1. Introduction
The placement of an endotracheal tube (ETT) in order to secure the airway is possible in the majority of patients. This can commonly be accomplished with direct laryngoscopy, however, there is a 0.3% incidence of failed intubations [1]. If airway management fails, the outcome can be catastrophic.
The GlideScope® (GS), a new video device for indirect laryngoscopy is available since 2002 [2]. This is a device which is handled similarly to the Macintosh laryngoscope (Mac) [2-5]. It has a high resolution camera near the tip of the blade with a light source next to it [3-5]. This blade has a 60 degree angle [2,4,5]. The GS does not depend on an external line of sight from the anesthesiologist’s eye to the glottis and can provide a better view of the larynx compared to direct laryngoscopy [2,3]. Good visualization of the glottis and easy advancement of the ETT through the vocal cords is vital for the safety and efficiency of the intubation. The intubation can be watched on an LCD screen, which makes it easier for another person to help with a difficult airway. It is also very useful for teaching purposes [3,4].
The purpose of this study was to assess the effectiveness of the GlideScope® in comparison with the Macintosh laryngoscope in a simulated difficult airway with blood in the airway and restricted range of motion of the neck.
To our knowledge, this study is the only prospective, randomized, crossover study, which simulates a difficult airway with blood in the oropharynx. Bleeding can occur in different situations, such as traumatic airways or malignancies in the airway. Investigating the performance of airway devices in bloody airways is essential. Frequently, suctioning cannot resolve the blood, because the bleeding is too intense or because the blood is too thick or coagulated.
Our hypothesis, based on the results of previous studies [2-4,6,7], was that the GS has a superior performance compared to the Macintosh laryngoscope in terms of a difference in intubation time of at least 20 seconds.
Our study is a prospective, randomized, crossover study to compare the GlideScope® video laryngoscope and the Macintosh laryngoscope in a simulated difficult, bloody airway situation, and restricted range of motion of the neck.
2. Methods
This study was conducted at the Milton S. Hershey Medical Center. The Institutional Review Board exempted the study, because it was conducted on a model and did not involve real patients. Twenty-nine anesthesiology attendings and ten senior anesthesiology residents were recruited. The participants were informed about the study and voluntary verbal consent was obtained.
The AirSim Bronchi® intubation simulation model (TruCorp Limited Belfast, UK) was used. To simulate a difficult airway we injected artificial blood through a soft suction tube which was inserted nasally into the pharynx. The artificial blood was created in our simulation laboratory by combining normal saline and red food dye to a very dark red liquid. Clear surgical lubrication gel was then added to this liquid to obtain the viscosity of real blood. This artificial blood had the color, transparency, and viscosity of real partially clotted blood. Additionally, to simulate coagulated blood, we soaked a gauze pad (4 × 4 Kendall Curity gauze sponge 12-Ply USP type VII gauze, Tyco Healthcare) in the artificial blood after cutting it in half and inserted it into the manikin’s airway with the help of the GlideScope® and Magill forceps, so that it came into position right above the glottis, covering about 50% to 70% of the view onto the glottis. This was done by the same person each time to ensure the same conditions for each procedure. This person was not a participant in the study. Furthermore we restricted the neck extension of the model mechanically to simulate situations such as cervical spine immobilization via inline stabilization. Figure 1 shows our simulated airway seen on the GS screen. For direct visual laryngoscopy a
Figure 1. Our simulated airway on the GlideScope® screen. Visible is the large artificial blood clot. The glottis opening is visible as a dark circle above the blood clot. On the upper edge of the screen one can see a part of the epiglottis.
Macintosh blade size 3 was used. For indirect laryngoscopy we used the GlideScope® Cobalt video laryngoscope (Verathon Medical Inc., Bothell, WA, USA). A size 7.5 mm high volume low pressure oral/nasal cuffed ETT (Covidien Mallinckrodt, Mansfield, MA, USA) was used for all intubations. For intubations with the Macintosh laryngoscope the participants were provided a stylet, which they were allowed to bend and adjust according to preference. For intubations with the GS the stylet provided by the GS manufacturer was used.
The intubating conditions were exactly the same for each intubation, for both devices. Each participant attempted endotracheal intubation once with each device. The order in which the devices were used was determined by a computer generated randomization list. The two attempts were performed at a time interval no shorter than 24 hours.
Our primary outcome was the time until full visualization of the glottis and the time until completed intubation. Timing began when the participant picked up the laryngoscope and was stopped when the ETT passed through the vocal cords into the trachea. This could simply be seen by the observer watching the trachea equivalent of the manikin or on the monitor of the GS. Our hypothesis was that there would be an intubating time difference of at least 20 seconds. We believe that a difference of 20 seconds intubating time can be clinically significant.
Secondary outcomes were the Cormack & Lehane score, the percentage of the glottis visually appreciated by the participants, and the subjective difficulty of the procedure on a scale of 0 - 10. In addition we documented the number of adjustment maneuvers, the number of attempts, and the number of failed intubations. Adjustment maneuvers were defined as up and down movements of the laryngoscope or the ETT without removing these devices from the patient’s airway or the application of cricoid pressure. Attempts were defined as removal of all instruments from the airway for any reason and reinsertion. Failed intubations were defined as esophageal intubations, intubations that lasted longer than 120 seconds, or simply failure to pass the ETT through the vocal cords.
Paired t-tests were used to compare the two timepoints with respect to intubation time and time to glottis view as well as the glottis view, subjective difficulty, and number of adjustments. The calculated sample size to show a mean intubating time difference of 20 seconds with a power of 80% for a paired t-test was 34 participants. This calculation was done for an alpha error of 0.05 and an expected standard deviation of the mean difference of 40 seconds. Our sample for the intubating time was 34 pairs, because among 35 participants who intubated with both devices one was not able to intubate the trachea once, resulting in no intubating time. For all other outcomes the sample size was 35.
3. Results
We recruited 39 participants. Thirty-five participants intubated once with the GS and once with the Macintosh laryngoscope. Four participants only performed one intubation with either the GS or the Macintosh and did not return for their second intubation with the other device. Nonetheless, their data were included in our analysis, except for the paired t-tests. There were 74 intubations total, 38 with the Macintosh and 36 with the GS. All individuals had more than 2 years of experience in anesthesia and performed more than 200 intubations with the Macintosh laryngoscope and an average of 27.5 intubations with the GS.
Table 1 summarizes our results.