Airflow-Generated Sound in a Hollow Canine Airway Cast: MATERIALS AND METHODS

Posted by James

Because of availability and ease of handling, we decided to use a canine model. The goal of the experiment was to make a model of the lungs accurately representing all airway levels from trachea to 1-mm airways and to measure sound generated within the lumina at several airways generations within a range of airflows in the inspirator) and expiratory directions. The sound amplitudes could then be related to the airway sizes at each flow rate and to flow rate at each airway size.

Model Construction

The intact health) lungs and trachea of a 25-kg mongrel dog were excised after the dog had been killed after an unrelated experiment. The specimen was thoroughly washed, suspended by the trachea, and connected to an air hose in parallel with a water manometer. Air pressure of 20 cm was then applied at the trachea. This resulted in a constant flow through the lung, with the air apparently exiting through pores in the pleura, since we found it unnecessary to make additional holes in the pleura. After 48 hours the lungs had completely dried in inflation, assuming the appearance and consis­tency of Styrofoam. The dried lungs were removed from the air hose and immersed in an ice-water bath, taking precautions to avoid admission of water into the trachea. After two hours of cooling, the airways were filled with melted Ostalloy 117 (Arconium Corp.) at a temperature of 50°C. Ostalloy 117 is a metal alloy, similar to Wood s metal, but it melts at 47°C (117°F). The lung was chilled prior to filling to help avoid entry of the metal into the alveolar spaces. After waiting 24 hours, the preparation, now filled with solid metal, was immersed in a 20 percent solution of NaOIl for three days to remove the pulmonary tissue by corrosion. This resulted in a negative cast that was then pruned to remove all casts of acini and airways smaller than 1 mm in diameter.

Once the negative cast was pruned, it was coated with a flexible film to serve as the positive cast when the metal was subsequently melted and removed. In previous attempts, dipping the cast into a plastic or Silastic solution did not yield satisfactory results because the liquid adhered in large agglomerations to groups of smaller airways. To avoid this, we prepared a thin solution of Silastic and painted it onto the negative cast in several coats until the coating was approximately 0.5 mm thick. When the coating was cured, the extreme tips of the small airway branches were clipped off”, and a radiograph was taken of the model to permit measurement of the luminal diameters. The model was then placed into a hot-water bath and worked by hand to melt and remove all of the metal core. The result was a complete positive cast of the airways down to those of 1 mm in diameter (Fig 1).
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Figure 1. A (top). Radiograph of airway model before removal of Ostalloy 117 core. В (bottom). Completed airway model.

Equipment

The positive cast was suspended from the trachea which was fitted to a No. 2 Fleisch pneumotachograph using a conical adapter with smooth internal contours designed to prevent gas turbulence. The pneumotachograph wits connected to either a compressed air or vacuum source through a series of tubes and a noise-attenuating system (Fig 2). The pneumotachograph played dual roles of meas­uring airflow and of attenuating turbulence at the entry point to the trachea. Preliminary experiments using a probe-tipped microphone during airflow of 2 L/s in the inspiratory direction revealed nearly silent flow immediately below the pneumotachograph and progres­sively more sound as the microphone was moved downstream (presumably as turbulent flow was established).

Sounds were picked up by an Electret condenser microphone (flat in free-field frequency response between 10 and 10,000 Hz) adapted to a glass conical probe tip, 8 mm in diameter at the microphone end and 0.5 mm at the tip. The probe was 10 mm in length. The small size of the probe assured that its resonant frequency would be over 30,000 Hz, well al>ove the frequencies anticipated (preliminary experiments revealed no detectable acous­tic energy above 2,500 Hz at any level in the model). To record the sounds, the microphone was held in a clamp, and the tip was introduced through a puncture hole in the wall of the airway, just deep enough to enter the lumen. Apcalis Oral Jelly

FIGURE 2. Experimental setup. Large cylinder is filled with sound-absorbing foam.

The sounds were amplified, low-pass filtered at 1,600 Hz, and recorded on an FM tape recorder. Later, they were digitized at a rate of 5,000 Hz by a 14-bit waveform analyzer (Data Precision model 6000A), stored in 1,024 point records, and submitted to a fast Fourier transformation from which the total amplitudes (derived from the integrated magnitude spectrum) were determined. The ratio between the maximal sound amplitude (measured in the trachea, in the expiratory direction) and the noise floor (at zero airflow) was 35 dB.

Sounds were recorded during zero airflow and at 0.5, 1.0, 1.5, 2.0, and 2.5 L/s in the inspiratory and expiratory directions. The complex branching limited the number of airways that could be accessed without distorting the model. Nevertheless, we were able to acquire 341 measurements in airways of the following diameters: 2.0, 2.5, 3.0, 3.5, 4.0, 5.0, 6.0, 7.0, 8.0, 10.0, 14.0, 15.0, 16.0, and 19.0 mm (the distal trachea). Because airflow is directly related to cross-sectional area, this was calculated and used for the analyses.
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