The role of time and pressure on alveolar recruitment

Abstract

Inappropriate mechanical ventilation in patients with acute respiratory distress syndrome can lead to ventilator-induced lung injury (VILI) and increase the morbidity and mortality. Reopening collapsed lung units may significantly reduce VILI, but the mechanisms governing lung recruitment are unclear. We thus investigated the dynamics of lung recruitment at the alveolar level. Rats (n = 6) were anesthetized and mechanically ventilated. The lungs were then lavaged with saline to simulate acute respiratory distress syndrome (ARDS). A left thoracotomy was performed, and an in vivo microscope was placed on the lung surface. The lung was recruited to three recruitment pressures (RP) of 20, 30, or 40 cmH(2)O for 40 s while subpleural alveoli were continuously filmed. Following measurement of microscopic alveolar recruitment, the lungs were excised, and macroscopic gross lung recruitment was digitally filmed. Recruitment was quantified by computer image analysis, and data were interpreted using a mathematical model. The majority of alveolar recruitment (78.3 +/- 7.4 and 84.6 +/- 5.1%) occurred in the first 2 s (T2) following application of RP 30 and 40, respectively. Only 51.9 +/- 5.4% of the microscopic field was recruited by T2 with RP 20. There was limited recruitment from T2 to T40 at all RPs. The majority of gross lung recruitment also occurred by T2 with gradual recruitment to T40. The data were accurately predicted by a mathematical model incorporating the effects of both pressure and time. Alveolar recruitment is determined by the magnitude of recruiting pressure and length of time pressure is applied, a concept supported by our mathematical model. Such a temporal dependence of alveolar recruitment needs to be considered when recruitment maneuvers for clinical application are designed.

Fig. 1.

Overview of our methods for measuring and graphing both microscopic and macroscopic lung recruitment. Three photomicrographs of subpleural alveoli before the application (T0) of recruitment pressure (RP) and at 2 (T2) and 40 s (T40) after the application of RP. Note the increased number of alveoli over time at the same RP. The gross lung is also depicted at the same time periods. The dark red areas were considered atelectatic, and the light pink areas were considered inflated. Note that there was more inflated (pink) areas of lung over time at the same pressure. The blue lines demonstrate how the increase in alveolar and lung recruitment was depicted graphically.

Fig. 2.

A: microscopic percent recruitment. Alveolar temporal recruitment at three different RPs (20, 30, and 40 cmH₂O). Application of a constant RP occurred immediately after time point 0 and was held for 40 s. The majority of alveolar recruitment occurs within the first 2 s (T2) during the application of each RP. Alveoli continue to recruit (positive slope) over 40 s during each RP. RP-40 recruits significantly more alveoli than RP-20 at 40 s. Data are means ± SE (*P < 0.05 vs. PR-20). B: gross whole lung recruitment at three separate RPs (20, 30, and 40 cmH₂O) over 40 s. Again, the majority of recruitment occurs within the first 2 s (T2) during each RP. There was a gradual temporal recruitment from T2 → T40 during RP. The higher the RP, the steeper the slope of the curve, suggesting increased recruitment. Data are means ± SE (*P < 0.05 vs. PR 20). C: simulated data from the mathematical model for each RP (20, 30, and 40 cmH₂O).

Fig. 3.

Total percent recruitment against starting pressures and recruiting pressures for gross (○) and microscopic (•) recruitment. Solid line represents the least squares regression for a straight line fitted to all data points. Broken line represents an integrated Gaussian function with mean of 16 cmH₂O and a standard deviation of 12 cmH₂O.