Effect of PEEP on breath sound power spectra in experimental lung injury

BACKGROUND Acute lung injury (ALI) is known to be associated with the emergence of inspiratory crackles and enhanced transmission of artificial sounds from the airway opening to the chest wall. Recently, we described the effect of ALI on the basic flow-induced breath sounds, separated from the crackles. In this study, we investigated the effects of positive end-expiratory pressure (PEEP) on these noncrackling basic lung sounds augmented during ALI. METHODS Lung sounds were recorded in six anesthetized, intubated, and mechanically ventilated pigs at three locations bilaterally on the chest wall. Recordings were obtained before and after induction of lung injury with oleic acid and during application of incremental positive end-expiratory pressure. RESULTS Oleic acid injections caused severe pulmonary edema predominately in the dependent-lung regions. Inspiratory spectral power of breath sounds increased in all lung regions over a frequency band from 150 to 1,200 Hz, with further power augmentation in dependent-lung areas at higher frequencies. Incremental positive end-expiratory pressure reversed the spectral power augmentation seen with ALI, reducing it to pre-injury levels with PEEP of 10 and 15 cmH2O in all lung regions at all frequencies. The application of positive end-expiratory pressure to normal lungs attenuated spectral power slightly and only over a band from 150 to 1,200 Hz. CONCLUSIONS We confirm a gravity-related spectral amplitude increase of basic flow-induced breath sounds recorded over lung regions affected by permeability-type pulmonary edema and show that such changes are reversible by alveolar recruitment with PEEP.