Indirect laryngoscopes, including videolaryngoscopes, are widely used for tracheal intubation in the clinical setting. They have a number of advantages for tracheal intubation in that they can be used successfully without needing to align the laryngeal, pharyngeal, and oral axes,1,2 and the optical camera attached to the tip of the scope enables more accurate tracheal intubation by visualizing the glottis from a short distance.3,4 Indeed, indirect laryngoscopy is reported to be superior to conventional direct laryngoscopy for tracheal intubation.5,6

Previous meta-analyses have shown that indirect laryngoscopes are also useful in patients in whom intubation is difficult, such as those requiring manual in-line stabilization7,8 and those with severe obesity.9 In addition, indirect laryngoscopes are considered useful for tracheal intubation by novice operators. Studies in mannequins have shown that the intubation rate is higher and intubation time is shorter when novices use an indirect laryngoscope rather than a direct laryngoscope.10,11 Nevertheless, an indirect laryngoscope may not be able to successfully guide the tracheal tube to the glottis, even if the glottis can be visualized,12 and the video images do not visualize the pharynx and hypopharynx, which can lead to visual and cognitive blind spots.13,14 These disadvantages suggest that indirect laryngoscopes may not always be effective in the hands of novice operators. Moreover, clinical studies in humans have not been able to determine whether indirect laryngoscopy is advantageous for tracheal intubation in inexperienced hands.15,16,17

We sought to undertake this systematic review and meta-analysis to determine whether indirect laryngoscopy has an advantage over direct laryngoscopy in terms of the tracheal intubation rate, glottic visualization, and intubation time when used by novice operators. We also aimed to compared the frequency of adverse events, including esophageal intubation, oropharyngeal injury, and desaturation, between indirect laryngoscopes and direct laryngoscopes.

Methods

The manuscript was prepared following the recommendations of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement.18 The study protocol was registered at PROSPERO (registration number, CRD42022309045; registered 4 September 2022).

Search strategy

We conducted a comprehensive literature search using the PubMed, Embase, and Cochrane Central Register of Controlled Trials databases. The search strategy used is shown in Electronic Supplementary Material (ESM) eAppendix 1. We also manually searched the reference lists in the reports and reviews extracted to identify further potentially eligible articles. No restrictions were imposed on article type or language of the publication. The search was performed in October 2022.

Selection of included studies

Articles were extracted by each of the authors working independently and assessed for suitability for inclusion in the systematic review. Disagreements regarding interpretation or analysis of the data in the extracted articles were resolved through discussion. In the event of duplicate reporting, only the report that analyzed the most recent data were included. If necessary, the authors of potentially eligible articles were contacted directly to obtain missing data and resolve any inconsistencies. For each included study, we searched online to confirm if the research protocol had been published, and if so, whether its content matched the results subsequently reported. A risk of bias was recorded if the study protocol had not been published.

Studies were eligible for inclusion if they had a prospective randomized design and compared the outcomes of tracheal intubation for adult patients by novice operators using an indirect laryngoscope or a direct laryngoscope. Information on success rate (first attempt), glottic visualization (Cormack–Lehane classification 1 vs ≥ 2), and intubation time was extracted from the eligible articles. The definition of failure of tracheal intubation was recorded for each study. Adverse events during tracheal intubation were also compared between the two types of laryngoscopes.

The research question was framed using the Population, Intervention, Comparison, Outcomes framework as follows: population = patients requiring oral tracheal intubation when undergoing surgery under general anesthesia; intervention = tracheal intubation with an indirect laryngoscope attempted by a novice operator; comparison = tracheal intubation with a direct laryngoscope attempted by a novice operator; and outcomes = tracheal intubation success rate, glottic visualization, and intubation time.

Studies with mannequins; studies in which tracheal intubation was performed during cardiopulmonary resuscitation or nasal intubation, and in pediatric patients; and studies that used double-lumen tubes were excluded. We also divided the indirect laryngoscopy groups and direct laryngoscopy groups into subgroups to compare outcomes according to whether or not a tracheal tube guide was used.

Critical appraisal of study quality

Risk of bias and quality of evidence

We evaluated the risk of bias with reference to the Cochrane Handbook19 (ESM eAppendix 2). The quality of evidence for the main outcomes was assessed using the Grading of Recommendations Assessment, Development and Evaluation approach20 (ESM eAppendix 3).

Data synthesis and analysis

Statistical analysis was performed using the DerSimonian and Laird random effects model. Binary variable pool effect estimates (success rate, glottic visualization, and adverse events) are expressed as the relative risk (RR) with 95% confidence interval (CI). The pooled difference in intubation time between the indirect and direct laryngoscope groups is expressed as the weighted mean difference (WMD) of the 95% CI. The heterogeneity of effect size was examined using the Cochran Q test and the I2 statistic.21

We also performed a trial sequential analysis (TSA) to assess sensitivity to prevent type I error arising from multiple tests of effect in the meta-analysis.22,23 First, we calculated the required sample size (required information size [RIS]) and set the risk of type I error to 5% and the risk of type II error to 10%. We set the minimum clinically meaningful risk ratio in TSA to 1.33 and the mean difference to ten seconds. Trial Sequential Analysis version 0.9.5.5 beta (Copenhagen Trial Unit, Centre for Clinical Intervention Research, Copenhagen, Denmark) was used for this analysis.

Publication bias was assessed by testing the symmetry of a funnel plot24 and by Begg’s test.25 A P value of < 0.1 from this test indicated publication bias.

Results

Characteristics of included studies

The literature search identified 332 potentially relevant articles. Eighty-six studies were immediately identified to be unrelated and excluded. The remaining 246 articles were carefully read to determine whether they met our eligibility criteria. A further 226 studies were excluded for the following reasons: trial performed in mannequins (n = 88); not a randomized controlled trial (n = 51); laryngeal mask airway used (n = 27); a review article (n = 15); a cardiopulmonary resuscitation trial (n = 14); indirect laryngoscope not used (n = 13); not involving novice operators (n = 9); other reason (n = 8); involving pediatric patients (n = 4); and nasal intubation used (n = 2). The remaining 15 articles (17 trials) met our inclusion criterion and contained the data necessary for comparison (Fig. 1). These 15 articles are summarized in Table 1.2,15,16,17,26,27,28,29,30,31,32,33,34,35,36,37

Fig. 1
figure 1

Systematic review and meta-analysis flow chart

RCT = randomized controlled trial

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Table 1 Characteristics of included studies
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The included studies were published between 2009 and 2018. The most common indirect laryngoscope used was the Airtraq™ (Mercury Medical®, Clearwater, FL, USA; six trials), followed by the GlideScope® (Verathon Inc., Bothell, WA, USA; four trials), the McGRATH™ (Medtronic PLC, Dublin, Ireland; three trials), the Pentax Airway Scope (Nihon Kohden Corp., Tokyo, Japan; two trials), the C-MAC® (Karl Storz SE & Co. KG, Tuttlingen, Germany; one trial), and the Truview EVO2 (Leica Geosystems AG, Heerbrugg, Switzerland; one trial). The definition of a novice operator was a resident in ten trials and a medical student in the remaining five trials. The preoperative condition of the airway was reported to be normal in all but one trial. All direct laryngoscopes used were Macintosh laryngoscopes (Table 1).

Meta-analysis results

In total, 1,169 patients were intubated using an indirect laryngoscope and 1,121 using a direct laryngoscope.

Intubation performance

In the 17 trials, the tracheal intubation success rate was significantly higher with an indirect laryngoscope than with a direct laryngoscope (RR, 1.15; 95% CI, 1.07 to 1.24; P = 0.0002; Cochrane’s Q = 134.2; I2 = 88%; Fig. 2). Absolute risk reduction was 17.7% (indirect laryngoscopy, 89.1% vs direct laryngoscopy, 71.9%). For success rate, our TSA revealed that the Z-curve crossed the efficacy boundary, although the RIS was not reached (ESM eFig. 4).

Fig. 2
figure 2

Forest plot of the success rate of tracheal intubation using indirect laryngoscopy versus direct laryngoscopy

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Glottic visualization was evaluated in nine trials and was better when an indirect laryngoscope was used (RR, 1.76; 95% CI, 1.36 to 2.28; P < 0.001; Cochrane’s Q = 45.5; I2 = 85%; Fig. 3). Absolute risk reduction was 36.6% (indirect laryngoscope 83.3% vs direct laryngoscope 47.6%). The Z curve did not reach the TSA monitoring boundary for benefit, and the accrued sample size (n = 984) was 22.7% of the required sample size (n = 4,328) (ESM eFig. 5).

Fig. 3
figure 3

Forest plot of glottic visualization with indirect laryngoscopy versus direct laryngoscopy (Cormack–Lehane grade 1 and 2 vs other grades)

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Intubation time was significantly shorter with an indirect laryngoscope than with a direct laryngoscope (WMD, −9.06 sec; 95% CI, −16.4 to −1.76; P = 0.02; Cochrane’s Q = 508.3; I2 = 98%; Fig. 4). The Z curve crossed the futility boundary. Trial sequential analysis revealed that the accrued information size (n = 1,990) was 76.5% of the estimated RIS (n = 2,600) (ESM eFig. 6).

Fig. 4
figure 4

Forest plot of intubation time for tracheal intubation using indirect laryngoscopy versus direct laryngoscopy

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Subgroup analysis

In addition, the indirect and direct laryngoscopy groups were classified and analyzed according to whether a tracheal tube guide was used. The subgroup analysis according to whether or not a tracheal tube guide was used found that successful intubation and glottic visualization rates were significantly better with both indirect laryngoscopes than with a direct laryngoscope (with tracheal tube guide, success rate: RR, 1.24; 95% CI, 1.06 to 1.44; P < 0.006; Cochrane’s Q = 68.7, I2 = 90%; glottic visualization: RR, 2.38; 95% CI, 1.59 to 3.57; P < 0.001; Cochrane’s Q = 14.7; I2 = 80%, without tracheal tube guide, success rate: RR, 1.11; 95% CI, 1.01 to 1.23; P = 0.03; Cochrane’s Q = 61.5, I2 = 88%; glottic visualization: RR, 1.76; 95% CI, 1.36 to 2.28; P < 0.001; Cochrane’s Q = 45.5; I2 = 85%) (Figs 2 and 3). Nevertheless, intubation time using an indirect laryngoscope with or without a tracheal tube guide was comparable to that using a direct laryngoscope (Fig. 4).

Adverse events

Adverse events during tracheal intubation were compared according to whether an indirect laryngoscope or direct laryngoscope was used. The incidence of all adverse events during tracheal intubation was significantly lower with an indirect laryngoscope (esophageal intubation: RR, 0.16; 95% CI, 0.04 to 0.61; P = 0.007; Cochrane’s Q = 2.18; I2 = 8%; oropharyngeal injury: RR, 0.42; 95% CI, 0.23 to 0.76; P = 0.004; Cochrane’s Q = 2.50; I2 = 0.0%; oxygen desaturation; RR, 0.51; 95% CI, 0.27 to 0.97; P = 0.04; Cochrane’s Q = 0.08; I2 = 0.0%; Table 2).

Table 2 Comparison of adverse events during tracheal intubation using indirect laryngoscopy versus direct laryngoscopy
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Quality of evidence

The quality of evidence for success rate, glottic visualization, and intubation time according to type of laryngoscope used by a novice operator was graded as “very low.” All of the included studies were found to have a moderate risk of bias because the operator could not be blinded to the type of laryngoscope used. Heterogeneity was high for all parameters, and there was publication bias in terms of the success rate and glottic visualization rate. Accordingly, the quality of evidence was downgraded to “very low” (Fig. 5).

Fig. 5
figure 5

The Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach

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Results of publication bias

The Begg’s test identified publication bias for success rate (Kendall’s statistic = 50.0; Z = 1.85; P = 0.02) and glottic visualization (Kendall’s statistic = 20.0; Z = 2.09; P = 0.06). No publication bias was found for intubation time (Kendall’s statistic =  −12.0; Z = 0.59; P = 0.4). Figure 6 summarizes the risks of bias.

Fig. 6
figure 6

Green circles, red circles, and yellow circles indicate “low risk of bias,” “high risk of bias,” and “unclear risk of bias,” respectively

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Discussion

This systematic review and meta-analysis found that tracheal intubation success rate, glottic visualization, and intubation time were improved when a novice operator used an indirect laryngoscope rather than a direct laryngoscope. Use of an indirect laryngoscope by a novice also reduced the risk of adverse events, including esophageal intubation, oropharyngeal injury, and desaturation.

In general, direct laryngoscopy enables tracheal intubation by aligning the oral, pharyngeal, and laryngeal axes.1,2 Nevertheless, indirect laryngoscopy can visualize the glottis without aligning them. Furthermore, use of an indirect laryngoscope allows the glottis to be confirmed in closer proximity by displaying the image obtained by the camera attached to the tip of the laryngoscope blade on an external monitor.3,4 These advantageous features of indirect laryngoscopes are considered to make tracheal intubation easier, thereby contributing to successful tracheal intubation by novice operators.27,38,39

Another advantage of using indirect laryngoscopes for novice operators is that information on the condition of the upper respiratory tract and the area near the glottis can be shared with supervisors during tracheal intubation.27,38,39 By sharing these images, the novice operator can receive appropriate advice and is thus more likely perform tracheal intubation successfully.

Novice operators can also learn to perform tracheal intubation more quickly using an indirect laryngoscope. Previous studies have also shown that the learning curve is less steep for an indirect laryngoscope than for a direct laryngoscope.15,31,40,41 Most of the randomized controlled trials included in the present systematic review and meta-analysis incorporated practicing tracheal intubation using both indirect and direct laryngoscopes on mannequins before use in patients. It may be easier for novices to master tracheal intubation with fewer preclinical exercises when using an indirect laryngoscope.

The subgroup analysis showed that the success rate and glottis visualization were significantly better for both indirect laryngoscopes regardless of whether a tracheal tube guide was used, compared with direct laryngoscope. Furthermore, intubation time was not significantly different between indirect and direct laryngoscopes, regardless of whether a tracheal tube guide was used. This finding suggests that tracheal intubation can be performed successfully using an indirect laryngoscope with or without a tracheal tube guide. Nevertheless, intubation time also varies depending on whether an intubation aid such as a stylet or gum-elastic bougie was used during tracheal intubation.42 In this systematic review and meta-analysis, we were unable to investigate the use of intubation aids, so we were unable to remove these biases. Further studies are warranted, as each study defined intubation time differently and the sample size was insufficient for analysis.

The incidence of adverse events was significantly lower when an indirect laryngoscope was used. The main reason for the reduced incidence of esophageal intubation with an indirect laryngoscope is that novice operators can share accurate information with their supervisor on a video screen and receive better guidance.27,38,39

A previous study found a higher incidence of adverse events, including soft tissue bleeding, oropharyngeal injury, and dental trauma, when video laryngoscopes were used.13 Indirect laryngoscopes create visual and cognitive blind spots that can increase the risk of oropharyngeal injury.14 Nevertheless, in our meta-analysis, the incidence of oropharyngeal injury was significantly lower when an indirect laryngoscope was used. Use of an indirect laryngoscope achieved successful tracheal intubation even if the pharyngeal lifting force of the laryngeal deployment was low.43,44 This low pharyngeal lifting force helps to protect against oropharyngeal injury. Also, indirect laryngoscope blades made of polyethylene are softer and less sharp than a stainless steel blade of an direct laryngoscope. This indirect laryngoscope blade configuration also helps to reduce incidence of oropharyngeal injury. When intubated without a stylet with a videolaryngoscope and an angled blade (GlideScope, McGRATH, C-MAC), it may be difficult to pass the tube through the vocal cords despite a good glottic view.42 On the other hand, the use of stylets contributes to oropharyngeal injury. This systematic review did not include studies that described the use of stylets, so we were unable to establish a clear relationship between stylet use and oropharyngeal injury. The shorter intubation time associated with use of an indirect laryngoscope may also decrease the risk of desaturation.

The results of our study show that indirect laryngoscopes are useful for tracheal intubation in novice operators. This result suggests the possibility of making tracheal intubation safer for residents and nonexperienced anesthesiologists, as well as making tracheal intubation safer for novice operators outside of operating rooms such as hospital wards and emergency departments.

Limitations

This systematic review and meta-analysis has several limitations. First, the type of laryngoscope used could not be blinded, which increased the risk of bias. Second, moderate to high heterogeneity was found in our results, which affected the study quality; however, subgroup analyses were performed. Third, the definition of a novice operator was not consistent between the included trials. Most operators were residents, but were medical students in four trials. Fourth, RIS was not reached for some results in the TSA analysis. Therefore, analysis of the glottis visualization was underpowered. Also, a separate per geometry analysis of individual indirect laryngoscopes was not possible, and detailed data on the preoperative airway status of individual patients were not available. Furthermore, patient age and height, anesthesia method, and definition of intubation time varied across the trials, and these differences also affected the study quality.

Conclusion

In this systematic review and meta-analysis, we found that the tracheal intubation success rate, glottic visualization, and intubation time were improved when novice operators used an indirect laryngoscope rather than a direct laryngoscope. Moreover, the risk of adverse events, including esophageal intubation, oropharyngeal injury, and desaturation, was lower when novices used an indirect laryngoscope. Trial sequential analysis indicated that the sample size was sufficient for examining the success rate and intubation time.