Endotracheal Intubation Criteria and Stress Response: Airtraq versus Macintosh Laryngoscopes - A Prospective Randomized Controlled Trial.

BACKGROUND: Airtraq® is a single-use video laryngoscope used to facilitate tracheal intubation in both expected and unexpected difficult airways. AIMS: We hypothesized that Airtraq laryngoscope would facilitate better intubation criteria and lower stress response to laryngoscopy in comparison to the Macintosh laryngoscope.
MATERIALS AND METHODS: In this randomized, single-blinded, prospective study, 70 adult patients were randomly assigned to be intubated with either Airtraq (Group AT) or Macintosh (Group M) laryngoscope (35 patients in each). The primary outcomes involved intubation time, first-attempt success rate, time to best laryngoscopic view, and percentage of glottic opening (POGO) score. Other recorded parameters involved the hemodynamic and intraocular pressure (IOP) responses to laryngoscopy and intubation and complications during and after laryngoscopy and after extubation. Serum samples were collected before anesthesia induction and 2 min after intubation and analyzed for epinephrine, cortisol, and glucose.
RESULTS: Group AT had significantly higher POGO score and significantly shorter intubation time and time to best laryngoscopic view than Group M (P < 0.001). The first-attempt success rate was 97.1% in Group AT and 94.3% in Group M (P = 0.55). Postoperatively, laryngospasm and sore throat were encountered in 2.9% of Group M patients compared to 0% in Group AT (P = 1.00). The heart rate, mean arterial pressure, IOP, serum epinephrine, and cortisol were significantly increased in Group M than Group AT.
CONCLUSION: In comparison to the Macintosh laryngoscope, Airtraq conferred significantly better intubation criteria and lesser stress response to laryngoscopy and intubation. Copyright:

INTRODUCTION

Endotracheal intubation is the standard means to secure the airway and apply mechanical ventilation.[12] In spite of recent developments in airway device technologies, the curved laryngoscope blade described by Macintosh remains the most commonly used device to facilitate endotracheal intubation. It is the one against which new laryngoscopy and endotracheal intubation devices are usually compared.[3]

Adequate glottic vision through Macintosh laryngoscopy involves distortion and compression of the airway anatomical structures.[4] These noxious stimuli together with intubation increase the intraocular pressure (IOP) and cause an intense sympathetic discharge which increases heart rate (HR) and blood pressure.[56]

The Airtraq® (Prodol Meditec S.A., Vizcaya, Spain) is a single-use rigid video laryngoscope with a built-in channel, in which the endotracheal tube (ETT) is installed before intubation. Due to its blade great curvature, Airtraq improves laryngeal visualization without alignment between the mouth, larynx, and trachea. However, its main disadvantages are blurred vision by secretions and the high cost.[7]

We hypothesized that Airtraq would provide better intubation criteria with lesser stress response than the Macintosh laryngoscope in routine anesthesia practice. To prove this hypothesis, we compared the performance of Airtraq and Macintosh laryngoscopes in terms of intubation criteria, stress response, and complications during and after intubation.

MATERIALS AND METHODS

We confirm that the present randomized controlled trial runs in concordance with the Consolidated Standards of Reporting Trials guideline. After approval of the Local Ethics and Research Committee of Anesthesia Department, Faculty of Medicine, Menoufiya University Hospital, Menoufia, Egypt (Chairperson Prof. Amin H) (June 2017), written informed consent was obtained from all patients. This study was registered at www.pactr.org (PACTR201708002460428). The study was conducted at Menoufia University Hospitals from September 2017 to February 2018.

This randomized, single-blinded, prospective study included 70 patients, 18–60 years old, with the American Society of Anesthesiologists physical status class I or II, having no criteria for suspected difficult intubation, scheduled for various types of nonophthalmic elective surgery requiring orotracheal intubation.

Patients with raised IOP or intracranial pressure, suspicion of difficult intubation, need for rapid sequence induction, gastric acid aspiration risk, suspicion or history of difficult intubation, cervical spine pathology, body mass index ≥35, cardiovascular, hyperreactive airway disease, and/or on β-blocker therapy were excluded from the study.

Patients were randomly assigned to be intubated with Macintosh laryngoscope blade of size 3 or 4 (Group M) or Airtraq (Group AT) (35 patients in each) using a computer-generated randomization program. The randomization codes were involved in closed opaque envelopes. The used laryngoscope size was left to the discretion of the performing anesthetist. The first author who has a clinical experience of >8 years with Macintosh and Airtraq laryngoscopes carried out all tracheal intubations.

The anesthesia technique was standardized in both groups. Patients monitoring involved noninvasive blood pressure, electrocardiography, SpO2, entropy, and train-of-four (TOF) stimulation of the ulnar nerve.

The patients were preoxygenated with 100% oxygen for 3 min. General anesthesia was induced with fentanyl (2 μg.kg-1), propofol (2 mg/kg) followed by bag-mask ventilation with isoflurane (1.2%) in 100% oxygen. After confirming adequate bag-mask ventilation, cisatracurium (0.15 mg.kg-1) was used to facilitate laryngoscopy and orotracheal intubation guided by TOF. The intubation position was achieved by flexing the lower cervical spine and extending the upper cervical spine.

After intubation, the lungs were mechanically ventilated using isoflurane in oxygen/air mixture targeting end-tidal (ET) isoflurane 1.2%, ETCO2 35–40 mmHg, and SpO2 >98%. An intubation attempt was considered a failed one when it reached 60 s duration without correct placement of the ETT. When an intubation attempt failed, bag-mask ventilation was resumed to prevent desaturation and maintain normocapnia before the next attempt. In all cases, a member of the nursing staff, who was blinded to the research protocol, recorded the timing using a stopwatch.

The first-attempt success rate (correct placement of the ETT within the first 60 s), number of intubation attempts till correct intubation, the time to best laryngoscopic view (the time from inserting the laryngoscope between the central incisors till achieving the best view for endotracheal intubation as declared by the intubating anesthetist), and the intubation time (time from inserting the laryngoscope between the central incisors till confirmed vision of ETT passage between the vocal cords) were our primary outcomes.

The best laryngoscopic view was quantified by the percentage of glottic opening (POGO) score reported by the intubator. A 100% POGO score is a full view of the glottis from the anterior commissure to the interarytenoid notch. A POGO score of 0 means that even the interarytenoid notch is not seen.

Any complications during laryngoscopy and intubation (desaturation, injuries to the lips, teeth, and/or tongue) or after extubation (laryngospasm, hoarseness of voice, and sore throat) were recorded. Any need for assistance during laryngoscopy and intubation was also recorded.

The secondary outcomes included the mean arterial pressure (MAP) and HR recorded at baseline (before induction of anesthesia), postinduction (immediately before laryngoscopy), immediately before ETT insertion, and 1, 3, and 5 min after intubation is completed. The IOP was measured by an ophthalmologist in both lubricated eyes using Schiotz tonometer: baseline (before induction of anesthesia), immediately after ETT placement, and 2, 4, and 6 min after intubation is completed. Serum samples for epinephrine, glucose, and cortisol were collected before induction and 2 min after intubation.

We planned a study of continuous response variable from independent control and experimental patients with one control per experimental patient. In a previous study,[8] the response within each patient group was normally distributed with standard deviation of 7.5. If the true difference in the experimental and control means was 5.9, we would need to study 35 experimental patients and 35 controls to be able to reject the null hypothesis that the population means of the experimental and control groups are equal with propability (power) 0.9. The type I propabilty error associated with this was 0.05.

The Statistical Package for the Social Sciences version 23 (IBM SPSS statistics for windows, version 23.0, IBM Corp., Armnok, NY, USA) was used for analysis. Descriptive statistics were expressed as number, percentage, mean, and/or standard deviation. Student's t-test was used for comparison of quantitative variables between two groups of normally distributed data. The Mann–Whitney's test was used for comparison of quantitative variables between two groups of nonnormally distributed data. The Chi-square test was used to study the association between qualitative variables. Whenever the expected cells were <5, Fisher's exact test was used. Two-sided P < 0.05 was considered statistically significant.

RESULTS

A total of 70 patients who fulfilled our inclusion criteria were enrolled in the study, 35 in each group [Figure 1]. Both groups had homogenous demographic data [Table 1].

Figure 1

Study flow chart

Table 1

Demographic data

	AT (n=35)	M (n=35)	P
Age (years)	40.43±9.93	41.62±5.22	0.53
Weight (kg)	69.50±13.18	68.15±11.11	0.64
Height (cm)	164.26±5.63	165.54±4.42	0.29
Sex male/female	19/16	22/13	0.46
ASA, n (%)
I	23 (65.7)	22 (62.9)	0.80
II	12 (34.3)	13 (37.1)

*P<0.05 indicates significance between the two group. Data were expressed as mean±SD or number of patients. n=Number of patients, ASA=American Society of Anesthesiologists, AT=Airtraq, M=Macintosh, SD=Standard deviation

The first-attempt intubation was successful in 97.1% and 94.3% of Group AT and M patients, respectively (P = 1). We have not encountered failed second-attempt intubation. The intubation time and the time to best laryngoscopic view were significantly longer in Group M than Group AT (14.18 ± 3.42 vs. 11.50 ± 4.36 and 8.87 ± 2.09 vs. 5.82 ± 1.17) (P < 0.001), respectively. However, the patients’ oxygenation status was not affected in any single patient. Group AT had a significantly higher POGO score than that of Group M (97.02 ± 2.41 vs. 90.60 ± 2.48) (P < 0.001) [Table 2].

Table 2

Intubation criteria and complications

	Mean±SD		P
	Group AT (n=35)	Group M (n=35)
POGO score	97.02±2.41	90.60±2.48	<0.001*
Intubation time (s)	11.50±4.36	14.18±3.42	<0.001*
Time to best laryngoscopic view (s)	5.82±1.17	8.87±2.09	<0.001*
Number of intubation attempts, n (%)
1	34 (97.1)	33 (94.3)	0.55
2	1 (2.9)	2 (5.7)
Blood staining of laryngoscope blade, n (%)	1 (2.9)	2 (5.7)	0.55
Sore throat, n (%)	0	1 (2.9)
Laryngospasm, n (%)	0 (0.0)	1 (2.9)	1.00
Need for external laryngeal manipulation, n (%)	2 (5.7)	4 (11.4)	0.39
Need to change head position, n (%)	0 (0.0)	3 (8.5)	0.23

*P<0.05 indicates significance between the two group. Data were expressed as mean±SD or n (%). n=Number of patients, AT=Airtraq, M=Macintosh, SD=Standard deviation, POGO=Percentage of glottic opening

External laryngeal manipulation was needed in 4 (11.4%) of Group M patients compared to 2 (5.7%) patients in Group AT (P = 0.39). None of both groups’ patients needed laryngoscope blade change or the use of bougie. Three (8.57%) patients in Group M needed better optimization of the head position compared to none in the AT group (P = 0.23) [Table 2].

The laryngoscope blade was found to be blood stained in 2 (5.7%) cases in Group M compared to 1 (2.9%) patient in Group AT (P = 0.55). One (2.9%) patient in Group M needed throat suction of blood-tinged secretions during laryngoscopy. Postoperatively, laryngospasm and sore throat were recorded in 1 (2.9%) of Group M patients compared to none in Group AT (P = 1, P = 0.31, respectively). None of both groups’ patients suffered esophageal intubation or had obvious injury in the oral cavity [Table 2].

Both groups were comparable as regard baseline HR (P = 0.29), MAP (P = 0.03), IOP (P = 0.5), serum epinephrine (P = 0.17), serum cortisol (P = 0.38), and serum glucose (P = 0.37) [Figures 2–4 and Table 3].

Figure 2

Mean arterial pressure of the studied groups

Figure 4

Heart rate changes of the studied groups

Table 3

Stress hormonal levels in the serum

	Mean±SD		P
	Group AT (n=35)	Group M (n=35)
Epinephrine before induction (nmol/l)	0.53±0.05	0.55±0.04	0.17
Epinephrine 2 min after intubation (nmol/l)	0.86±0.11	1.04±0.15	<0.001*
Cortisol before induction (nmol/l)	233.72±103.17	307.40±200.44	0.38
Cortisol 2 min after intubation (nmol/l)	465.25±175.05	563.27±186.33	0.02*
Glucose before induction (mg/dl)	91.92±6.02	93.05±4.57	0.37
Glucose 2 min after intubation (mg/dl)	106.50±3.75	108.15±8.64	0.30

*P<0.05 indicates significance between the 2 group. Data were expressed as mean±SD. SD=Standard deviation, AT=Airtraq, M=Macintosh

After induction of anesthesia, both groups showed reduction in MAP and HR without statistically significant difference between them. These parameters started to increase again with laryngoscopy until it became significantly higher in M group than AT group immediately before ETT insertion, and 1, 3, and 5 min after ETT insertion. Five min after intubation, the HR and MAP returned to near their baseline values in both groups [Figures 2 and 4].

The IOP increased significantly in Group M than Group AT immediately after ETT placement, at 2, 4, and 6 min after ETT placement [Figure 3]. Serum epinephrine and cortisol increased significantly in Group M than Group AT patients at 2 min after intubation (1.04 ± 0.15 nmol/l vs. 0.86 ± 0.11 nmol/l) (P < 0.001) and (563.27 ± 186.33 nmol/l vs. 465.25 ± 175.05 nmol/l) (P = 0.02), respectively. Two minutes after intubation, serum glucose was insignificantly higher in Group M than Group AT (108.15 ± 8.64 vs. 106.5 ± 3.75 mg/dl) (P = 0.3) [Table 3].

Figure 3

Intraocular pressure of the studied groups

DISCUSSION

The present study showed that endotracheal intubation was not only accomplished within a shorter time with Airtraq in comparison to Macintosh laryngoscope but also it provoked a lesser stress response. As all the intubations were performed by the same anesthetist and the two groups had comparable demographic data and standardized anesthesia technique, the assessed parameters should be primarily related to the used laryngoscope. In consistency with our results, Airtraq facilitated better glottis visualization, shorter intubation time, and better intubation criteria in other studies.[7910]

Although the difference in the first-attempt success was statistically insignificant between AT and Macintosh in the study population, in a population at risk of aspiration it is clinically very significant. Similar results were recorded in a recent meta-analysis.[11]

In consistency with other studies,[1213] our study showed that in comparison to Macintosh, Airtraq facilitated significantly better glottis visualization. We chose the POGO score to quantify the laryngeal view as it is more sensitive than Cormack–Lehane grading and has a better intra- and inter-rater reliability.[14] Despite the Airtraq facilitated better laryngeal views, external laryngeal manipulation was needed in two patients to allow passage of the ETT between the vocal cords. Furthermore, one of these patients needed a second intubation attempt during which slight clockwise rotation with a backward inclination of the Airtraq combined with external laryngeal manipulation resulted in better centralization of the glottic opening within the view which helped easy passage of the ETT. This problem is mostly related to the preformed curvature and the dedicated channel for loading the ETT which limit the intubator's ability to manipulate the ETT, instead, the Airtraq itself should be manipulated. To overcome such a problem, other authors[15] advised to retract the ETT, and then introduce a gum elastic bougie through the ETT into the trachea, then the ETT can be introduced over the bougie. Similar findings were previously recorded in pediatrics[16] and simulated laryngoscopic difficulty.[17] In consistency with our findings, lesser optimization maneuvers were needed with Airtraq in comparison to Macintosh in other studies.[1819]

Chalkeidis et al.[8] recorded unsuccessful intubation of three patients with Airtraq because of the blurred visual field in two patients and accidental extubation during withdrawing the AT from the mouth after successful intubation in the 3rd patient. This could be related to improper familiarity of the laryngoscopist with the device or improper use of the Airtraq. Vlatten et al.[18] reported a 20% better glottic visualization and a 25% longer intubation time with Airtraq than Macintosh laryngoscopes despite having comparable time to best laryngoscopic view in pediatrics. Although the investigators were experienced laryngoscopists, yet they were not experienced enough in using the Airtarq.

In comparison to Macintosh, the needed force to visualize the laryngeal opening is lesser and the applied pressure on tongue base is more uniformly distributed with video laryngoscopes.[20] In this study, insignificantly more frequent airway trauma and sore throat were encountered with Macintosh. This may be related to the relatively greater force required to visualize the laryngeal opening. Al-Ghamdi et al.[21] also presented similar results in routine airway management. However, they reported longer intubation and laryngoscopy times with Airtraq in comparison to Macintosh. This may be related to the limitation and diversity of the intubators experience and skills in Airtraq use (25 staff anesthetists whose experience with Airtraq were a simulation course before the study).

Direct laryngoscopy and endotracheal intubation precipitate a significant pressor response and increase the IOP. The epilaryngeal and laryngotracheal stimulation occurring during laryngoscopy results in reflex sympathoadrenal discharge. This discharge increases the atrial blood pressure and HR. It also increases the IOP by increasing the resistance to aqueous humor drainage, causing vaso- and venoconstriction, and increasing the central venous pressure, which is closely related to the IOP.[569]

We found, as did other studies,[692223] a significant increase in HR, MAP, and IOP with Macintosh compared to Airtraq laryngoscopes. The more favorable hemodynamic profile with Airtraq can be attributed to the shorter intubation time and the lesser pressor response resulting from minimal stimulation of airway stress receptors. This, in turn, could be attributed to the special design of the optical components and the curvature of the Airtraq that allowed adequate glottic vision without alignment of the oral, pharyngeal, and tracheal axes. Another contributing factor is the lesser traction needed to lift the mandible.[9] Furthermore, the ETT insertion through the vocal cords was atraumatic due to better glottic vision and alignment of the tube to the tracheal axis.[24] The Airtraq associated hemodynamic stability may be advantageous in geriatric patients and those with coronary heart disease, dysrhythmias, or hypertension.[10]

On the contrary, Saraçoǧlu et al.[25] found no difference in pressor response or intubation time between Airtraq and Macintosh. They also recorded a significantly higher incidence of airway trauma and sore throat with Airtraq relative to Macintosh. They found blood spots on the AT blade in four cases with negative detailed oral examination. They have attributed the blood spots to vallecular trauma. This was not proved by vallecular video examination to avoid affection of the sore throat assessment later on. They stated that the higher requirements for airway manipulations with Airtraq may have contributed to these traumas. Again, the intubators experience in Airtraq use is questionable as they defined experience as the performance of >500 intubations with Macintosh or 50 intubations with fiberoptic bronchoscope.

The current study showed a comparable airway complications rate between Airtraq and Macintosh during laryngoscopy and after extubation. Stress hormones increase significantly subsequent to laryngoscopy with and without tracheal intubation.[26] In this study, there was a lesser increase in serum epinephrine, cortisol, and glucose levels with Airtraq in comparison to Macintosh indicating a lesser magnitude of stress response with Airtraq. The difference between the two groups was significant for epinephrine and cortisol but not for glucose. We assume that if serum glucose was measured later, this difference may have become significant as it may take longer time for serum glucose to rise.

The IOP increased significantly with Macintosh than Airtraq immediately after intubation and at 2, 4, and 6 min after intubation. Compared to baseline, IOP measured immediately after intubation increased significantly in both groups. However, the mean increase was 1.07 mmHg in the AT group and 4.67 mmHg in M group. This should not be deleterious to a normal eye but can be harmful for a patient with glaucoma or impending perforation. This is consistent with the results of other studies.[627]

In the current study, endotracheal intubation was confined to a single anesthetist to minimize the interintubator bias. However, there was an obvious but unavoidable risk of bias due to the inability to blind the intubator to the laryngoscope being used. Furthermore, adequate experience and familiarity with both laryngoscopes might result in a difference in results with less experienced users. The study was conducted on patients without significant cardiovascular disease or suspicion of difficult intubation, and hence, its results cannot be applied to patients having these criteria.

CONCLUSION

We conclude that in comparison to Macintosh, Airtraq is superior in the provision of better intubation criteria and hemodynamic stability. We recommend its routine use for tracheal intubation, especially in patients having raised IOP and those at risk for cardiovascular events.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.