Dr. Schumann et al.1 make several useful suggestions regarding the development of an expiratory support system. During quiet breathing in a resting adult, expiration is passive and driven mainly by the elastic recoil of lungs.2 However, many factors such as altered lung mechanics, increased ventilatory demand, and psychological factors can induce expiratory muscle activity. Because the expiratory flow simulation analysis that considers expiration as a passive activity is inadequate for an expiratory support system, dissociation between the subject's spontaneous breathing and the expiratory support system is possible. If the flow of an expiratory support system is higher than that of the subject's spontaneous breathing, negative intratracheal pressure is generated as a result of overcompensation. This is 1 of the problems of expiratory support systems. In our system, if the intratracheal pressure becomes negative, the inspiratory demand valve of the ventilator supplies gas to prevent a negative intratracheal pressure.
The precise assessment of intratracheal pressure is very important. We proposed calculating the intratracheal pressure from the airway pressure and flow in the expiratory system.3 However, the calculated intratracheal pressure is occasionally inaccurate because of inspissated secretions or conformational changes. Thus, direct measurement of the intratracheal pressure is better. If a small tip transducer is implanted into an endotracheal tube wall, direct measurement of intratracheal pressure would be possible.
Tracheal intubation is an invasive procedure. A smaller sized endotracheal tube may reduce the incidence of pharyngeal and laryngeal injury and the discomfort caused by intubation.4 Therefore, we selected 5-mm internal diameter (ID) tubes as the target tube size for expiratory tube compensation. In a tube 6-mm ID or less, the pressure decrease across the tube is >8 cm H2O even at low respiratory flow levels (20–30 L/min).5 A negative pressure source is necessary to compensate for resistance of such a small tube during expiration. Automatic tube compensation (ATC) is a very useful system for tube compensation. It offers the possibility of resolving the problem of endotracheal tube resistance during the expiratory phase. Because the commercially available ATC system does not have a negative pressure source, its capacity is too small to support expiration through 5-mm-ID tubes. Fabry et al.6 used a negative airway pressure up to −18 cm H2O to support expiration in the ATC of their prototype ventilator. The negative pressure level of −18 cm H2O is considered to be inadequate for mechanical ventilation using a smaller-sized tube. If the ATC system has an adequate negative pressure source, we speculated that it may be more appropriate for expiratory tube compensation than our expiratory support system. Our expiratory support system is thought to be one of methods of creating negative airway pressure. Further research is needed to determine the negative pressure sources that are suitable for expiratory support.
Today, there are no ventilators to generate adequate expiratory support. Moreover, there are many problems and questions regarding expiratory support: negative pressure source, airway pressure control algorithm, and the precise monitoring of intratracheal pressure. We think that further studies regarding expiratory support and its clinical effectiveness are required.
Akinori Uchiyama, MD, PhD
Department of Anesthesiology and Intensive Care
Osaka University Graduate School of Medicine
Suita, Osaka, Japan
1. Schumann S, Harberthuer C, Guttmann J. Compensating for endotracheal tube resistance. Anesth Analg 2010;110:639–40
2. Grassino AE, Goldman MD. Respiratory muscle coordination. In: Fishman AP, Macklen PT, Mead J, Geiger SR, eds. Handbook of physiology, section 3: the respiratory system volume 3, Part 2: mechanism of breathing. Bethesda: American Physiological Society, 1986:463–80
3. Guttmann J, Eberhard L, Fabry B, Bertschmann W, Wolff G. Continuous calculation of intratracheal pressure in tracheally intubated patients. Anesthesiology 1993;79:503–13
4. Colice GL. Resolution of laryngeal injury following translaryngeal intubation. Am Rev Respir Dis 1992;145:361–4
5. Deen S, Bishop MJ. Airway management. In: Tobin MJ, ed. Principles and practice of mechanical ventilation. 2nd ed. New York: McGraw-Hill, Inc., 2006:779–99
6. Fabry B, Guttmann J, Eberhard L, Wolff G. Automatic compensation of endotracheal tube resistance in spontaneously breathing patients. Technol Health Care 1994;1:281–91