a b s t r a c t

Study objective: This study sought to determine and compare the utility of the Airway scope (AWS; Pentax Corporation, Tokyo, Japan) and the conventional Macintosh laryngoscope (MLS) for intubation in the prehospital setting.

Methods: In this randomized controlled trial in the prehospital setting, the primary outcome was time required for intubation, and the secondary outcomes were ultimate success, first attempt success, and difficulty of intubation. The intent-to-treat principle was used to analyze time to intubation. Ultimate success was defined as intubation completed within 600 s regardless of the device ultimately used.

Results: A total of 109 patients, primarily with cardiac arrest, were randomly assigned to the AWS or MLS arms. Median time (interquartile range) to intubation was 155 (71-216) s with the AWS versus 120 (60-170) s with the MLS (P = .095). Ultimate success rate was slightly lower with the AWS (96.4%) than with the MLS (100%) (P = .496), while the First attempt success rate was significantly lower (46% and 75%, respectively; P =

.002). There was no significant difference in difficulty of intubation (P = .066). Multivariate logistic regression analysis revealed that Cervical immobilization and oral contamination, such as vomit, was associated with first attempt success (odds ratio [95% confidence interval]: 0.11 [0.01-0.87] and 0.43 [0.18-0.99], respectively). Conclusion: Despite its many advantages seen in other settings, the AWS did not show superior efficacy to the MLS in relation to time required for intubation, ultimate or first attempt success rate, or difficulty level of intubation in the prehospital setting.

(C) 2013

Introduction

Among the video laryngoscopes used for emergency tracheal intubation, the Airway scope (AWS, Pentax Corporation, Tokyo, Japan) is a common adjunctive device for intubation in Japan. Its efficacy has been reported in patients under general anesthesia in the operating room and with a manikin [1-7]. The most recent studies report on other video laryngoscopes and compare their utility with that of the conventional Macintosh laryngoscope (MLS) either in the emergency department (ED) or intensive care unit [8-11]. However, few studies have focused on the use of the AWS in the prehospital setting, where most patients who require intubation are suffering cardiac arrest.

The AWS has several advantages. First, it enables the operator to see the passage of the tracheal tube through the vocal cords via a display, even in the case of a difficult airway [1,6,7,12]. Second, patients requiring intubation in the prehospital setting are typically lying on the ground, meaning that an optimal table height for

* Corresponding author. Emergency Department Funabashi Municipal Medical Center 1-21-1, Kanasugi, Funabashi City, Chiba, Japan (260-8677). Tel.: +81 47 438

3321; fax: +81 47 438 7323.

E-mail address: [email protected] (T. Arima).

intubation cannot be set. However, the AWS allows intubation without alignment of the oral, pharyngeal, and tracheal axes, and intubation with the AWS is therefore less influenced by the patients’ position and limited work space than with the MLS [13]. Third, it is reported that the AWS is advantageous with patients with cervical immobilization [2,3] and, fourth, that it might be less affected by chest compressions in comparison with the MLS [14].

The purpose of this study was to determine and compare the utility of the AWS and MLS for intubation in the prehospital setting in order to evaluate the efficacy of the AWS.

Methods

This randomized controlled trial ran from February 2012 to March 2013. The study protocol was approved by the Ethics Committee of Funabashi Municipal Medical Center, Funabashi City, Japan.

Funabashi City operates a physician car system where physicians and paramedics travel together by ambulance to incident sites. Around 500 cases of cardiac arrest are responded to annually. The physicians that staff the service are those on day shifts at Funabashi Municipal Medical Center’s ED, and they are dispatched according to their current duties in the ED, with no regularity in their dispatch. All

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T. Arima et al. / American Journal of Emergency Medicine 32 (2014) 40–43 41

physicians have more than 3 years of working experience and are sufficiently skilled and trained in ED medicine and the use of both the AWS and MLS devices in daily practice.

Patients sequentially selected for inclusion in this study were those age >=18 years and requiring emergency tracheal intubation in the prehospital setting only during the day shift. The operators were told which of the two devices had been allocated to them to use only when en route to the incident in the ambulance. The allocation was changed in a serial manner and was controlled by personnel at the physician car system center. If the first intubation failed, the operator could decide to try the same device again or change to the other device. A suction device and Magill forceps were available for use at any time. Intubation was performed only by the physician dispatched. The primary outcome was time required for intubation, and the secondary outcomes were ultimate success, first attempt success, and difficulty of intubation. Ultimate success was defined as intubation

completed within 600 s regardless of the device ultimately used.

Time to intubation was measured from the insertion of the blade between the teeth to the confirmation of endotracheal tube placement by capnograph. If the blade was inserted to perform oral suction, time measurement was started at this point. endotracheal tube position was confirmed by visualization, auscultation, esopha- geal detector devices, and then the capnograph. If the intubation failed and the device for intubation was changed, time was measured from the insertion on the first attempt to the success on the second or successive attempts. Time records were kept by the paramedics.

In order to evaluate difficulty of intubation, we used the intubation difficulty score (IDS) [15]. Soon after intubation, the operator recorded on the medical sheet the following: IDS, difficult airway characteristics, and patient demographics, including occurrence of cardiac arrest, cervical supine immobilization, and oral contamination by vomit, foreign body, blood, or massive secretion.

All statistical analysis was performed using JMP 10 (SAS Institute Inc, Cary, NC). Continuous variables were compared between the AWS and MLS with Student t test or the Mann-Whitney U test, as appropriate. Categorical variables were compared between the groups with the ?² test or Fisher’s test, as appropriate. The intent-to-treat principle was used to analyze time to intubation. Based on the preliminary research, we calculated a sample size to detect 30 seconds reduction for time to

5 excluded (insufficient records)

58

MLS

2 excluded

(age <18)

121

enrolled patients

53

MLS intubation

53

enrolled in MLS

intubation, with a two-sided ? of .05, and a power of 80.0%. Lastly, in order to find the cause of intubation failure, we performed a multivariate regression analysis. Predictor variables included cardiac arrest, cervical immobilization, and oral contamination.

Results

Of 121 patients enrolled in this study, 12 were excluded due to missing data, age b 18 years, or problems with the device used, leaving 109 for final analysis: in 56 cases intubation was started with the AWS and in 53 cases it was started with the MLS (Figure). Eleven physicians, 6 with a background in anesthesia, were involved in responding to the patient incidents included in the study.

The patient characteristics of both groups are shown in Table 1. Most of the patients had cardiac arrest, but the groups did not differ significantly in terms of age, proportion of men, cases of cardiac arrest, requiring cervical immobilization, or proportion of operators with background in anesthesia. Only oral contamination with vomit, blood, foreign body, or massive secretion differed significantly between the groups, being higher in the AWS group (58% vs 30.2%; P = .021).

Differences in the primary and secondary endpoints between the groups are shown in Table 2. There was no difference in terms of the median time to intubation, ultimate success rate, or difficulty of intubation. However, the first attempt success rate was significantly lower with the AWS than with the MLS, at 46% (26/56 cases) and 75% (40/53 cases), respectively (P = .002).

In 2 cases, intubation was not completed within 600 s regardless of the device ultimately used: one case involved Massive bleeding in the mouth and the other case involved limited mouth opening. Initial intubation with the AWS failed in 20 cases but was followed by successful intubation with the MLS. The number of attempts before switching was none in 3 cases, one in 14 cases, 2 in one case, and 3 in 2 cases. Median time (interquartile range [IQR]) using the AWS before switching was 60 (12-120) s. According to the medical sheets recorded by the operators, the reasons for difficult intubations with the AWS that required switching to the MLS were oral contamination in 12 cases (60.0%), poor visualization of the glottis in 4 cases (20.0%), unable to insert the AWS blade in one case (5.0%), and obscured

61

AWS

34

AWS intubation

56

enrolled in AWS

5 excluded

(3 insufficient records,

2 equipment problems)

2

> 600 seconds

22

AWS-> MLS intubation

Fig. Patient flow through the study indicating patients intubated with the AWS and/or MLS.

42 T. Arima et al. / American Journal of Emergency Medicine 32 (2014) 40–43

Table 1

Patient characteristics

	AWS (n = 56)	LS (n = 53)	P
Mean age, y	74.4+-13.6	74.1+-13.0	NS
Sex male, %	60.7 (34)	71.7 (38)	NS
Cardiac arrest, %	96.4 (54)	88.7 (47)	NS
Cervical supine immobilization, %	5.3 (3)	3.8 (2)	NS
Oral contamination, %	51.8 (29)	30.2 (16)	.021
Background in anesthesia, %	51.8 (29)	47.2 (25)	NS

display because of strong sunlight in one case (5.0%). Two cases (10%) had unknown causes.

Multivariate logistic regression analysis revealed that cervical immobilization and oral contamination were associated with the first attempt success rate (odds ratio [95% confidence interval]: 0.11 [0.01- 0.87] and 0.43 [0.18-0.99], respectively; Table 3).

Limitations

This study was a single-center trial and as such the potential for bias exists especially in regard to the operators. Basically, all 11 operators involved in this study were skilled physicians, 6 of whom had a background in anesthesia, and they all generally perform N 100 intubations each per year. Drawing parallels with intubation performed by paramedics is therefore open to interpretation.

The information recorded after intubation (eg, IDS, occurrence of oral contamination, and reasons for difficult intubation) was, to some extent, subjective. Additionally, standard preoperative predictors of difficult airway, such as obesity, neck mobility, thyromental distance, and Mallampati Score, were not recorded because all intubations were performed in an emergency situation. The IDS was originally created to evaluate difficulty with MLS, especially focusing on Corrmack grade and lifting force required [15]. Therefore it has not been widely validated for the AWS.

The major associating factor of successful intubation identified on multiple logistic regression analysis was cervical immobilization and oral contamination. However, this may be due to the small sample number for these variables. Further studies are required to verify these findings.

Discussion

Intubation should obviously be performed correctly and promptly in any situation, but intubation performed in the prehospital setting is often more difficult and challenging than that performed in the operating room, ED, or intensive care unit because of factors such as preexisting airway compromise, limited workspace, tricky position- ing, lack of Sedative agents, and chest compressions required during cardiopulmonary resuscitation. In comparison to previous studies on the utility of the AWS, the most remarkable difference with the present study is that our sample population was composed of mainly cardiac arrest patients (101/109 cases). This is likely because our study is the only one to have evaluated the AWS in the real prehospital setting.

In contrast to our findings that the AWS was not superior to the MLS, a number of manikin studies in the prehospital setting

Table 2

Primary and secondary endpoints

	AWS (n = 56)	MLS (n = 53)	P
Median time (IQR), s	155 (71-216)	120 (60-170)	.095
Ultimate success, %	96.4	100	.496
First attempt success, %	46.4	75.5	.002
Median IDS (IQR)	0 (0-1)	1 (0-2)	.066

Table 3

Logistic regression model for successful intubation

Variable First attempt success

	Odds ratio	95% confidence interval
Cardiac arrest	2.72	0.42-53.55
Cervical supine immobilization	0.11	0.01-0.87
Oral contamination	0.43	0.18-0.99
Background in anesthesia	0.48	0.20-1.13

have suggested the AWS is more advantageous to use than the MLS [5,6], and other studies have reported advantages of using a video laryngoscope over the MLS in routine and emergency situations [8-11].

First, there was no difference between AWS and MLS in time required for intubation. One possible reason was that the operators were permitted to switch to the MLS device that was personally more comfortable for them because intubation needed to be completed urgently in a life-threatening situation. In addition, they knew that the time was being recorded by paramedics and tried to complete the intubation as quickly as possible even if that meant it was achieved by switching devices. In the intent-to-treat analysis, 20 of 56 cases were started with the AWS but were ultimately intubated with the MLS. In some of these cases, the operators reported giving up on the first intubation attempt with the AWS at an early stage and switched devices. If we had forced them to use only a single device for intubation, the result would not have been the same.

Second, although no significant difference in ultimate success was seen between the two devices, the first attempt success rate was strikingly lower with the AWS than with the MLS. Moreover, it was lower than those of other studies using video scopes, at 46% and 79% to 91% [8-11]. Multiple logistic regression analysis showed that the major associating factors for successful intubation in the present study were cervical immobilization and oral contamination. The latter frequently occurs in cardiac arrest patients and was actually found in 45 of the total 109 cases (41%) in this study. Even if oral suction is performed once, vomit or secretion regurgitated from the esophagus or trachea due to increased intra-thoracic pressure from chest compressions can recur. When the AWS lens was obscured, the AWS needed to be removed from the mouth and cleaned, which required extra time and possibly influenced the time required for intubation and success rate. On the other hand, the suction device is easier to use with the MLS than with the AWS when performing intubation, effectively meaning that the MLS did not need to be removed in the case of oral contamination.

There is the possibility that the present results were influenced by the fact that the two groups were not equivalent in terms of oral contamination and possibly by a difference in the operators’ skill level at using the two devices. Moreover, even though the patients were randomized, the AWS group contained a significantly higher proportion of cases with oral contamination than the MLS group. A possible reason for this is that when using the AWS the operators were more aware of oral contamination problems because oral contamination directly obscures the lens and thus the view. However, since the occurrence of oral contamination was reported after intubation, this information could have been influenced by recall bias. Turning now to the operators’ intubation skills, 6 physicians had generally performed N 100 intubations per year as they had previously worked as anesthetists. The number of AWS intubations they have performed is not precisely known, but is estimated to be in the range of 15 to 30 AWS intubations per physician per year. The remaining 5 physicians had done an anesthesia rotation and had performed at least 50 intubations, but with relatively fewer experiences with AWS intubation. Additionally, their intubation skills had been checked by experienced anesthetists in advance of this study, and they had attended a lecture and training session with a manikin. Plus, in

T. Arima et al. / American Journal of Emergency Medicine 32 (2014) 40–43 43

general, using a video laryngoscope is easy enough to learn [5,6,10,16]. We therefore consider that all of the participating physicians were equally skilled to perform intubations with both the MLS and AWS.

Lastly, there were other problems with AWS aside from oral contamination, such as difficulty inserting the blade. The AWS blade is thicker than that of MLS, so the AWS was difficult to use with patients with limited mouth opening [12]. Another problem was that the display was difficult to see in strong sunlight and thus interfered with intubation, as has been previously reported [17].

Conclusion

Despite its many advantages seen in other settings, the AWS did not show superior efficacy to the MLS in relation to time required for intubation, ultimate or first attempt success rate, or difficulty level of intubation in the prehospital setting, where most of patients required intubation for cardiac arrest. A major cause of failure on first attempts was cervical immobilization and oral contamination. In the prehos- pital setting, the MLS appears to be the first choice for intubation.

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