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Eur J Cardiothorac Surg 2006;30:415-416
© 2006 Elsevier Science NL
Letter to the Editor |
Department of Anaesthesiology and Intensive Care Medicine, University Hospital Schleswig-Holstein, Campus Kiel, Schwanenweg 21, D-24105 Kiel, Germany
Received 10 April 2006; accepted 9 May 2006.
* Corresponding author. Tel.: +49 431 597 2991; fax: +49 431 597 3002. (Email: schaedler{at}anaesthesie.uni-kiel.de).
Key Words: Weaning from mechanical ventilation • Closed-loop control of ventilation • Coronary artery bypass surgery
We read with interest the recent article by Hendrix et al. [1] on automated weaning of patients from mechanical ventilation using the Siemens 300 A ventilator. We have several concerns related to methodology and interpretation of the findings, which we would like to comment on.
The authors compared automode with protocol-driven conventional weaning in patients after coronary artery bypass grafting (10 patients in groups A and B, respectively). After the transfer to the ICU, all patients were ventilated with the tidal volume of 10 ml/kg of body weight with an inspiration-to-expiration time ratio of 1:1. Although it is questionable if these settings reflect the current practice in ventilator therapy, we would first of all like to point out the lacking detailed information on other ventilator settings in a study where the weaning time is the primary end-point. Only the upper pressure limit of 30 cmH2O was given, whereas the positive end-expiratory, mean and peak airway pressures, mechanical breathing rate were not mentioned. These settings decisively influence the length of the weaning period. In the follow-up phases ‘Spontaneous Breathing I and II’, all patients in group B were ventilated with an identical pressure support of 10 cmH2O. Was this setting suitable for all patients? Furthermore, no data concerning patients’ gas exchange were given.
We would also like to question the adequacy of the ‘time from tracheal intubation until extubation’ chosen as a variable to determine the effectiveness of automated weaning. The time between the ICU admission and extubation would have probably been more appropriate.
The authors claim that ‘the 2-h difference between groups A and B in time to extubation as well as the significantly higher cardiac index in group A patients demonstrated a marked trend to a benefit for patients automatically ventilated’. Although they acknowledge the limitations of their study (small number of patients, unblinded study design, limitations in randomization), their conclusions are misleading. The fact is that, with the significance level of p = 0.069, this is the second study demonstrating that automode failed to significantly reduce the postoperative duration of mechanical ventilation [2]. At the most, the results indicate a tendency for reducing the time to extubation and even this finding, as well as the claimed positive cardiovascular effects, is equivocal because of the highly significant age difference between the groups and insufficient documentation of the ventilator settings.
Finally, the study was performed already nine years ago. Automode only enabled ventilation with either mandatory ventilation or assisted spontaneous breathing. Neither a combination of these modes nor a stepwise decrease in mechanical breathing rate was available. More up-to-date methods for automated weaning like adaptive support ventilation (Galileo, Hamilton Medical, Bonaduz, Switzerland) [3] or SmartCare/PS (Evita XL, Dräger Medical, Lübeck, Germany), aimed at decreasing the work of breathing and keeping the patient in a zone of respiratory comfort, were shown to significantly decrease the weaning time [4,5]. In this respect the results presented by Hendrix et al. are outdated and do not correspond to the modern guidelines for weaning.
References
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