Lung sound analysis to detect recruitment processes during mechanical ventilation

Introduction

The optimal setting of positive end-expiratory pressure (PEEP) during mechanical ventilation in the ICU is still an open problem [1]. Recruitment processes are important influence factors that are difficult to measure in patients [2, 3]. It is proposed to use lung sound analysis to identify recruitment during mechanical ventilation.

Methods

A special low-noise microphone and amplifier has been developed (Figure 1, left) and integrated into a standard stethoscope. A series of experiments were performed to obtain sound data from isolated animal lungs. A standard protocol was implemented using the Evita4Lab equipment (Dräger Medical, Lübeck, Germany). After preparation of the lung, a low-flow manoeuvre was performed as a first inflation to open the atelectatic lung. Visual inspection allows us to identify the amount of recruitment. At most two more low-flow manoeuvres were performed while recording the lung sounds until the whole lobe below the stethoscope was opened. Finally, a PEEP wave manoeuvre was used to record normal breaths at different PEEP levels starting from zero PEEP up to a peak pressure of 50 mbar.

Figure 1

(Left) Isolated lung during experiment. (Middle) Crackle as a wave form. (Right) Power spectrum.

Results

Crackle sounds (Figure 1, right) could be identified and analysed in the recorded sound data offline using Matlab (Mathworks, Natick, MA, USA) for the analysis and visualization. With a window technique, the power of the sound signal in a specific frequency range for crackles (700–900 Hz) was determined. At the beginning of the low-flow manoeuvre, trains of crackles were found that increased in intensity with pressure. At higher pressure levels the crackle intensity decreased, especially in the second and third manoeuvres.

Conclusion

Sound analysis in isolated lungs can reliably detect increasing crackle activities that seem to correlate with visually observable opening of atelectasis.