The Science Journal of the American Association for Respiratory Care

2005 OPEN FORUM Abstracts

SHORT EXPIRATORY TIMES WITH PRESSURE-RELEASE VENTILATION ARE INADEQUATE TO PREVENT ALVEOLAR DERECRUITMENT IN SURFACTANT-DEPLETED LUNG

Joseph DiRocco, MD, David Carney, MD, Francoise Boubert, BA, Nils-Tomas McBride, Katy Hojnowski, Ryan LaFollette, Jillian Norton, Kathy Snyder, Gary Nieman, BA. Department of Surgery, SUNY Upstate Medical University, Syracuse NY

Rationale: Protective strategies for ventilatory management of patients with acute respiratory distress syndrome (ARDS) minimize ventilator-associated lung injury (VALI). A central mechanism of VALI is alveolar instability whereby repeated recruitment and derecruitment of alveoli exacerbates local injury. One protective strategy is the maintenance of lung recruitment by various methods. Pressure-release, inverse ratio ventilation is hypothesized to reduce alveolar instability through a reduction of expiratory time, thus preventing alveolar collapse. This study examined the difference of alveolar instability between conventional pressure-control and pressure-release ventilation using direct imaging of alveoli by in vivo microscopy.

Methods:
Sufactant-depletion injury was induced in a Sprague-Dawley rat by saline lung lavage. Ventilation was performed using standard pressure-control (PCV: RR 24, I:E=1:3) or pressure-release ventilation (PRV: time at peak 6s, time at PEEP 0.2s) with equivalent pressures (peak pressure 30cmH2O, PEEP 5cmH2O) during microscopy. Three distinct 1mm2 areas of subpleural alveoli were recorded by in vivo videomicroscopy with each ventilatory mode. Alveolar instability was measured as the %decrease in area of patent alveoli per field from peak-inspiration to end-expiration.

Results:
Alveolar instability was present but variable irrespective of ventilatory mode (PCV 41.4±23.2% vs. PRV 51.7±24.2%, p> 0.1). Despite the variability in alveolar instability among fields, the degrees of instability observed with each ventilatory mode were comparable (Figure). There was no significant difference in alveolar recruitment at inspiration when measured by the number of patent alveoli (PCV 81±22 vs. PRV 66±13, p> 0.1) or % of field aerated (PCV 48.2±10.5% vs. PRV 49.7±5.8%, p> 0.1).

Conclusion:
In this model, pressure-release ventilation did not confer any protection through a reduction in alveolar instability. Alveolar derecruitment occurred rapidly and alveolar instability persisted despite the short expiratory time (0.2s).

 

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