Alveolar inflation during generation of a quasi-static pressure/volume curve in the acutely injured lung

Abstract

The normal lung does not increase in volume by simple isotropic (balloon-like) expansion of alveoli, as evidenced by the horizontal (no change in alveolar area with increases in airway pressure) pressure/area curve. After surfactant deactivation, the alveolar inflation pattern becomes very complex, with each alveolar type (I, II, and III) displaying a distinct pattern. None of the alveolar pressure/area curves directly parallel the quasi-static lung pressure/volume curve. Of the 16, only one type III atelectatic alveolus recruited at the first inflection point and only five recruited concomitant with the second inflation point, suggesting that neither inflection point was due to inflection point was due to massive alveolar recruitment. Thus, the components responsible for the shape of the pressure/volume curve include all of the individual alveolar pressure/area curves, plus changes in alveolar duct and airway size, and the elastic forces in the pulmonary parenchyma and the chest wall.

Total number and mean size of alveoli in mammalian lung estimated using fractionator sampling and unbiased estimates of the Euler characteristic of alveolar openings

“…A comparative study of cat, dog and rat lungs filled with fluid and air indicate that the density of interalveolar surface forces is the major determinant of lung distensibility in the air-filled state (Haber et al, 1983). In vivo microscopy of a small portion of the ventilatory unit in normal pigs showed little change in alveolar size over most of the pressure volume curve (2-18 mm Hg) (Schiller et al, 2003). In contrast to these observations at the local ventilatory unit level, the distribution of alveolar size in air-filled, frozen dog lungs positioned head up showed a gradient of decreasing size from upper to lower lungs (Glazier et al, 1967).…”

Ventilating the Lungs Safely: What’s New for Infants and Children?

“…However, recent studies demonstrated that alveolar recruitment is a continuum phenomenon that occurs along the entire inflation limb of the P-V curve. A superior correlation was found between alveolar de-recruitment and the deflation limb of the P-V curve [5,6]. This finding may be explained by ventilation heterogeneity that has been observed in both healthy and injured lungs [7,8].…”

A minimal model of lung mechanics and model-based markers for optimizing ventilator treatment in ARDS patients

“…Recent in vivo studies have shown that the recruitment and derecruitment of alveoli occurs even in healthy lungs and that once a unit is recruited, it does not change its size significantly [15,16,17]. This result suggests that recruitment and derecruitment is the dominant cause of volume change, rather than isotropic "balloon like", expansion of alveoli as had been traditionally thought, which is discussed in [16].…”

“…However, that model included four different types of lung units, based directly on work by Schiller et al [17], and all different types of unit required unique unit compliance and threshold pressure distributions using results from the literature. The model therefore required as many as 42 patient specific parameters to be identified [11].…”

“…Second, using the hypothesis that most of the volume change is caused by recruitment and derecruitment [e.g. 15,16,17], the unit compliance has a relatively smaller contribution to the overall PV curve shape. As a result, the unit compliance curve defined in Equation 1 can be fixed at generic values.…”