Changes in regional distribution of lung sounds as a function of positive end-expiratory pressure

Abstract

Introduction: Automated mapping of lung sound distribution is a novel area of interest currently investigated in mechanically ventilated, critically ill patients. The objective of the present study was to assess changes in thoracic sound distribution resulting from changes in positive end-expiratory pressure (PEEP). Repeatability of automated lung sound measurements was also evaluated.

Methods: Regional lung sound distribution was assessed in 35 mechanically ventilated patients in the intensive care unit (ICU). A total of 201 vibration response imaging (VRI) measurements were collected at different levels of PEEP between 0 and 15 cmH2O. Findings were correlated with tidal volume, oxygen saturation, airway resistance, and dynamic compliance. Eighty-two duplicated readings were performed to evaluate the repeatability of the measurement.

Results: A significant shift in sound distribution from the apical to the diaphragmatic lung areas was recorded when increasing PEEP (paired t-tests, P < 0.05). In patients with unilateral lung pathology, this shift was significant in the diseased lung, but not as pronounced in the other lung. No significant difference in lung sound distribution was encountered based on level of ventilator support needed. Decreased lung sound distribution in the base was correlated with lower dynamic compliance. No significant difference was encountered between repeated measurements.

Conclusions: Lung sounds shift towards the diaphragmatic lung areas when PEEP increases. Lung sound measurements are highly repeatable in mechanically ventilated patients with various lung pathologies. Further studies are needed in order to fully appreciate the contribution of PEEP increase to diaphragmatic sound redistribution.

Figure 1

Schematic diagram describing the elements of the system. The patient lies on the acoustic sensor array and the flow sensor is inserted in the breathing circuit. The vibration response imaging (VRI) system collects acoustic information simultaneously from the sensor array and pressure and flow waveforms from the ventilator.

Figure 2

An example of acoustic data as displayed for a recording obtained from a 77-year-old male with myasthenia gravis. A representative peak-inspiratory image (left panel); synchronized sound energy graph and ventilator airway pressure and flow waveforms (middle panel); sound energy distribution in the six lung regions as automatically provided by the software in percentage of weighted pixel count (right panel). VRI = vibration response imaging.

Figure 3

Mean ± standard deviation of sound energy distribution in 34 mechanically-ventilated patients recorded at three levels of PEEP (0, 5 and 10 cmH₂O). Significant P values are indicated (paired t-tests). LL = lower left; LR = lower right; ML = middle left; MR = middle right; PEEP = positive end-expiratory pressure; UL = upper left; UR = upper right.

Figure 4

Individual sound energy distribution in diaphragmatic lung areas in 34 mechanically-ventilated patients recorded at PEEP levels 0 and 10 cmH₂O. Sound energy distribution increased from 17 ± 11% to 23 ± 12% (P < 0.0001) in (a) 26 'responder' patients and decreased from 30 ± 17% to 27 ± 17% (P < 0.001) in (b) eight 'non-responder' patients. PEEP = positive end-expiratory pressure.

Figure 5

Representative frames (or maps) at peak-inspiratory flow obtained from five individual patients at PEEP levels 0, 5, 10 and 15 cmH₂O. (a) A 74-year-old female with respiratory failure. (b) A 19-year-old male with right pneumothorax. (c) A 83-year-old male with sternal wound infection. (d) A 77-year-old male with myasthenia gravis. (e) A 57-year-old male with acute pancreatitis. PEEP = positive end-expiratory pressure.

Figure 6

Representative frames (or maps) at peak-inspiratory flow obtained from two patients ventilated with different ventilator settings. (a) A 72-year-old female with chronic obstructive pulmonary disease recorded at positive end-expiratory pressure (PEEP) level of 5 cmH₂O and at two levels of tidal volume (VT; left = 330 mL, right = 560 mL); (b) A 24-year-old male with bilateral chest contusion recorded at PEEP level of 7 cmH₂O, VT of 600 mL and at two levels of respiratory rate (RR) and inspiratory/expiratory ratio (i:e; left: i:e = 2:3 and RR = 12 breaths/minute, right: i:e = 5:2 and RR = 16 breaths/minute). TL = total left lung; TR = total right lung.