Quantitative computed tomography in porcine lung injury with variable versus conventional ventilation: Recruitment and surfactant replacement*

Abstract

Biologically variable ventilation improves lung function in acute respiratory distress models. If enhanced recruitment is responsible for these results, then biologically variable ventilation might promote distribution of exogenous surfactant to nonaerated areas. Our objectives were to confirm model predictions of enhanced recruitment with biologically variable ventilation using computed tomography and to determine whether surfactant replacement with biologically variable ventilation provides additional benefit in a porcine oleic acid injury model. Prospective, randomized, controlled experimental animal investigation. University research laboratory. Domestic pigs. Standardized oleic acid lung injury in pigs randomized to conventional mechanical ventilation or biologically variable ventilation with or without green dye labeled surfactant replacement. Computed tomography-derived total and regional masses and volumes were determined at injury and after 4 hrs of ventilation at the same average low tidal volume and minute ventilation. Hemodynamics, gas exchange, and lung mechanics were determined hourly. Surfactant distribution was determined in postmortem cut lung sections. Biologically variable ventilation alone resulted in 7% recruitment of nonaerated regions (p < .03) and 15% recruitment of nonaerated and poorly aerated regions combined (p < .04). Total and normally aerated regional volumes increased significantly with biologically variable ventilation, biologically variable ventilation with surfactant replacement, and conventional mechanical ventilation with surfactant replacement, while poorly and nonaerated regions decreased after 4 hrs of ventilation with biologically variable ventilation alone (p < .01). Biologically variable ventilation showed the greatest improvement (p < .003, biologically variable ventilation vs. all other groups). Hyperaerated regional gas volume increased significantly with biologically variable ventilation, biologically variable ventilation with surfactant replacement, and conventional mechanical ventilation with surfactant replacement. Biologically variable ventilation was associated with restoration of respiratory compliance to preinjury levels and significantly greater improvements in gas exchange at lower peak airway pressures compared to all other groups. Paradoxically, gas exchange and lung mechanics were impaired to a greater extent initially with biologically variable ventilation with surfactant replacement. Peak airway pressure was greater in surfactant-treated animals with either ventilation mode. Surfactant was distributed to the more caudal/injured lung sections with biologically variable ventilation. Quantitative computed tomography analysis confirms lung recruitment with biologically variable ventilation in a porcine oleic acid injury model. Surfactant replacement with biologically variable ventilation provided no additional recruitment benefit and may in fact be harmful.