Role of alveolar recruitment in lung inflation: influence on pressure-volume hysteresis

Abstract

The behavior of terminal lung units (alveoli) with changes in lung volume is controversial. For example, different investigators using similar techniques have suggested that alveoli expand homogeneously or, conversely, get smaller with increases in lung volume. We studied this problem by filling excised dog lobes with monodisperse aerosol and observing deposition at zero airflow. Under these conditions, the deposition of particles is inversely proportional to a mean alveolar linear dimension (ALD). With this technique, changes in ALD were assessed as the lung ventilated along its pressure-volume (PV) curve. PV curves were generated using a rapid cycling technique that minimized trapping and allowed reversible regulation of inflation-deflation hysteresis. Irreversible changes in PV hysteresis were assessed by rinsing the lung with Tween. With significant PV hysteresis, the ALD progressively decreased with inflation to total lung capacity (TLC). With deflation from TLC, the ALD was unchanged until low volumes were reached, when it decreased markedly. When PV hysteresis was minimized (reversibly or irreversibly), inflation and deflation ALD were superimposed. These data are consistent with progressive alveolar recruitment with inflation to TLC and derecruitment with deflation. The correlation between alveolar dimensions and PV hysteresis suggests that shifts in the PV curve can be accounted for by changes in the population of units. The number open at any given point is determined by the dynamic history of inflation.

Lung Density Changes With Growth and Inflation

“…18 Th us, unless the lung volumes are matched carefully, it is not possible to compare CT scan densities from diff erent individuals. TLC is based on a voluntary eff ort, and there is considerable variability in this eff ort among individuals that is likely greater in people with lung disease.…”

Kinematically irreversible acinar flow: a departure from classical dispersive aerosol transport theories

“…By matching this degree of geometric hysteresis, we have generated physiologically realistic asynchrony in physical models (41) and computational models (16), which demonstrated that geometrical hysteresis, even if small, can produce stretchand-fold patterns and, consequently, induce substantial acinar flow irreversibility. In a related work, Smaldone et al (31), using gravitational sedimentation of aerosol particles to estimate mean linear intercepts, showed significant geometric hysteresis in excised lungs with large-volume excursions from minimal volume and interpreted their data in terms of respiratory unit recruitment and derecruitment. Those studies, however, being restricted to single-volume histories, did not address the issue of mixing during cyclic ventilation and so are not strictly comparable to the present work.…”

Crackle sounds and lung recruitment

“…The distribution of the recruited ∆Vs has been predicted to be a power law with an exponent of 2 [21] and has recently been measured indirectly [6]. Our data reflect a direct assessment of the distribution of alveolar recruited ∆Vs.…”

“…Nevertheless, for the first inflation, the data were in excellent agreement with the model prediction [21] since the theoretical value of the exponent is 2, whereas the estimated average one was 2.02. The distribution obtained from the second inflation had an exponent of 1.88, which is also close to the theoretical value of 2.…”

“…However, in animal models of the ARDS [16,21] and in human subjects with the ARDS [22,23], the P−V curve often exhibits a lower knee, even though the lung is inflated from functional residual capacity (FRC). The interpretation of the lower knee in the P−V curve of the injured lung is generally the same as for the first inflation of normal lungs from the degassed state [23,24].…”