THE EFFECT OF CONTINUOUS POSITIVE AIRWAY PRESSURE VERSUS POSITIVE END-EXPIRATORY PRESSURE ON THE DIAPHRAGM

Continuous positive airway pressure (CPAP) and positive end-expiratory pressure (PEEP) both increase lung volume and hence may compromise diaphragm function. However, the effects of these two positive airway pressure modalities on inspiratory work of breathing are conflicting. In this study, we compared the effect of CPAP versus PEEP on diaphragm function in spontaneously breathing anesthetized dogs. Eight sodium pentobarbital-anesthetized dogs were randomly exposed to various levels of CPAP and PEEP. Measurements of diaphragmatic shortening, transdiaphragmatic pressure swings, and diaphragmatic electromyogram (EMG) were made. The change in lung volume and diaphragm length was similar at equivalent airway pressures during PEEP or CPAP. Therefore, expiratory muscle recruitment in the two conditions was equivalent. However, tidal diaphragmatic EMG and transdiaphragmatic pressure swings increased markedly during PEEP compared with CPAP. At a PEEP of 18 cm H2O, crural and costal EMG activities were 185% ± 16% and 163% ± 8% of control, respectively, whereas during CPAP the EMG activity was 66% ± 11 % of control for both the costal and the crural diaphragms (±SE). During PEEP, the duration of neural inspiration (TIEMG) was greater than the duration of inspiration as measured by airflow (TIV). On the other hand, during CPAP, TIEMG was less than TIV. We conclude that although expiratory muscle recruitment is comparable and tidal volume greater during CPAP, the inspiratory activation of the diaphragm decreases with CPAP but increases markedly with PEEP. © 1991.