The Science Journal of the American Association for Respiratory Care

2006 OPEN FORUM Abstracts

CHANGES IN AIRWAY VOLUME INDUCED BY PEEP CAN LEAD TO REDUCTION IN ALVEOLAR MINUTE VENTILATION


Lara Brewer, M.S.1 Joseph Orr, Ph.D.1 and Dinesh Haryadi, Ph.D.2

1.Anesthesiology, University of Utah, Salt Lake City, Utah, United States. 2.Respironics Inc, Wallingford, Connecticut, United States.

Background: Ventilation of patients with Acute Lung Injury (ALI) often necessitates the use of positive end-expiratory pressure (PEEP). Anatomic Deadspace measurements can be used to track the expansion of the airway with additional PEEP. If this volume increases significantly relative to the tidal volume, the effective alveolar minute ventilation can be detrimentally affected. Materials and

Methods:
Measurements of airway deadspace (VDaw) and effective alveolar minute volume (MValv) were obtained from a NICO2 Respiratory Profile Monitor (Respironics, Inc.; Wallingford, CT) in six mechanically ventilated swine (38-49 kg). Volume control ventilation was used throughout the study. PEEP was increased stepwise by 5 cmH2O in the range of 0 to 15 cmH2O. Tests were done first in healthy lungs and then following an oleic acid-induced lung injury which was created by intravenously infusing 0.09 ml/kg of oleic acid over a 15-minute period.

Results:
In healthy lungs, the average correlation of VDaw with PEEP was r2 = 0.961 ±2O. In injured lungs, the average r2 was 0.914 ± 0.083 and the compliance was 2.6 ± 0.594 mL/cm H2O. With a PEEP of 15 cm H2O, the MValv decreased by an average of 17% in healthy lungs and 26% in ALI.

Discussion:
The VDaw was positively correlated with changes in PEEP. The VDaw also decreased during lung injury. Monitoring changes in airway deadspace of intubated patients shows potential for assessment of mucus accumulation, airway contractions and other factors that impact airway volume. This study was conducted with volume control ventilation (VCV); different MValv results may have been observed with pressure controlled ventilation. These subjects were ventilated with 10 mL per kilogram. If the ARDSnet recommendation of 6 mL/kg had been used, the reduction in effective alveolar minute ventilation would have been more significant since a greater percent of the tidal volume of each breath would have been dead space. Thus, monitoring MValv during VCV ventilation with PEEP could be useful when the clinician is also interested in maintaining sufficient alveolar ventilation.



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