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spiratory resistance imposed by the pneumotachograph used in patients. This most probable explanation should remind us that even small increases in airway resistance (i. e., due to a pneumotachograph) may have tremendous effects on breathing workload. This is especially true for endotracheal tubes which have a clearly higher resistance than a pneumotachograph. In a laboratory study we have shown that the WOBimposed for the EVITA-2 during CPAP was comparable to the numbers found in Dr. Calzias study (51.2 ± 6.0 mJ/L), but the WOB imposed by the ventilator and an 8.0 mm ID endotracheal tube was 293 ± 7.4 mJ/L. In this study PS was not able to reduce the total WOBimposed below 142 ± 11.5 mJ/L although up to 10 mbar PS were used [5]. We do not query the results of Dr. Calzia's study but we want to emphasize that in the intubated patient the endotracheal tube imposes relatively more workload than modern ventilators and that PS might not be an optimal solution to compensate for this.

References 1. Calzia E, Lindner KH, Stahl W, Martin A, Radermacher P, Georgieff M (1998) Work of breathing, inspiratory flow response, and expiratory resistance during continuous positive airway pressure with the ventilators EVITA-2, EVITA-4 and SV 300. Intensive Care Med 24: 931±938 2. Brochard L, Rua F, Lorini H, Lemaire F, Harf A (1991) Inspiratory pressure support compensates for the additional work of breathing caused by the endotracheal tube. Anesthesiology 75: 739±745 3. Brochard L, Harf A, Lorino H, Lemaire F (1989) Inspiratory pressure support prevents diaphragmatic fatigue during weaning from mechanical ventilation. Am Rev Respir Dis 139: 513±521 4. Kuhlen R, Guttmann J, Nibbe L, Max M, Reyle HS, Rossaint R, Falke K (1997) Proportional pressure support and automatic tube compensation: new options for assisted spontaneous breathing. Acta Anaesthesiol Scand Suppl 111: 155±159

5. Nibbe L, Kuhlen R, Pappert D, et al. (1995) Compensation of the added work of breathing using inspiratory pressure support and automatic tube compensation. In: Roussos CH (ed) Proceedings of the 8th European Congress of Intensive Care Medicine. Monduzzi, Bologna, pp 641±644

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R. Kuhlen ( ) × M. Max × R. Rossaint Klinik für Anaesthesiologie, Medizinische Einrichtungen der RWTH Aachen, Pauwelstrasse 30, D-52074 Aachen e-mail: [email protected] Tel. + 49 24 18 08 81 79 Fax: + 49 24 18 88 84 06

E. Calzia W. Stahl P. Radermacher

Reply Received: 25 November 1998 Accepted: 30 November 1998 Sir: We thank Drs. Kuhlen, Max and Roissant for their interest in our recently published article. We absolutely agree with them that the automated tube resistance compensation (ATC) system [1] very precisely compensates for the resistive load imposed by the endotracheal tube. Generally speaking, there is no doubt that the principle which the ATC system is based on is more suitable than the ªclassicalº pressure support mechanism for compensating an additional workload imposed by resistors placed in the insipiratory limb of the breathing circuit such as the endotracheal tube. In fact, the support applied by the ventilator with ATC is continuously adapt-

ed to the pressure drop along the endotracheal tube as it is calculated based on the actual flow. Furthermore, any additional inspiratory resistance, e. g. as caused by the use of heat-and-moisture exchangers or by increased density of the inspired gas mixture, could probably be compensated in the same manner by the ATC mechanism. In our investigation, however, the rationale for adding a small pressure support of 2 mbar when using the two EVITA models was to achieve comparable conditions between the ventilators tested, since the SV 300 automatically provides for such an additional support. Although the ATC system would probably have elegantly compensated for the additional inspiratory work, it would also have influenced other parameters investigated in our study, e.g the inspiratory flow delivery, and, hence, it would have distorted the comparison between the different ventilators.

References 1. Fabry B, Haberthür C, Zappe D, Guttmann J, Kuhlen R, Stocker R (1997) Breathing pattern and additional work of breathing in spontaneously breathing patients with different ventilatory demands during inspiratory pressure support and automatic tube compensation. Intensive Care Med 23: 545±552

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E. Calzia ( ) × W. Stahl × P. Radermacher Universitätsklinik für Anästhesiologie, Sektion anästhesiologische Pathophysiologie und Verfahrensentwicklung, Parkstrasse 11, D-89 075 Ulm/Donau, Germany e-mail: [email protected] Tel. + 49 (731) 5 02 51 40 Fax + 49(7 31)5 02 51 43