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Cervical-spine motion with the AirWay Scope or Macintosh laryngoscope
- David Turnbull, Matthew J Wilson, Louise Powell (10 October 2008)
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David Turnbull, Anaesthetist Sheffield Teaching Hospitals Trust, Matthew J Wilson, Louise Powell
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Dear editor, We read the recent paper by Muramaya and colleagues with interest1 , but we have reservations about the study design. A review of the literature on aids to intubation reveals a surfeit of small studies. The primary objective of an aid to intubation is to prevent difficult and failed intubation, a rare event of low prevalence around 1% or less. The recent review and metanalysis of R. Mihai2 provided an excellent summary of the design flaws in previous clinical trials and sets standards for future studies. These methodological objectives provide a benchmark against which future clinical trials can be measured and can be summarised as follows: • Studies should be powered to detect a reduction in failed or difficult intubation either in a normal population or a population where intubation is compromised. • The standard Macintosh should be the control laryngoscope against which all test devices should be compared. • The intubation aids can be considered under three broad descriptive headings: 1. Videolaryngoscopes (Glidescope, WuScope, Airway Scope etc.) 2. Optical Bougies (Bonfils, Shikani). 3. Conduits (Airtraq, CTrach). Mihai and colleagues recommended that more than one test device from each of the three groups should be compared against the standard. This reduces the opportunity for a conflict of interest, where the manufacturers of the test device may influence the outcome. Though we are not suggesting a conflict of interest existed in the study reported by Maruyama, it did receive support from The Pentax Corporation. • Measured outcomes should be time to intubation and the incidence of failed intubation, not change in view at laryngoscopy. If this is the standard against which clinical trials of intubation aids are to be judged, the study by K. Maruyama falls short on several criteria. 1. The authors’ primary objective is to demonstrate that the AirWay Scope reduces C-Spine movement during intubation that may cause spinal cord compromise where the C-Spine is injured. Their reference to support this objective3 provides no support for the conjecture that intubation with a Macintosh Laryngoscope leads to a worsening of spinal cord injury where the cervical spine is compromised. 2. The sample is taken from a normal population, where the incidence of difficult intubation is around 0.5%. If it was acceptable to reduce the incidence of difficult intubation to 0.25%, a sample size calculation estimates about 200 subjects per group. The Muramaya sample size was therefore not powered to demonstrate a reduction in the incidence of a difficult intubation. 3. The sample size estimate is derived from the change in c-spine movement, set at an arbitrary 10%, with no references to support this. Clinical research projects’ are increasingly difficult to undertake with a burdensome bureaucracy and limitations on clinical time. We are sympathetic to the problems facing clinical researchers, however this should not excuse flawed study design. 1 Maruyama K, Yamada T, Kawakami R, et al. Randomized cross-over comparison of cervical-spine motion with the AirWay Scope or Macintosh laryngoscope with in-line stabilization: a video-fluoroscopic study Br. J. Anaesth 2008; 101: 563-567 2 Mihai R, Blair E, Kay H, et al. A quantitative review and meta- analysis of performance of non-standard laryngoscopes and rigid fibreoptic intubation aids. Anaesthesia 2008; 63: 745-60 3 Crosby ET. Airway management in adults after cervical spine trauma. Anesthesiology 2006; 104: 1293-318 Conflict of Interest:None declared |
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