Alveolar leak develops by a rich-get-richer process in ventilator-induced lung injury

Abstract

Acute respiratory distress syndrome (ARDS) is a life-threatening condition for which there are currently no medical therapies other than supportive care involving the application of mechanical ventilation. However, mechanical ventilation itself can worsen ARDS by damaging the alveolocapillary barrier in the lungs. This allows plasma-derived fluid and proteins to leak into the airspaces of the lung where they interfere with the functioning of pulmonary surfactant, which increases the stresses of mechanical ventilation and worsens lung injury. Once such ventilator-induced lung injury (VILI) is underway, managing ARDS and saving the patient becomes increasingly problematic. Maintaining an intact alveolar barrier thus represents a crucial management goal, but the biophysical processes that perforate this barrier remain incompletely understood. To study the dynamics of barrier perforation, we subjected initially normal mice to an injurious ventilation regimen that imposed both volutrauma (overdistension injury) and atelectrauma (injury from repetitive reopening of closed airspaces) on the lung, and observed the rate at which macromolecules of various sizes leaked into the airspaces as a function of the degree of overall injury. Computational modeling applied to our findings suggests that perforations in the alveolocapillary barrier appear and progress according to a rich-get-richer mechanism in which the likelihood of a perforation getting larger increases with the size of the perforation. We suggest that atelectrauma causes the perforations after which volutrauma expands them. This mechanism explains why atelectrauma appears to be essential to the initiation of VILI in a normal lung, and why atelectrauma and volutrauma then act synergistically once VILI is underway.

Fig 1. Blood-gas barrier permeability by ventilation group and dextran conjugate size.

Permeability is defined as the ratio of bronchoalveolar lavage fluid fluorescence to serum fluorescence for each size of dextran conjugate (3 kDa, 70 kDa, and 2000 kDa). *Significant increase in permeability from all other groups within dextran size. ^#Significant increase from Control (p<0.05). ZEEP: zero end-expiratory pressure, PEEP3: positive end-expiratory pressure = 3 cmH₂O, Short: ventilation time 30 min, Mid: ventilation time 60 min, ≥2xH: ventilation until elastance (H) at least doubled.

Fig 2. Propidium iodide (PI) as indicator of lung cell injury.

Representative images from the Control (A) and ZEEP/≥2xH group (B) groups showing PI+ nuclei (red cells, white arrows). Fluorescence intensity increases ~20-fold when PI binds to nucleic acids. ZEEP/≥2xH group was ventilated with zero end-expiratory pressure until elastance at least doubled; control group was not ventilated.

Fig 3. Injured fraction of lung cells (PI+/DAPI+) by ventilation group.

*Significantly less than all groups (p≤0.036). PI: propidium iodide, marker of cell membrane disruption, ZEEP: zero end-expiratory pressure, PEEP3: positive end-expiratory pressure = 3 cmH₂O, Short: ventilation time 30 min, Mid: ventilation time 60 min, ≥2xH: ventilation until elastance (H) at least doubled.

Fig 4. Lung function.

Change in elastance measured at PEEP = 0 between the start and end of ventilation (ΔH₀) versus (A) permeability of 3 kDa dextran (BALF/Blood fluorescence, cohort 1 mice) and (B) fraction of injured lung cells (PI+/DAPI+, cohort 2 mice). PI: propidium iodide, marker of cell membrane disruption, ZEEP: zero end-expiratory pressure, PEEP3: positive end-expiratory pressure = 3 cmH₂O, Short: ventilation time 30 min, Mid: ventilation time 60 min, ≥2xH: ventilation until elastance (H) at least doubled.

Fig 5. Measured and predicted barrier permeability.

Comparison between the predicted (bars, α = 1.0) and measured (points) blood-gas barrier permeability to 3kDa, 70 kDa, and 2000 kDa dextrans. Error bars show the standard deviation of the experimental measurements.

Fig 6. Rich-get-richer model predictions.

Histogram of hole diameters for the rich-get-richer simulation run until m = 30,000 (closed symbols) and m = 100,000 holes (open symbols). Straight lines were fit to each set of points after discarding the first 3 in each case. The slopes of the relationships beyond the first 3 points are -2.9 (SE 0.1) for m = 30,000 and -2.9 (SE 0.2) for m = 100,000.