Correlation between lung sound distribution and functional residual capacity: preliminary findings

Introduction

Vibration response imaging (VRI) is a bedside lung sound monitoring system. The VRI measurement has been proven sensitive to changes in ventilator settings, including changes in the mode of mechanical ventilation or in positive end-expiratory pressure (PEEP). In the present study, we correlate lung sound distribution with functional residual capacity (FRC).

Methods

Thirty-nine lung sound measurements were performed on seven mechanically ventilated critically ill patients at different levels of PEEP before and after recruitment maneuver. The FRC was obtained for each measurement. Lung sound distribution was monitored using the sound distribution index (SDI), an index revealing the lung sound distribution at peak inspiration and computed from the percentages of lung sound distribution in four lung segments (right lower (RL), left lower (LL), right upper (RU), left upper (LU)) as SDI = 100 - abs(RL + LL - RU - LU)/2 - abs(RU + LU - RL - LL)/2 (range 0% to 100%). P values were obtained using the Wilcoxon two-sample test.

Results

Significantly increased mean (± SD) SDI was registered in cases with higher FRC. In two out of five measurements with FRC above 2.9 l, a low SDI was obtained (~70%).

Conclusion

Lung SDI significantly increased with FRC. At very high volumes, a decreased SDI may indicate possible hyperinflation.