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TECHNOLOGY, COMPUTING, AND SIMULATION

An Expiratory Assist During Spontaneous Breathing Can Compensate for Endotracheal Tube Resistance

From the Department of Anesthesiology and Intensive Care Medicine, Osaka University Graduate School of Medicine, Yamadaoka, Suita, Japan.

Address correspondence and reprint requests to Akinori Uchiyama, MD, PhD, Department of Anesthesiology and Intensive Care Medicine, Osaka University Graduate School of Medicine, 2-15 Yamadaoka, Suita, Osaka Prefecture 565-0871, Japan. Address e-mail to auchiyama{at}hp-icu.med.osaka-u.ac.jp.

BACKGROUND: Although inspiratory assist of spontaneous breathing in intubated patients is common, expiratory assist functions have rarely been reported. Effective expiratory support (ES) could be used to compensate for endotracheal tube (ETT) resistance during spontaneous breathing. In this study, we examined the performance of a new system designed to provide both inspiratory support (IS) and ES during spontaneous breathing with the goal of reducing the effective resistance of the ETT.

METHODS: The ES system consisted of a ventilator demand valve and a computer-controlled piston cylinder, which aspirated gas from the respiratory circuit during the expiratory phase. The movement of the piston was synchronized with spontaneous breathing. We compared the pressures at the tip of the ETT and in the breathing circuit during spontaneous breathing through an ETT of internal diameter (ID) 5 mm with that of an ETT with ID 8 mm in nine healthy adult male volunteers. The ventilatory mode was set to maintain a continuous airway pressure of 0 cm H₂O. Three ventilator settings (no support, IS only, and IS plus ES) were compared using ID 5 mm ETT.

RESULTS: We monitored pressure in the breathing circuit (P_aw), ETT tip pressure (P_tip), and respiratory flow. The P_tip of the ID 5 mm ETT showed a large negative deflection during inspiration and a positive deflection during expiration without support. IS alone did not improve the respiratory pattern through the small ETT. However, IS plus ES resulted in negative P_aw during expiration in addition to positive deflection of P_aw during inspiration, making the pressure characteristics of P_tip similar to those of ID 8 mm ETT. Moreover, IS plus ES produced a respiratory pattern through the ID 5 mm ETT that was similar to that through the ID 8 mm ETT.

CONCLUSIONS: In this study of healthy volunteers, IS plus ES compensated for the airway resistance imposed by a ID 5.0 mm ETT to create pressure changes at the tip of the ETT similar to those of an ID 8.0 mm ETT.