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 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.