0344 Acoustic Measurements may Conveniently Monitor the Degree of Lung Inflation in CPAP Therapy

Abstract Introduction Continuous positive airway pressure (CPAP) is the predominant mode of therapy for obstructive sleep apnea (OSA). Recently, it became clear that excessively high pressure in CPAP overinflates the lung and may have harmful effects, but there is no bedside applicable method to monitor lung inflation and avoid excessively high CPAP. Here we describe a new method based on acoustic delay to monitor lung inflation. Methods A special sound wave was digitally designed such that its frequency smoothly fluctuated between 700 Hz and 900 Hz, and its amplitude fluctuated sinusoidally. This wave was played in loop by a small speaker into a small tube connected to the CPAP's main tubing near the face. Small button microphones were secured on the neck midline and on the lower chest wall and recorded breathing sounds that carried the specially designed sound wave. The time delay between the acoustic signals recorded from the neck and the chest wall was computed from the phase delay in cross-correlational analysis. Results We carried out preliminary measurements in normal human subjects. The acoustic time delay between the neck and the lower chest wall decreased by about 1.5 ms when CPAP increased from 4 to 15 centimeters of water. Conclusion Our preliminary results suggested that acoustic measurements can conveniently monitor the degree of lung inflation from CPAP in OSA treatment. Further measurements in sleeping OSA patients are under way. Support (if any)