydas
To assess respiratory neuromuscular function and load compensating ability
in patients with chronic airway obstruction (CAO), we studied eight stable
patients with irreversible airway obstruction during hyperoxic CO2
rebreathing with and without a 17 cmH2O X l-1 X s flow-resistive
inspiratory load (IRL). Minute ventilation (VE), transdiaphragmatic
pressure (Pdi), and diaphragmatic electromyogram (EMGdi) were monitored.
Pdi and EMGdi were obtained via a single gastroesophageal catheter with
EMGdi being quantitated as the average rate of rise of inspiratory (moving
average) activity. Based on the effects of IRL on the Pdi response to CO2
[delta Pdi/delta arterial CO2 tension (PaCO2)] and the change in Pdi for a
given change in EMGdi (delta Pdi/delta EMGdi) during rebreathing, two
groups could be clearly identified. Four patients (group A) were able to
increase delta Pdi/delta PaCO2 and delta Pdi/delta EMGdi, whereas in the
other four (group B) the IRL responses decreased. All group B patients were
hyperinflated having significantly greater functional residual capacity
(FRC) and residual volume than group A. In addition the IRL induced percent
change in delta Pdi/delta PaCO2, and delta VE/delta PaCO2 was negatively
correlated with lung volume so that in the hyperinflated group B the higher
the FRC the greater was the decrease in Pdi response due to IRL. In both
groups the greater the FRC the greater was the decrease in the ventilatory
response to loading. Patients with CAO, even with severe airways
obstruction, can effect load compensation by increasing diaphragmatic force
output, but the presence of increased lung volume with the associated
shortened diaphragm prevents such load compensation.
