Increased spatial distribution of airflow in lungs with low-level pressure support ventilation compared with maintenance ventilation

Introduction

Vibration response imaging (VRI) is a novel technology that utilizes sophisticated software and surface skin sensors placed on the back to record, analyze and display vibrations as a non-invasive measure of lung ventilation.

Hypothesis

Compared with assist volume control ventilation (AVC), pressure support ventilation (PSV) will result in a greater spatial distribution of lung airflow.

Methods

We performed serial VRI during maintenance AVC and immediately following initiation of a spontaneous breathing trial using low-level PSV in 26 mechanically ventilated patients. Recordings were performed over 12–20 s periods of respiration. Respiratory cycles free of noise or motion artifacts were chosen for analysis, and images at mid-inspiration were analyzed. Areas of images were calculated digitally using the program ImageJ. The areas of right and left lung measured at mid-inspiration were summed, and compared, AVC vs PSV. Statistical analysis was performed using a t test and the t distribution.

Results

The tidal volume (VT) was 534.8 ± 69.4 ml for AVC and decreased to 407.6 ± 152.1 ml for PSV (P = 0.00015). The mean areas of both lungs were 68.75 ± 11.06 and 71.57 ± 10.50 (mean in kilopixels ± SD) in the AVC and PSV images, respectively (P = 0.00985). There was a mean increase of 4.73 ± 9.09% in the areas of the lungs during PSV compared with AVC (P = 0.039).

Conclusions

Despite a lower VT, PSV (compared with ACV) produced a greater spatial distribution of lung airflow. Possible mechanisms of this PSV-associated increase are the increase in patient-generated negative intrapleural pressure during inspiration and better synchronization of patient-negative pressure with positive pressure from the ventilator. VRI allows a non-invasive quantitation of airflow distribution during different modes of mechanical ventilation.