An Inquiry on Airway Management by McCoy Blade with Elevated Tip and Miller Straight Blade with Paraglossal Technique: Relevance for Difficult Airway Management in Current Infectious Times.

Background: Modifications of curved and straight laryngoscope blades have been used for airway management since a long time. While McCoy blade with an elevated tip is commonly used to intubate patients with anticipated difficult airway, the Miller's straight blade is used for intubations in children and less commonly adults. In this study, we revisit the paraglossal technique of Miller's straight blade as a method to improve laryngeal view especially in difficult intubations. Aim: This study aimed to compare laryngoscopic view and ease of intubation (EOI) using McCoy blade elevated tip and Miller's straight blade paraglossal technique. Materials and
Methods: A prospective single-blind study was conducted on 170 patients undergoing elective surgery under general anesthesia. They were randomly allotted to two groups. In Group A, laryngoscopy was performed by Miller's blade paraglossal approach, whereas in Group B, laryngoscopy was performed by McCoy blade with an elevated tip. Laryngeal view was graded using the modified Cormack-Lehane grading, and EOI was graded using EOI score. These were compared with preoperative intubation prediction score. Statistical analysis was done using "Medcalc" version 19.0.3. Numerical and categorical data were analyzed by Student's t-test and Chi-square test, respectively. A P < 0.05 was considered statistically significant.
Results: The paraglossal approach with Miller's blade offered better laryngoscopic view as compared with McCoy blade with an elevated tip in normal (54.1% vs. 25.9%) and difficult airway (44.7% vs. 11.8%). Tracheal intubation was easier with McCoy blade with an elevated tip although the success rate of intubation improved with the assistance of a bougie with Miller's straight blade paraglossal approach.
Conclusion: The laryngeal view was significantly better with the paraglossal approach of Miller's straight blade even in difficult airway. McCoy blade with an elevated tip was also found to be a useful tool to have in difficult airway, as EOI is significantly higher. The study also highlights the usefulness of adjuncts such as a gum elastic bougie while intubating. Copyright:

INTRODUCTION

Securing the airway is an intrinsic part of anesthesia, and conventional laryngoscopy with endotracheal intubation is one of the most common clinically practiced interventions in hospitals worldwide. While there are several indications for intubation, laryngoscopy is commonly used for administration of general anesthesia, controlled ventilation, protecting the airway, removing secretions from the airway, and isolating the lung during surgeries in the thorax (one-lung ventilation). Sustained innovations and research in the field of airway biomedical engineering in recent decades have led to the advent of a plethora of laryngoscope blades and airway instruments. However, when it comes to conventional laryngoscopy, the curved and the straight blades have stood the test of time and remain one of the most trusted and commonly used instruments.

Various modifications in the design of blades and the techniques of laryngoscopy have been suggested all along for optimizing intubating conditions and laryngeal view. McCoy blade[1] with an elevated tip is a modification of the curved blade, wherein the hinged tip is postulated to facilitate the elevation of epiglottis without causing a significant hemodynamic response to laryngoscopy. Paraglossal approach with a Miller's straight blade, on the other hand, is a modified technique of laryngoscopy employed to enhance the laryngeal view in both anticipated and unanticipated difficult airway. In this approach, the blade is advanced from the right corner of the mouth in the groove between the tongue and the tonsil using leftward and anterior pressure to displace the tongue to the left. The tip of the blade is passed underneath the posterior surface of the epiglottis and elevated anteriorly to expose the glottis. An assistant retracts the cheek to create space for the passage of the endotracheal tube. The paraglossal approach with Miller's blade and the McCoy blade with its elevated tip have independently demonstrated an improvement in laryngeal view in several studies and have been reported to be useful in several difficult airway scenarios.[2] However, there were no previous studies comparing the McCoy elevated tip and Miller's blade with paraglossal approach in terms of the laryngeal views obtained and ease of intubation (EOI) and that was precisely what we set out to do. While our study was conducted in the pre-COVID era, the analysis of results and the subsequent brainstorming while preparing the manuscript made us realize the importance of both techniques vis a vis aerosol exposure during the process of airway instrumentation in the infectious times that we all live and operate in. We therefore aim to present the results of our study in tandem with its importance to improving safety both in terms of managing difficult airway situations and enhancing safety for the operator.

MATERIALS AND METHODS

After due approval from the institutional ethics committee (TMH/FRM/QMS/ALL/19, dated November 5, 2014) and obtaining informed consent from the patients in accordance with the Declaration of Helsinki, we conducted a prospective, randomized, single-blind study on 170 patients undergoing elective surgery under general anesthesia. The study was conducted between January 2015 and October 2016 (1 year and 10 months). Patients of either sex aged between 18 and 70 years and belonging to an American Society of Anaesthesiologists (ASA) Physical Status classification classes I and II were included in this study. The exclusion criteria comprised higher ASA Physical Status classes (III and IV), emergency surgery, and the patients not unwilling to participate in the study.

A detailed preanesthetic checkup including an extensive airway assessment was performed in all the patients. Significant parameters pertinent to our study which were noted included demographic parameters, mouth opening, modified Mallampatti grade,[3] atlanto-occipital joint extension (using goniometer),[4] sternomental distance, and length of mandible (mandibular space).[5] Each of the above airway predictive tests was graded and each grade was given a score (intubation prediction score or [IPS]),[6] as summarized in Table 1.

Table 1

Intubation prediction score

Parameters	1 point	2 point	3 point	4 point
Mallampatti grading classified by the observer on the basis of the pharyngeal structures visualized. Then the view is graded	Grade 1: Soft palate, fauces, uvula, anterior and posterior tonsillar pillars	Grade 2: Soft palate, fauces, uvula	Grade 3: Soft palate, base of uvula	Grade 4: Soft palate not visible at all
AOJE angle between the erect and extended planes of the occlusal surface of the upper teeth quantitates the degree of AOJE	Grade 1: AOJE ≥35°	Grade 2: AOJE ≥22° and<35°	Grade 3: AOJE ≥13° and <22°	Grade 4: AOJE <13°
Mandibular space it includes the TMD and the horizontal LM	Grade 1: TMD ≥6 cm and LM ≥9 cm	Grade 2: TMD ≥6 cm and LM<9 cm	Grade 3: TMD <6 cm and LM ≥9 cm	Grade 4: TMD <6 cm and LM <9 cm

Intubation prediction score[6] Addition of the above points generates a nominal score (IPS) and is classified as: Grade 1: Easy intubation is predicted (3-4 points), Grade 2: Moderately difficult intubation is predicted (5-8 points), Grade 3: Difficult intubation is predicted (9-12 points). IPS=Intubation prediction score, AOJE=Atlanto-occipital joint extension, TMD=Thyromental distance, LM=Length of the mandible

The patients were randomly allocated to two groups by a computer-generated random number function using the Microsoft excel 2003 spreadsheet. Group A included patients in whom laryngoscopy was performed by using a Miller's blade with paraglossal approach [Figure 1] and Group B included patients in whom laryngoscopy was performed by using McCoy blade with its elevated tip [Figure 2].

Figure 1

Paraglossal approach with Miller's straight blade (look for position of the Miller blade of laryngoscope, tongue and teeth in the molar or retromolar variation of the paraglossal straight blade technique)

Figure 2

McCoy blade in its elevated tip position (the hinge at the distal end reduces the force required to lift the epiglottis)

Inside the operating room after establishing standard ASA monitoring and recording the baseline hemodynamic parameters (heart rate, blood pressure, and oxygen saturation), preoxygenation was performed with 100% oxygen for 3 min or till the end tidal oxygen levels were 98% (UNMED AG300). General anesthesia was induced with fentanyl 2 μg.kg−1 and propofol 2–3 mg.kg− 1 intravenously till loss of verbal response, and muscle relaxation was achieved with vecuronium 0.1 mg.kg− 1. After 3 min of controlled ventilation, laryngoscopy and intubation was performed by an experienced anesthesiologist (more than 5 years’ experience in anesthesia). The same anesthesiologist performed the laryngoscopy and intubation in all the cases. The study was designed to be single blind. Hence, the glottic view obtained after laryngoscopy was marked by the anesthesiologist on the diagram of the larynx in the study pro forma. This was interpreted and classified according to the Modified Cormack–Lehane (CL) grading [Figure 3] [7] by an anesthesiologist not involved in the study. Intubation was done with the recommended size of endotracheal tube for the gender of the patient. Anesthesia was maintained with isoflurane titrated to achieve a minimum alveolar concentration of 1.2 with a mixture of 60% nitrous oxide and 40% oxygen. At the end of surgery, neuromuscular block was reversed with appropriate dose of neostigmine (0.5 mg.kg− 1) and glycopyrrolate (10 μg.kg− 1).

Figure 3

Modified Cormack–Lehane grading for laryngeal view. Grade 1- most of the cords visible; Grade 2A - posterior cords visible.; Grade 2B - only arytenoids visible; Grade 3A - epiglottis visible and liftable; Grade 3B - epiglottis adherent to pharynx; Grade 4- no larynx structure seen

The following parameters were recorded: demographic variables, laryngeal views obtained with laryngoscopy [Modified CL scale; Figure 3], and EOI grade[8] on a 4-point scale where Grade 1 was easy intubation, Grade 2 was intubation requiring an increased anterior lifting force and assistance to pull the right corner of the mouth upward to increase space, Grade 3 was intubation requiring more than one attempt and/or bougie or a curved stylet, and Grade 4 was failure to intubate with the assigned laryngoscope (defined as three attempts at failed intubation with the assigned blade). In such patients, intubation was completed with the Macintosh blade. The number of attempts at laryngoscopy (insertion of the laryngoscope blade and visualization of glottic aperture), number of attempts at intubation (insertion of endotracheal tube into the glottic aperture and confirmation of the same by ETCO2), and total laryngoscopy duration (in seconds) (time interval between the insertion of laryngoscope blade in the right angle of the mouth and confirmation of successful placement of endotracheal tube by ETCO2 graph on the monitor) were observed. Hemodynamic parameters and complications were also noted.

Statistical methods

All statistical calculations were done using “(MedCalc Software Ltd, Acacialaan 22, 8400 Ostend, Belgium)“ version 19.0.3. Numerical data were expressed as mean and standard deviation and categorical data were expressed as percentages. Normality of the distribution was assessed by Kolmogorov–Smirnov test. Numerical data were compared for significance using Student's t-test. Categorical data were compared using Chi-square test. P ≤ 0.05 was considered statistically significant.

This was a prospective, randomized, single-blind study, and a pilot study was first undertaken on ten patients with both intubation modalities with two-sided 95% confidence interval and 95% power of study to estimate the accurate sample size for both groups. Sample size was estimated using software G Power version 3.1.9.2 (Heinrich-Heine-Universität Düsseldorf, Universitätsstr. 1, 40225 Düsseldorf) and it was estimated to be 82 each in the group. To detect the difference in Ease of Intubation score of Grade 1, reported as 37% and 70% in Group A and Group B respectively, and to detect the difference in laryngeal view of Cormack Lehane Grade 1, reported as 52% and 25% in Group A and Group B respectively, the estimated sample size for each of the group came out 82-82. Hence, we made a sample size of 85 patients in each of the two groups.

RESULTS

Out of 225 patients that were eligible for participation in the study, 170 patients met the inclusion criteria and were randomized to the two study groups [Figure 4]. Both the groups (85 patients in each group) were comparable with respect to the demographic parameters, baseline hemodynamic parameters, as well as IPS [Table 2]. A better laryngeal view (modified CL grading of 1) was observed in significantly higher number of patients who were intubated using the paraglossal approach with the Miller's straight blade (54.1%) when compared with those intubated with the McCoy blade with its elevated tip (25.9%) [P = 0.003, Table 3].

Figure 4

Consort flow diagram

Table 2

Demographic profile of patients

	Group A (n=85)	Group B (n=85)	P
Age (years)	47.89±15.0	47.51±13.0	0.8620
Height (cm)	165.03±4.83	164.25±4.76	0.3320
Weight (kg)	62.04±9.16	61.64±9.0	0.7749
BMI (kg/cm)	22.73±2.74	22.77±2.66	0.6894
Males	38 (44.7)	28 (32.9)	0.15
ASA I	31 (36.5)	29 (34.1)	0.872
IPS 1	21 (24.7)	27 (31.8)	0.724
IPS 2	57 (67.0)	57 (67.0)
IPS 3	7 (8.3)	1 (1.2)
Baseline systolic BP	136.7±15.8	133.4±17.4	0.21
Baseline diastolic BP	83.5±8.5	85.0±8.4	0.24
Baseline heart rate	85.1±12	88.0±14	0.15

Values are expressed as n (%) or mean±SD. A P=0.05 or less was considered significant. Group A=Miller blade paraglossal technique, Group B=McCoy blade elevated tip. BMI=Body mass index, SD=Standard deviation, ASA=American Society of Anaesthesiologist physical status classification, IPS=Intubation prediction score, BP=Blood pressure

Table 3

Comparison of laryngeal view and ease of intubation in both the groups

	Group A (n=85), n (%)	Group B (n=85), n (%)	P
Laryngeal view (CL grading)
CL-1	46 (54.1)	22 (25.9)	0.003*
CL2a	24 (28.2)	33 (38.8)	0.193
CL2b	10 (11.7)	10 (11.8)	1.0
CL3a	4 (4.7)	13 (15.3)	0.038*
CL3b	1 (1.3)	6 (7.0)	0.117
CL4	0	1 (1.2)	1.0
EOI scores
EOI 1	27 (31.8)	62 (72.9)	0.00000767*
EOI 2	16 (18.8)	13 (15.3)	0.6834
EOI 3	38 (44.7)	10 (11.8)	0.000009503*
EOI 4	4 (4.7)	0	0.1261

*Statistically significant P≤0.05 Values expressed as n (%). P≤0.05 considered significant. CL=Modified Cormack-Lehane grading, EOI=Ease of Intubation grading

A review of intubation scores (EOI) among both intubation modalities revealed some surprises. Even though laryngeal views were superior with the Miller's blade paraglossal approach, easy intubation (EOI Grade 1) was more common observed with the McCoy blade with its elevated tip. In addition, a higher percentage (44.7%) of patients in Group A demonstrated significant rates of Bougie-assisted intubation (EOI score 3) as compared to patients in Group B (11.8%) (P = 0.0000095) [Table 3].

Now, while considering airways with moderate difficulty, it was observed that the Miller's blade paraglossal approach resulted in a higher number of patients with CL Grade 1 views, but the EOI scores in the same set of patients were better with McCoy blade with its elevated tip, with 66.7% of the patients showing the EOI score of 1 [Table 4].

Table 4

Comparison of best view and ease of intubation in both the groups in patients with moderately difficult airway (intubation prediction score 2)

	Group A, n (%)	Group B, n (%)	P
CL Grade 1	26 (45.6)	12 (21.0)	0.0098
EOI score: 1	18 (31.6)	38 (66.7)	0.0003

Values expressed as n (%). P≤0.05 considered significant. CL=Modified Cormack-Lehane grading, EOI=Ease of Intubation grading

The mean total duration of laryngoscopy was significantly lesser in Group A as compared to Group B (13.7 ± 1.25 s vs. 16.5 ± 3.71 s, P = 0.03).

DISCUSSION

This study was undertaken with the aim to compare the laryngeal views and EOI with two intubation modalities viz Miller's blade paraglossal approach and the McCoy blade with its elevated tip. Our study showed that while laryngoscopy with the Miller's blade paraglossal approach provided significantly better laryngoscopic view (CL grade 1) compared to McCoy blade with its elevated tip, intubation though was easier with the McCoy blade (EOI Grade 1). What was more remarkable was the fact that these findings were evident in subjects in whom airway was predicted to be moderately difficult. While there are previous studies comparing intubating conditions with both these blades, ours is the first study investigating the paraglossal approach of the Miller's blade vis a vis the McCoy's elevated tip technique.[9]

The McCoy's blade and the Miller's blade have been recommended for and found useful in multiple anticipated difficult airway scenarios.[910] Airway scenarios where these blades have found relevance include a stiff epiglottis wherein its elevation is difficult, cervical spine trauma with restricted permissible neck extension which was well in craniofacial abnormalities affecting the airway such as oral or facial tumors, and Pierre Robin Syndrome.[1112] In our study, the Miller's blade paraglossal approach produced a better laryngoscopic view. Our results are in concordance with several studies which have compared the conventional approach to laryngoscopy with the Miller's blade paraglossal approach and found the latter to result in unequivocally better laryngeal views. It has been postulated that placing the blade as far to the corner of the mouth as possible when attempting to bring the glottis into view (as opposed to being in the midline) minimizes the distance to the glottis and hence results in enhanced visibility of the vocal cords.[9] However, while the laryngoscopic view was better with the Miller's blade with paraglossal approach, it did not really translate into an easier intubation as assessed by the EOI score. This finding is similar when a video laryngoscope is used as it invariably requires use of an adjunct to facilitate the endotracheal intubation as observed by Panwar et al.[13] The seeming paradox of a difficult intubation in the setting of an enhanced laryngeal visibility could be attributed to the fact that while the laryngeal view was better with Miller's straight blade, it also required the use of gum elastic bougie more often than the McCoy's blade. The need for a gum elastic bougie to aid intubation with the Miller's straight blade with paraglossal approach is a result of the reduced space available to manipulate the endotracheal tube, resulting in difficulty in guiding the endotracheal tube toward the glottis. In the EOI scoring system, the score increases to 3 (EOI 3) if there is use of any adjunct such as bougie or stylet, hence explaining why the Miller's blade with paraglossal approach resulted in a higher EOI score even though visibility of the larynx is better with this approach. The first step toward a successful intubation is obtaining a good view of the laryngeal inlet (as scored by the Modified CL Grading). Once a good view is obtained, the larynx can be successfully intubated under direct vision in the shortest period even if an adjunct such as a gum elastic bougie is required (as scored by the EOI score). Therefore, the Miller's blade paraglossal technique gives a definite advantage over the McCoy blade with its curved tip because it provides a better laryngeal view.

In the current scenario of the COVID-19 pandemic affecting the way how medicine is practiced, minimizing aerosol generation during intubation is a significant consideration. Hence, familiarity with the alternative blades and techniques other than conventional ones, can be very useful in anticipated and unanticipated difficult airway situations. The findings assume even more importance for anesthesiologists and intensive care personnel working in a COVID setup where due to personal protective equipment (PPE) (such as the use of N95mask, face shield, goggles, PPE suit), the visibility of the glottis during laryngoscopy is limited and thus use of any instrument and techniques which increases the visibility and consequently enhances the chances of successful intubation should be encouraged.[14]

Another area where the results of our study are highly relevant are resource depleted settings where the availability of video laryngoscope and fiber optic bronchoscope is scarce. Herein, the better visibility of the glottis afforded by Miller's straight blade with paraglossal approach has the potential of offering a viable, readily available, and inexpensive intubation modality albeit, with the help of an adjunct, in anticipated and unanticipated difficult airway scenarios.

Furthermore, due to better laryngeal visualization when paraglossal approach with Miller's blade is used, the intubation time is significantly reduced which will limit the aerosol exposure when the anesthesiologist is intubating a COVID-19 patient.

Hence, it can be postulated that even though resulting in apparently difficult intubation scores, intubation with Miller's straight blade paraglossal approach results in enhanced visibility of the larynx and is still a worthy and safe alternative especially when used in conjunction with commonly available inexpensive airway adjuncts such as the gum elastic bougie. It has the potential to be of great help in expediting routine and difficult airway situations in the operating room and in the emergency department while increasing safety in decreasing intubation times (and logically aerosol exposure) for the airway operators.

The present study, however, was conducted at a single center and hence lacked population with varied ethnicity. A multi-centric trial is warranted to replicate the usefulness of the technique in populations with varied ethnicity.

CONCLUSIONS

The Miller's straight blade with a paraglossal approach and McCoy blade with elevated tip were compared for laryngoscopic view and EOI. The laryngeal view was much better with the paraglossal approach of the Miller's straight blade even when difficult airway was anticipated. This improved view can be extremely useful for a successful intubation in difficult settings like the current times of COVID-19. McCoy's blade with its elevated tip was also found to be a useful tool to have when difficult airway is encountered, as EOI is significantly higher with the elevated tip. The study also highlights the usefulness of adjuncts such as a gum elastic bougie to have while intubating. The markedly improved successful intubation rate when the bougie was used with the straight blade makes it a great combination to have for every anesthesiologist for securing the airway and offers a clear advantage over conventional blades in various settings especially in the context of decreasing aerosol exposure in the times of highly infectious diseases that we live in. Nevertheless, our recommendation is that anesthesiologists in their routine capacity as airway managers should consider practicing and becoming proficient in using alternative laryngoscope blades and approaches specially the Miller blade paraglossal technique, to overcome a difficult airway scenario especially when it is unanticipated.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.