Validation of the airflow perturbation device and the pressure flow characteristics of excised sheep lungs

Diagnosis of obstructive lung diseases is generally associated with high airway resistance, resulting in higher work of breathing. One non-invasive instrument designed to measure real time airway resistance in humans and animals is the Airflow Perturbation Device (APD). A study was conducted to validate the APD by mechanically ventilating twenty excised matured sheep lungs (mean animal weight 110 lb.) in a custom built respiratory chamber. Alveolar capsules were attached on the excised lung surface to directly determine the airway resistance by measuring the alveolar pressures. As a comparison, the Forced Oscillation technique (FO) was also used to determine lung airway resistance. Resistance of the excised lungs determined through the APD was compared against those obtained directly with the alveolar capsules and the FO technique. Numerous pulmonary catheters were also implanted in the excised lungs using a bronchoscope to measure intra-lung pressures and to determine partial airway resistance. Sheep lung casts were prepared to confirm the precise locations of the pulmonary catheters implanted in the lungs. Several methods were explored to determine energy transfer beyond the lung surface as result of APD perturbations or forced oscillations induced at the mouth. The methods pursued included affixing miniature accelerometers, video-graphic imaging of the lung surface, and frequency suppression of a differential pressure transducer. Attempts were also made to generate iso-volume pressure flow characteristics of the excised lungs. The airways resistance (Mean +/- SE) measured by the APD, alveolar capsules and the FO technique were 3.09 +/- 0.127, 1.83 +/- 0.127 and 2.50 +/- 0.425 cmH2O.sec/L, respectively. It is concluded that the APD perturbations induced at the mouth are transmitted beyond the alveolar regions or the lung surface into the respiratory chamber. Compared to the alveolar capsule resistance, the higher resistance indicated by the APD and the FO technique suggest the measurement of pulmonary resistance and total respiratory resistance respectively. It is also concluded that the FO technique transfers about two times more energy beyond the lung surface as the APD.