The sound speed through the lungs is influenced by the lungs' material properties and density. Probing the lungs of intubated infants with sound may provide a means to identify changes in lung structure and composition that occur as a result of prolonged ventilation therapy. To examine the feasibility of this approach we measured the sound transit times in 6 ventilator dependent infants (birth weight 640-4960g, age 12-81 days, study weight 990-5140g) suffering from early or advanced chronic lung disease. A piezoelectric ceramic disc mounted at the entrance of the endotracheal tube was used to produce brief 3 KHz pulses that propagate into the lungs via the tube. The onset, duration, and intensity of the pulse was registered with an electret microphone at the tube entrance. A lightweight quartz accelerometer secured on the chest at the mid-thoracic level registered sound transmission across the lungs and chest wall. Transit times for the pulse to propagate from the tube entrance to the chest surface was estimated with spectral averaging and cross-correlation. The estimated transit times varied between 1.47 and 2.27 ms with an average of 1.72 ms (0.28 SD). These times correspond roughly to sound speeds of 28 (9) meters/s. The transit times correlated poorly with the body weight or chest circumference but correlated well (r= +0.90) with patient age. These results suggest that changes in the lungs material properties that occur with prolonged ventilation therapy may be a predominant factor determining parenchymal sound speed in infants with chronic lung disease.

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Durand, M., Sullivan, K., McEvoy, C. et al. TRANS-THORACIC SOUND SPEED IN VENTILATOR DEPENDENT INFANTS: A PILOT STUDY. Pediatr Res 32, 636 (1992).

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