Preoxygenation by spontaneous breathing or noninvasive positive pressure ventilation with and without positive end-expiratory pressure.

Abstract

Editor, We read with great interest the study by Hanouz et al.1 and commend the authors on their work. The study addresses an important aspect of emergency anaesthesia and critical care practice and contributes to the evidence available on preoxygenation. However, there are several issues with the trial and conclusions that we feel are worthy of comment and further discussion. The authors present findings of a comparison of time taken to reach a point of satisfactory preoxygenation prior to tracheal intubation, as identified by an end-tidal oxygen fraction of 90%, using three different techniques. They concluded that preoxygenation with positive pressure ventilation, both with and without the application of positive end expiratory pressure, led to shorter preoxygenation times in patients undergoing elective surgical procedures compared with spontaneous ventilation via a mask. The authors proceed to highlight the importance of this finding in populations wherein rapid airway control – such as emergency surgery and haemodynamically unstable critically ill patients – is desirable.2 This is a pertinent area to study, but we perceive several issues regarding patient selection and exclusion criteria for the trial, and question the transferability of findings from this trial to contemporary practice. The authors only included American Society of Anesthesiology physical status I and II patients undergoing elective surgical procedures in their study.1 In fact, the strict inclusion criteria prevented enrolment of patients from most clinical situations in which thorough and rapid preoxygenation would be desirable – such as rapid sequence induction of anaesthesia and emergency tracheal intubations – and also in patients with potentially difficult airways. As a result, the reader is left questioning whether the patient population selected is appropriate for the clinical question posed. It is certainly clear that the patients studied are not representative of emergency surgical or critically ill patient populations, and thus it is difficult to justify extrapolating the findings of this study to those particular groups. We would also question the authors’ use of noninvasive positive pressure ventilation without the application of positive end-expiratory pressure as a method of preoxygenation. Bilevel noninvasive ventilation, in which both positive end-expiratory pressure and positive pressure ventilation are applied, has been demonstrated to improve gas exchange in a wide variety of clinical situations, including in preoxygenation prior to tracheal intubation.3 The application of positive end-expiratory pressure – or continuous positive airway pressure – alone has also been demonstrated to improve oxygenation and measured indices of respiratory function in a wide range of patients in the perioperative period.4 The use of positive pressure ventilation without positive end-expiratory pressure, however, is an infrequently used modality of ventilatory support, and it seems curious that the authors chose to study this mode of support rather than continuous positive airway pressure. It would be interesting and useful to learn if the addition of positive pressure ventilation to a basic level of positive end-expiratory pressure had a favourable effect on preoxygenation, and unfortunately in this regard, we feel that this study represents something of a missed opportunity. The time taken from tracheal intubation to desaturation to oxygen saturations of 93% was used as a secondary endpoint in this study and no significant difference was found between the three methods of preoxygenation. However, we would question the relevance of this finding to clinical practice. Rapid desaturation as a complication of intubation is a common and difficult problem in emergency situations.5 However, the study population display few, if any, of the risk factors for desaturation following intubation, and thus no real conclusions can be drawn on the efficacy of any of the methods of preoxygenation in preventing this significant clinical problem. So where does this study leave us in terms of guiding our clinical practice? The authors make no reference at all in their discussion of the study population being very different to that highlighted in both the introduction and discussion as the population of interest. Is this a feasibility study for trialing the technique before its use in sicker patients more in need of rapid and efficient preoxygenation? If not, it does not provide robust evidence to suggest that we should be changing our clinical practice with regard to preoxygenation and high-risk inductions. Further work – perhaps comparing continuous positive airway pressure, bilevel noninvasive ventilation or oxygen delivered by high flow nasal cannulae6 to standard mask preoxygenation – in emergency and critically ill patients is warranted. Acknowledgements related to this article Assistance with the letter: none. Financial support and sponsorship: none. Conflicts of interest: none.