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Mechanism by which a sustained inflation can worsen oxygenation in acute lung injury
Guido Musch1, R Scott Harris, Marcos F Vidal Melo
1Department of Anesthesia and Critical Care, CLN 309, Massachusetts General Hospital, 55 Fruit Street, Boston, Massachusetts 02114, USA. gmusch@partners.org
Background:
Sustained lung inflations (recruitment maneuvers [RMs]) are occasionally used during mechanical ventilation of patients with acute lung injury to restore aeration to atelectatic alveoli. However, RMs do not improve, and may even worsen, gas exchange in a fraction of these patients. In this study, the authors sought to determine the mechanism by which an RM can impair gas exchange in acute lung injury.
Methods:
The authors selected a model of acute lung injury that was unlikely to exhibit sustained recruitment in response to a lung inflation. In five sheep, lung injury was induced by lavage with 0.2% polysorbate 80 in saline. Positron emission tomography and [13N]nitrogen were used to assess regional lung function in dependent, middle, and nondependent lung regions. Physiologic data and positron emission scans were collected before and 5 min after a sustained inflation (continuous positive airway pressure of 50 cm H2O for 30 s).
Results:
All animals showed greater loss of aeration and higher perfusion and shunting blood flow in the dependent region. After the RM, Pao2 decreased in all animals by 35 +/- 22 mmHg (P < 0.05). This decrease in Pao2 was associated with redistribution of pulmonary blood flow from the middle, more aerated region to the dependent, less aerated region (P < 0.05) and with an increase in the fraction of pulmonary blood flow that was shunted in the dependent region (P < 0.05). Neither respiratory compliance nor aeration of the dependent region improved after the RM.
Conclusions:
When a sustained inflation does not restore aeration to atelectatic regions, it can worsen oxygenation by increasing the fraction of pulmonary blood flow that is shunted in nonaerated regions.
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