Correlation of lung vibration and airflow

Introduction

Airflow into a mechanically ventilated patient is easily measured in the inspiratory limb of the ventilator. Regional airflow inside the lungs, up to this point, is a black box. Vibration response imaging (VRI) is a novel technology that measures vibration energy from the lungs to create a real-time structural and functional image of regional vibration during respiration. Sophisticated surface skin sensors are placed on the subject's back to record, analyze and display vibrations noninvasively. Our goal was to assess the correlation of vibration measured at the chest wall with airflow into the lungs.

Methods

To assess the effect of constant inspiratory flow on lung vibration, VRI was performed on a mechanically ventilated patient on assist volume control, and airflow in the tubing was recorded concurrently. To assess the effect of increasing flow rates on lung vibration, healthy subjects were recorded several times with VRI while taking tidal volumes of 200–1,300 ml at the same respiratory rate. The inspiratory tidal volume was recorded.

Results

In the mechanically ventilated patient, when there is minimal flow, the vibration was at its lowest. When flow begins at the ventilator, the vibration measured over the lungs increases and when the flow stops, the vibration decreases. An inspiratory hold was performed to separate inspiratory from expiratory vibrations (Figure 1). As the subject takes increasing tidal volumes, the vibration during the breath cycle increases linearly. A sample subject is shown in Figure 2 (R² = 0.81).

Figure 1

(abstract P203)

Figure 2

(abstract P203)

Conclusion

Vibration measured using VRI correlates with lung airflow. Given the difficulty in assessing airflow in the lungs, measuring lung vibration could potentially serve as a surrogate for regional lung airflow.