Journal of Applied Physiology, Vol 77, Issue 2 819-827, Copyright © 1994 by American Physiological Society
Effects of hyperinflation and CPAP on work of breathing and respiratory failure in dogs
E. Shade, Y. Kawagoe, R. G. Brower, S. Permutt and H. E. Fessler
Division of Pulmonary and Critical Care Medicine, Johns Hopkins Asthma and Allergy Center, Baltimore, Maryland 21224.
Increased end-expiratory lung volume (EELV) and airway resistance are both
characteristic features of obstructive lung disease. Increased EELV alone
loads the respiratory muscles and may cause respiratory failure, changes
that could be reversed by continuous positive airway pressure (CPAP). To
study the effects of elevated EELV on respiration without increased airway
resistance, we used a mechanical analogue of airway closure to increase
EELV in six spontaneously breathing anesthetized dogs. Hyperinflation of
0.84 +/- 0.11 liter for 30 min decreased minute ventilation from 4.8 +/-
0.37 to 3.5 +/- 0.21 l/min and increased arterial PCO2 from 40.3 +/- 1.5 to
73.2 +/- 8.1 Torr (both P < 0.01). Inspiratory work per breath increased
3-fold, work per liter increased 3.7-fold, and work per minute increased
2.8-fold (all P < 0.01). CPAP at 15 cmH2O restored minute ventilation to
4.3 +/- 0.3 l/min and reduced arterial PCO2 to 54 +/- 6.6 Torr (NS vs.
baseline). All measurements of inspiratory work were also restored to
baseline, but cardiac output was reduced (baseline 3.09 +/- 0.36,
hyperinflation 2.71 +/- 0.36, hyperinflation + CPAP 1.94 +/- 0.29 l/min; P
< 0.05, baseline vs. hyperinflation + CPAP). We conclude that increases
in EELV mimic important features of airway obstruction, increase
inspiratory work, and can cause respiratory failure independent of
increased airway resistance. This respiratory failure is reversed by CPAP
at the potential expense of hemodynamic compromise.
