Central to peripheral sound propagation in excised lung.

This research examines how sound travels from the trachea to the pleura in different gas-filled, excised lungs from horses and a dog, to better understand acoustic phenomena in the lungs.

Research Methodology

• The researchers used five intact, excised horse lungs and one dog lung in this study. These lungs were inflated with different gases, including helium, air, carbon dioxide, and sulfur hexafluoride.
• The travel time of audible sound from the trachea to the pleura was recorded and their speed was calculated based on the straight-line distance from the carina (a ridge within the trachea where it splits into the two bronchi) to the pleura (the membrane covering the lungs).
• The total airway length was also considered as a predictor of time delay for the sound propagation.
• In one of the lungs, the airway length was partitioned according to airway diameter to observe its impact on time delay prediction.

Findings

• The regression estimates showed that sound travels fastest in helium (775 +/- 60.5 m/s), followed by air (282 +/- 23.5 m/s), carbon dioxide (219 +/- 25.5 m/s), and sulfur hexafluoride (142 +/- 43.5 m/s).
• Except for the case of sulfur hexafluoride, the sound speeds obtained were 15% to 20% less than the free-field speed of sound in each corresponding gas.
• The correlation between total airway length and straight line distance was found to be high (r = 0.94), indicating that both are closely related predictors for time delay. However, it could not be determined unequivocally which predictor was more efficient.
• Partitioning airway length by diameter significantly improved the predictability of time delay, providing better accuracy than an unpartitioned model.
• Free-field sound speeds were observed to be equal to the sound speed in the trachea, whereas, in air-filled airways of diameter 1 to 25 mm, the sound speed was lower (268 +/- 44 m/s).
• The researchers concluded that for at least 90% of the distance from the trachea to the pleura, sound travels in the airways and spends at least 87% of total travel time there.