Regional distribution of acoustic-based lung vibration as a function of mechanical ventilation mode

Abstract

Introduction: There are several ventilator modes that are used for maintenance mechanical ventilation but no conclusive evidence that one mode of ventilation is better than another. Vibration response imaging is a novel bedside imaging technique that displays vibration energy of lung sounds generated during the respiratory cycle as a real-time structural and functional image of the respiration process. In this study, we objectively evaluated the differences in regional lung vibration during different modes of mechanical ventilation by means of this new technology.

Methods: Vibration response imaging was performed on 38 patients on assist volume control, assist pressure control, and pressure support modes of mechanical ventilation with constant tidal volumes. Images and vibration intensities of three lung regions at maximal inspiration were analyzed.

Results: There was a significant increase in overall geographical area (p < 0.001) and vibration intensity (p < 0.02) in pressure control and pressure support (greatest in pressure support), compared to volume control, when each patient served as his or her own control while targeting the same tidal volume in each mode. This increase in geographical area and vibration intensity occurred primarily in the lower lung regions. The relative percentage increases were 28.5% from volume control to pressure support and 18.8% from volume control to pressure control (p < 0.05). Concomitantly, the areas of the image in the middle lung regions decreased by 3.6% from volume control to pressure support and by 3.7% from volume control to pressure control (p < 0.05). In addition, analysis of regional vibration intensity showed a 35.5% relative percentage increase in the lower region with pressure support versus volume control (p < 0.05).

Conclusion: Pressure support and (to a lesser extent) pressure control modes cause a shift of vibration toward lower lung regions compared to volume control when tidal volumes are held constant. Better patient synchronization with the ventilator, greater downward movement of the diaphragm, and decelerating flow waveform are potential physiologic explanations for the redistribution of vibration energy to lower lung regions in pressure-targeted modes of mechanical ventilation.

Figure 1

An example of a normal vibration response image. A maximal energy frame from a vibration response image recording of a healthy, 30-year-old, male non-smoker is shown.

Figure 2

Selection of maximal inspiratory frames for analysis. Examples of frame selection in various vibration response imaging (VRI) waveform patterns are shown. The dot on the VRI waveform represents the area from which the maximal energy frame was chosen for analysis. (a) When inspiratory and expiratory vibrations are clearly separated, the maximal energy frame during inspiration (first peak) is chosen. (b) When nspiratory and expiratory vibrations merge into one peak, the highest energy frame is chosen. (c) When inspiratory and expiratory vibrations form a plateau, first frame at zero slope is chosen. (d) When no clear separation exists between inspiratory and expiratory vibrations, and the frame nearest the inflection point of the shoulder is chosen.

Figure 3

Vibration response images on various modes of mechanical ventilation. Maximal energy frames extracted from recordings of a 73-year-old mechanically ventilated female with respiratory failure secondary to pancreatitis are shown. Chest radiography reported pleural fluid in both lungs. Assist volume control, assist pressure control, and pressure support are shown from left to right. L, left lung; R, right lung.

Figure 4

Separation of inspiratory and expiratory signals in a vibration response imaging (VRI) waveform. Separation of inspiratory and expiratory signals produced by application of an inspiratory hold during the second breath in a mechanically ventilated patient is shown. Flow was sampled directly from the ventilator and synchronized with VRI. The three waveforms depict pressure, flow, and vibration as a function of time. Exp., expiratory; Insp., inspiratory.

Figure 5

Mean area and vibration among individual patients. Mean areas of each patient (a) and mean vibration intensity values of each patient (b) on assist volume control (VC), assist pressure control (PC), and pressure support (PS) are presented.

Figure 6

Total area and vibration intensity among modes. Mean total areas (a) and mean total vibration intensity values (b) on assist volume control (VC), assist pressure control (PC), and pressure support (PS) are presented. Total area differed significantly between VC and PC as well as between VC and PS. Data are presented as mean ± standard error of the mean.

Figure 7

Distribution of area and vibration intensity between modes. Percentage changes in total areas (a) and percentage changes in total vibration intensity (b) between assist volume control (VC), assist pressure control (PC), and pressure support (PS) are shown. Percentage change in total area differed significantly between VC and PC modes as well as between VC and PS. VC to PC and VC to PS showed a significant difference in percentage change in total vibration intensity between modes.

Figure 8

Redistribution of area and vibration intensity. Relative percentage changes in area (a) and relative percentage changes in vibration intensity (b) in different lung regions between assist volume control (VC), assist pressure control (PC), and pressure support (PS) are presented as mean percentage changes ± standard error. Gray represents VC-PC, white represents VC-PS, and black represents PC-PS. The asterisks indicate p values of less than 0.05, considered to be statistically significant. The relative percentage change in area in the middle and lower regions changed significantly from VC to PC and PS modes (a). A difference in relative percentage change in vibration between VC and PS was observed in the lower lung region.

Figure 9

The effect of tidal volume/airflow on vibration intensity. There is a strong correlation and linear relationship between tidal volume and lung vibration intensity.