The classic model of the respiratory system (RS) is comprised of a Newtonian resistor in series with a capacitor and a viscoelastic unit including a resistor and a capacitor. The flow interruption technique has often been used to study the viscoelastic behavior under constant inspiratory flow rate. To study the viscoelastic behavior of the RS during complete respiratory cycles and to quantify viscoelastic resistance (Rve) and compliance (Cve) under unrestrained conditions, we developed an iterative technique based on a differential equation. We, as others, assumed Rve and Cve to be constant, which concords with volume and flow dependency of model behavior. During inspiration Newtonian resistance (R) was independent of flow and volume. During expiration R increased. Static elastic recoil showed no significant hysteresis. The viscoelastic behavior of the RS was in accordance with the model. The magnitude of Rve was 3.7 +/- 0.7 cmH2O.l-1 x s, i.e., two times R. Cve was 0.23 +/- 0.051 l/cmH2O, i.e., four times static compliance. The viscoelastic time constant, i.e., Cve.Rve, was 0.82 +/- 0.11s. The work dissipated against the viscoelastic system was 0.62 +/- 0.13 cmH2O x 1 for a breath of 0.56 liter, corresponding to 32% of the total energy loss within the RS. Viscoelastic recoil contributed as a driving force during the initial part of expiration.

We measured the pulmonary clearance of technetium-99m-labeled diethylenetriamine pentaacetatic acid (99mTc-DTPA) for 3 h after perturbation of the surfactant system by administration of the detergent dioctyl sodium sulfosuccinate in aerosol. Forty-two rabbits were anesthetized with pentobarbital sodium. Tracheostomies were performed, and the rabbits were mechanically ventilated. Increasing concentrations of detergent (0.125-2%) or vehicle were given for 5 min, and clearance measurements were performed immediately or 60 min after detergent administration. No animals developed respiratory distress. After vehicle, the clearance was monoexponential with a half-life of 153 min. Detergent induced a biexponential clearance with a rapidly clearing additional pool of radioactivity with a half-life of 5-15 min. The relative amount of radioactivity clearing rapidly increased with detergent concentration. The detergent effect was partly reversible. We conclude that detergent induces a biexponential clearance of 99mTc-DTPA by accelerating the transfer of tracer across the alveolocapillary barrier in a proportion of lung units in a dose-related manner.

With the use of positron emission tomography, alveolar ventilation (VA), lung density, and pulmonary blood volume (VB) were measured regionally in eight nonsmokers in the supine posture and one nonsmoker in the prone posture during quiet breathing in a transaxial thoracic section at midheart level. Regional values of alveolar volume (VA) and extravascular tissue volume (VEV) were derived from the inherent relationships between different compartments in the lung. Ratios proportional to gas volume (VA/VEV) and ventilation (VA/VEV) per alveolar unit, respectively, were calculated. No differences between right and left lung were found. Variations in the vertical direction could explain approximately 65% of the total within-group variation in VA, VB, and ln (VA), whereas the corresponding value for horizontal variation was only 3-9% (right lung, supine subjects). Similar gravitational gradients were found in the single prone subject. There was a significant linear correlation between VA and ln (VA). When VA and VA are related to a given number of alveolar units (VEV), the data are consistent with a linear relationship between VA/VEV and VA/VEV, indicating that ventilation might be explained by the elastic properties of lung tissue according to Salazar and Knowles (J. Appl. Physiol. 19: 97-104, 1964). Regional VB was closely associated with the gradient of regional alveolar volume (VA/VEV) (by virtue of weight of blood and competition for space) and therefore, indirectly, closely associated with the vertical gradient of ventilation.

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