EFFECTS OF HYPERINFLATION AND CPAP ON WORK OF BREATHING AND RESPIRATORY-FAILURE IN DOGS

Increased end-expiratory lung volume (EELV) and airway resistance are both characteristic features of obstructive lung disease. Increased EE LV alone loads the respiratory muscles and may cause respiratory failu re, changes that could be reversed by continuous positive airway press ure (CPAP). To study the effects of elevated EELV on respiration witho ut increased airway resistance, we used a mechanical analogue of airwa y closure to increase EELV in six spontaneously breathing anesthetized dogs. Hyperinflation of 0.84 +/- 0.11 liter for 30 min decreased minu te ventilation from 4.8 +/- 0.37 to 3.5 +/- 0.21 l/min and increased a rterial PCo2 from 40.3 +/- 1.5 to 73.2 +/- 8.1 Torr (both P < 0.01). I nspiratory 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 cmH(2)O restored minute ventilation to 4.3 +/- 0.3 l/min and red uced arterial Pco(2) to 54 +/- 6.6 Torr (NS vs. baseline). All measure ments 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 impor tant features of airway obstruction, increase inspiratory work, and ca n cause respiratory failure independent of increased airway resistance . This respiratory failure is reversed by CPAP at the potential expens e of hemodynamic compromise.