Central to peripheral sound propagation in excised lung

Area of Science:

• Respiratory Physiology
• Acoustics
• Biomechanical Engineering

Background:

• Understanding sound propagation within the respiratory system is crucial for diagnosing lung conditions.
• Previous research has focused on sound transmission in the larger airways, but less is known about its speed and behavior in the smaller airways and lung tissue.

Purpose of the Study:

• To measure the time it takes for audible sound to travel from the trachea to the pleura in excised animal lungs.
• To determine the speed of sound in different gases within the respiratory tract.
• To assess the influence of airway geometry on sound travel time.

Main Methods:

• Excised horse and dog lungs were inflated with various gases (helium, air, carbon dioxide, sulfur hexafluoride).
• The time for sound to travel from the trachea to the pleura was measured.
• Regression analyses were used to estimate sound speed and assess predictors of time delay, including straight-line distance and partitioned airway lengths.

Main Results:

• Sound speeds at total lung capacity (TLC) varied by gas: helium (775 m/s), air (282 m/s), carbon dioxide (219 m/s), and sulfur hexafluoride (142 m/s).
• These speeds were generally 15%-20% lower than free-field speeds, except for sulfur hexafluoride.
• Partitioning airway length by diameter significantly improved time delay prediction, indicating airway geometry is important.
• Sound speed in the trachea matched free-field speed, while in air-filled airways (1-25 mm diameter) it was 268 m/s.

Conclusions:

• The initial sound reaching the lung surface travels predominantly within the airways, covering at least 90% of the distance and consuming at least 87% of the total travel time.
• Airway structure significantly influences sound propagation time within the lungs.
• These findings have implications for understanding lung acoustics and developing new diagnostic tools.