Pneumothorax effects on pulmonary acoustic transmission

Abstract

Pneumothorax (PTX) is an abnormal accumulation of air between the lung and the chest wall. It is a relatively common and potentially life-threatening condition encountered in patients who are critically ill or have experienced trauma. Auscultatory signs of PTX include decreased breath sounds during the physical examination. The objective of this exploratory study was to investigate the changes in sound transmission in the thorax due to PTX in humans. Nineteen human subjects who underwent video-assisted thoracic surgery, during which lung collapse is a normal part of the surgery, participated in the study. After subjects were intubated and mechanically ventilated, sounds were introduced into their airways via an endotracheal tube. Sounds were then measured over the chest surface before and after lung collapse. PTX caused small changes in acoustic transmission for frequencies below 400 Hz. A larger decrease in sound transmission was observed from 400 to 600 Hz, possibly due to the stronger acoustic transmission blocking of the pleural air. At frequencies above 1 kHz, the sound waves became weaker and so did their changes with PTX. The study elucidated some of the possible mechanisms of sound propagation changes with PTX. Sound transmission measurement was able to distinguish between baseline and PTX states in this small patient group. Future studies are needed to evaluate this technique in a wider population.

Keywords: acoustic; pneumothorax; transmission.

Fig. 1.

A: experimental setup showing sensor position, and sound source and data acquisition equipment connections. B: raw and smoothed power spectral density for a typical case of pneumothorax. Note the smooth spectra have conserved the main spectral features.

Fig. 2.

A: spectra of sounds transmitted from the mouth to chest wall of all study subjects for the control (solid line) and pneumothorax (PTX) (dashed line) states. A spectral drop in acoustic transmission due to PTX can be seen at certain frequencies. B: average spectra of transmitted sounds in the control (solid line) and PTX (dashed line) states. Error bars show the 95% confidence interval.

Fig. 3.

A: spectral drop in acoustic transmission with PTX for all study participants. There was a relatively consistent drop in acoustic energy (P < 0.01, Wilcoxon signed rank sum test) for frequencies >300 Hz. There appears to be a smaller change in amplitude in the 0–300 Hz range. B: average drop in transmitted spectra with PTX for all study participants. Error bars show the 95% confidence interval. It appears that the drop in spectral energy with PTX is most pronounced in the 400–600 Hz range. The drop in energy was smaller outside this frequency range.

Fig. 4.

Difference between the control and PTX power spectral density in the 400–600 Hz frequency range. A: maximum drop with PTX. B: mean drop with PTX. C: maximum increase with PTX. One can observe a consistent spectral drop with PTX and no spectral increase in this frequency range because all data points in A and B are positive, whereas those in C are negative.

Fig. 5.

The acoustic spectrum was divided into three bands. Band 1 = 50–250 Hz; band 2 = 400–600 Hz; band 3 = 1,300–1,500 Hz. The energy ratio between bands 1 and 2 (ER21 = energy in band 2 ÷ energy in band 1); and that between bands 2 and 3 (ER23 = energy in band 2 ÷ energy in band 3) were calculated for the control and PTX states. A: ER21 and ER23; dashed lines connect data points for the same subject. There is a trend toward decreased ratios with PTX. B: change in ER21 and ER23 with PTX. All data fell in the third quadrant, which corresponds to a decrease in both energy ratios. The borderline case (the case where E23 change is close to zero) corresponds to the case with the relatively smaller PTX air pocket.