Cardiovascular intubation responses with the Airway Scope® and the Macintosh laryngoscope.

Abstract

In a small randomised controlled trial, Koyama et al. [1] demonstrated that the Airway Scope® (AWS, Pentax, Tokyo, Japan), compared with the Macintosh laryngoscope, can attenuate the cardiovascular response to tracheal intubation in normotensive, but not hypertensive, patients. However, notably absent are the times required for intubation with the two devices. Previous studies have shown that there is a progressive increase in mean arterial pressure during the first 45 s of intubation with a straight laryngoscope blade [2], suggesting that prolonged intubation may cause a stronger stimulus to the airway. Regarding the forces applied to the pharynx by the AWS and Macintosh laryngoscope, the reference cited by the authors (number 12) in their article does not provide any direct evidence. To the best of our knowledge, there is no study comparing the two devices in terms of force applied to the pharynx during intubation in patients. Recently, a manikin study demonstrated that the AWS generates less pressure on the tongue than the Macintosh laryngoscope [3]. Similarly, we believe the evidence that the AWS produces less stimulation than conventional laryngoscopy is inconclusive. The technique using the AWS is similar to that for a Miller laryngoscope, with the blade’s tip inserted posterior to the epiglottis and directly elevating it [4]. The use of a Miller straight blade with elevation of the vagally innervated posterior aspect of the epiglottis can result in significantly higher systemic blood pressure than with a curved blade (Macintosh or McCoy) [5]. Moreover, with regard to the sympathetic response, laryngoscopy itself is only moderately stimulating, whereas passing a tracheal tube is far more stimulating [6]. For example, tracheal intubation with a lighted stylet [7], flexible fibreoptic bronchoscope [8] or intubating laryngeal mask airway [9] fails to prevent cardiovascular responses, despite avoidance of laryngoscopy. The AWS has a tube-guiding channel and a targeted crisscross mark delivering the tracheal tube to the glottis [4] and we assume that advancing the tube along the guiding channel may align the tube with the trachea, resulting in less tracheal stimulation. In contrast, when performing intubation with the Macintosh laryngoscope, the tube’s tip has to pass around an angle at the base of the tongue to enter the larynx, with greater potential to contact the anterior tracheal wall, producing a stronger tracheal stimulus. Finally, 2% sevoflurane was used during induction of anaesthesia. In the results, the authors did not describe whether the same end-tidal concentration of sevoflurane was achieved before intubation in all patients. Because the cardiovascular intubation responses depend to some extent upon depth of anaesthesia [6], we are concerned that non-standardisation of anaesthetic depth before intubation may have affected the results.