1. Introduction and need for study

Airway management is crucial for anesthetizing patients undergoing surgical procedures. It consists of laryngoscopy, tracheal intubation and ventilation. Optimal laryngoscopy should provide a good view of the glottis, room to easily pass the tracheal tube, along with minimal hemodynamic changes. However, laryngoscopy depends on several factors, such as the skill of the operator, the patient’s airway anatomy, and the laryngoscope blades used.¹

Airway management includes securing, preserving, and protecting the airway with tracheal tube during induction, maintenance and recovery from anaesthesia.  If the airway is not managed, catastrophic results such as brain damage or death can occur.²

Although laryngoscope blades of different sizes and shapes are available, the Miller and the Macintosh blades are most commonly used for laryngoscopy and tracheal intubation.³

McCoy blade is a modification of the standard Macintosh blade. The tip of the McCoy blade is hinged and when the blade is inserted into the vallecula, the tip acts on the hyoepiglottic ligament and lifts the epiglottis out of view to expose more of the glottis by compression of a lever attached to the proximal end of the blade.

Miller is a straight blade with a slight upward curve near the tip. The Miller blade is usually the preferred blade among the pediatric population and less commonly used in adults.⁴

The grade of laryngoscopic view and ease of intubation will vary with different laryngoscopic blades. So this study aims to compare the laryngoscopic view and ease of tracheal intubation between the Miller and McCoy blades among adult patients.

2. Review of literature

Nalini KB et al.¹ conducted a comparative crossover study where a total of 172 patients  undergoing elective surgeries with general anaesthesia were chosen. Patients were distributed in two groups (Macintosh/Miller and Miller/Macintosh), where laryngoscopy was first done with Macintosh blade, followed by Miller blade in the Macintosh/Miller group and vice-versa in Miller/Macintosh group. Grading of laryngoscopic views, number of attempts, ease of intubation and use of backward, upward, rightward pressure (BURP) were noted. Miller blade showed better laryngoscopic view compared to Macintosh (32.6% vs. 15.1%). Intubation with Miller blade was easier with regards to ease of intubation and number of attempts. They concluded that Miller blade showed better laryngoscopic view compared to Macintosh blade and also intubation with Miller blade was easier with regards to ease of intubation and number of attempts.

Kulkarni et al.² conducted a prospective randomized study. The authors aimed to compare glottic view and ease of intubation with Macintosh, Miller, McCoy blades and the Trueview® laryngoscope. Visualisation of glottis [Cormack Lehane (CL) grade], ease of intubation, number of attempts; need to change the blade and need for external laryngeal manipulation was noted. Grade 1 view was obtained most often (87% patients) with Trueview® laryngoscope. Intubation was easier (Grade 1) with Trueview® and McCoy blades (93% each). Seven patients needed two attempts; one patient in Miller group needed three attempts. No patient in McCoy and Trueview® Groups required external laryngeal manipulation. The authors concluded that in patients with normal airway, glottis was best visualised with Miller blade and Trueview® laryngoscope however, the trachea was more easily intubated with McCoy and Macintosh blades and Trueview® laryngoscope.

Arino JJ et al.³ conducted a study which included 500 patients scheduled to undergo elective surgery with general anaesthesia, who were divided into 5 groups, undergoing intubation with Macintosh, Miller, McCoy, Belscope and Lee-Fiberview blades. The laryngeal view was classified according to CL grading. The degree of difficulty with intubation was rated as: Grade 1, intubation easy; Grade 2, intubation requiring an increased anterior lifting force and assistance to pull the right corner of the mouth upwards to increase space; Grade 3, intubation requiring multiple attempts and a curved stylet; Grade 4, failure to intubate with the assigned laryngoscope. Grade I laryngoscopic view obtained with Belscope (98%) and Miller (96%) laryngoscopes were better than the other types of laryngoscopes. Ease of intubation was better with McCoy (97%) and Macintosh (91%), whereas with Miller it was only 75%. They concluded that laryngoscopy was better with straight blades but curved blades provided better intubating conditions.

Yadav P et al.⁵ conducted a randomized control study in a total of 75 children aged 2–6 years, either gender, with ASA grades I or II who were scheduled for elective surgery under general anaesthesia. They were randomly allocated to groups A, B and C to be intubated with Macintosh, Miller and McCoy blades respectively. Intubation Difficulty Score (IDS) was significantly lower in group B (0.6 ± 0.7) as compared to group A (1.4 ± 0.9) and group C (1.3 ± 1.1); majority of patients in group B (48%) had Cormack–Lehane grade Ι unlike group A (0%) and group C (20%) and Percentage of Glottic Opening (POGO) score was higher in group B (86 ± 23.4) when compared with groups A (68.2 ± 20.5) and C (59.8 ± 28.9). They concluded that Miller blade may be considered superior to Macintosh and McCoy blades in terms of glottic visualization and ease of intubation in paediatric patients.

Samel S et al.⁶ conducted a prospective observational study where 105 ASA grade I and II patients randomly divided into three groups were intubated using Macintosh, McCoy and Miller blades. Cormack and Lehane grade of glottic view was obtained. 18 patients (51.4%) were CL grade I and 17 (48.6%) were CL grade II in Macintosh, 24 (68.6%) were CL grade I and 11 (31.4%) were CL II in McCoy and, 32 (91.4%) were CL I and 3 (8.6%) were CL II in Miller group. They concluded that Miller blade provides best visualization of larynx out of the three blades.

3. Objectives

Primary objectives

1.     To compare the laryngoscopic view between Miller and McCoy blades in adults.

2.     To compare the ease of tracheal intubation between Miller and McCoy blades in adults.

4. Methodology

Source of data: The study group comprises of patients admitted in teaching hospital of Mandya Institute of Medical Sciences, Mandya, scheduled for surgery requiring general anaesthesia with orotracheal intubation.

Study setting: Department of Anaesthesiology, Mandya Institute of Medical Sciences, Mandya.

Study design: Randomized control trial

Study period: 12 months (May 2023 to April 2024)

Sample size:  83 in each group

Sample size is calculated using formula:

n = (p_oq_o+p₁q₁)(z _1-α/2 +z_1-β/2 )²/(p₁-p₀)²

Based on the results of one of the previous study (Study by Yadav P et al.⁶), POGO scores in group A (Macintosh) and group B (Miller) was 68.2 (P_o) and 86 (P₁) respectively.

z_(1-α)/2  = 1.96 = value of standard normal distribution corresponding to a significance level of α

z_(1-β)/2  = 0.84 = value of the standard normal distribution corresponding to the desired level of power

p_o = proportion of controls = 68.2

q_o = (1- p_o)

p₁ = proportion of cases = 86

q₁ = (1- p₁)

So, n = 83.4

Sample size taken for study is 166 patients, with 83 patients in each group.

Sampling Method: Simple random sampling

Inclusion Criteria:

Patients fulfilling the following criteria

ï‚· Patients aged 18-60 years.

ï‚· Patients with ASA class I and class II.

ï‚· Patients willing to participate in the study with informed consent.

Exclusion Criteria

ï‚· Systemic hypertension

ï‚· Morbid obesity(Body Mass Index > 30)

ï‚· Coronary artery disease

ï‚· H/o cerebrovascular accidents

ï‚· Valvular heart diseases

ï‚· If rapid sequence induction is required

Data collection:

Study population will include 166 patients fulfilling our inclusion and exclusion criteria posted for surgeries under general anaesthesia. The study will be a randomized control trial. The procedure will be explained and informed consent will be obtained. Patients requiring orotracheal intubation will be randomly allocated into two groups each containing 83 patients

Group I: patients undergoing laryngoscopy and intubation first with Miller blade and then with McCoy blade.

Group II: patients undergoing laryngoscopy and intubation first with McCoy blade and then with Miller blade.

A day prior to the planned procedure, detailed history of the patient will be taken during the pre-operative assessment visit. A thorough clinical examination will be conducted and necessary investigations will be sent and results will be noted. Based on the pre-anesthetic airway assessment, patients’ airway will be classified into different grades integrating three predictive tests. The predictive tests used will be

1) Modified Mallampati’s grading.³- Measures the relative tongue/pharyngeal size. The observer classifies the airway according to the pharyngeal structures seen:

·       Grade 1 = soft palate, fauces, uvula, anterior and posterior tonsillar pillars           (1 point)

·       Grade 2 = soft palate, fauces, uvula (2 points)

·       Grade 3 = soft palate, base of uvula (3 points)

·       Grade 4 = soft palate not visible at all (4 points)

2) Atlanto-occipital joint extension.³(AOJE)- When the AOJ is extended, the angle between the erect and extended planes of the occlusal surface of the upper teeth quantitates the degree of AOJE

·       Grade 1 = AOJE ≥ 35° (1 point)

·       Grade 2 = AOJE ≥22° and < 35° (2 points)

·       Grade 3 = AOJE ≥ 13° and < 22° (3 points)

·       Grade 4 = AOJE < 13° (4 points)

3) Mandibular space.³- Includes the thyromental distance (TMD) and the horizontal length of the mandible (LM)

·       Grade 1 = TMD≥ 6 cm and LM ≥ 9 cm (1 point)

·       Grade 2 = TMD ≥ 6 cm and LM < 9 cm (2 points)

·       Grade 3 = TMD < 6 cm and LM ≥ 9 cm (3 points)

·       Grade 4 = TMD < 6 cm and LM < 9 cm (4 points).

Addition of the points generates a nominal score (intubation prediction score.³) 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)

After classifying patients to different grades based on the intubation prediction score, patients in each grade will be randomly allocated into either Group I or Group II.

The randomization for each group will be done using computer generated randomization table.

All patients will be kept nil per oral for 6 hours before surgery and premedicated with tablet alprazolam 0.25mg and capsule omeprazole 20mg orally at bedtime. On the day of planned procedure, patients will be shifted to the operation theatre and connected to the standard monitors like Electrocardiograph (ECG), pulse oximeter, non-invasive blood pressure (NIBP) and baseline values of heart rate, blood pressure, SpO₂ will be noted. Intravenous access will be obtained using 18G intravenous cannula. Anesthetic technique will be standardized. Patients will be pre-oxygenated with 100% O₂ for 3 minutes using the circle system through face mask. Patients will be premedicated intravenously with midazolam 0.01mg/kg, fentanyl 2µg/kg, lignocaine hydrochloride (preservative free) 1.5mg/kg, followed by intravenous induction with propofol 2 mg/kg. After confirming the ability to mechanically ventilate the patient using circle system through face mask, intravenous vecuronium 0.1 mg/kg will be administered to facilitate endotracheal intubation. Mechanical ventilation with 100% oxygen and 1-2% Sevoflurane will be done for 3 minutes. Heart rate, systolic blood pressure, diastolic blood pressure and mean arterial pressure will be noted (value just before laryngoscopy). Laryngoscopy will then be performed by an experienced anaesthesiologist first with Miller blade in Group I or McCoy blade in Group II and patients will be intubated with appropriate size endotracheal tube.  CL grade and ease of intubation will be assessed and hemodynamic parameters will be noted for 10 minutes. After 10 minutes endotracheal tube will be removed, bag and mask ventilation will be continued and intravenous Propofol will be given in titrated doses if necessary to bring the hemodynamic parameters to near baseline values. Now, laryngoscopy and intubation will be performed in the same patient for the second time with McCoy blade in Group I or with Miller blade in Group II.  CL grade and ease of intubation will be assessed and hemodynamic response will be noted for 10 minutes.

While performing laryngoscopy with McCoy blade, the tip will be gently introduced in the vallecula followed by upward and forward movement of the blade for visualizing the larynx. Laryngeal view will be assessed, first without using lever and then with lever and CL grade will be noted in both instances.

Miller laryngoscope blade will be gently introduced and tip of the blade will be passed behind the epiglottis. By lifting the laryngoscope upward and forward, the epiglottis will be lifted directly exposing the larynx. Intubation with Miller blade will always be done using intubating stylet to straighten the endotracheal tube.

The laryngeal view will be classified according to Cormack and Lehane.⁷ as follows:

·       Grade 1: full view of glottis

·       Grade 2a: partial view of the glottis

·       Grade 2b: arytenoids or posterior of the vocal cords only just visible

·       Grade 3: only epiglottis visible

·       Grade 4: no glottis structure visible

The degree of difficulty with intubation will be rated as:

·       Grade 1- intubation easy

·       Grade 2- intubation requiring an increased anterior lifting force and assistance to pull the right corner of the mouth laterally to augment space

·       Grade 3- intubation requiring multiple attempts, BURP maneuver and stylet (use of stylet only in case of McCoy blade)

·       Grade 4- failure to intubate with the assigned laryngoscope

Time taken for tracheal intubation will be defined as time taken from insertion of laryngoscope blade into the oral cavity to successful passage of endotracheal tube into the glottis.

Heart rate, blood pressure values (systolic blood pressure, diastolic blood pressure, mean arterial pressure) and SpO₂ just before (0^th minute) and 1^st, 3^rd, 5^th, 10^th minute after laryngoscopy and intubation with each blade will be noted.

Plan for data analysis:

The collected data will be analyzed using Microsoft Excel software with SPSS trial version. Descriptive statistics {mean, standard deviation, proportions etc.}, inferential statistics {t- test (to know the difference between means), chi-square test (to know the association)}, and other relevant statistical tests will be used.

5. References

1.     Nalini KB, Gopal A, Iyer SS, Chanappa NM. A Comparative Crossover Randomized Study of Miller and Macintosh Blade for Laryngoscopic View and Ease of Intubating Conditions in Adult. Archives of Anesthesiology and Critical Care 2021;7:58-62.

2.     Kulkarni AP, Tirmanwar AS. Comparison of glottic visualisation and ease of intubation with different laryngoscope blades. Indian J Anaesth 2013;57:170-4.

3.     Arino JJ, Velasco JM, Gasco C, Lopez-Timoneda F. Straight blades improve visualization of the larynx while curved blades increase ease of intubation: a comparison of the Macintosh, Miller, McCoy, Belscope and Lee-Fiberview blades. Can J Anaesth 2003;50:501-6.

4.     Dorsch JA, Dorsch SE. Laryngoscopes. In: Understanding Anesthesia Equipment, 4th Edition. Baltimore: Williams and Wilkins 1998; 505-6.

5.     Yadav P, Kundu SB, Bhattacharjee DP. Macintosh, Miller and McCoy laryngoscope in paediatric patients. Indian J Anaesth 2019;63:15-20.

6.     Samel S, Patil BM, Roy A. Comparison of glottic view and haemodynamic stress response associated with laryngoscopy using the Macintosh, McCoy and miller blades in adult patients. International Journal of Contemporary Medical Research 2019;6:I11-I16.

7.     Cormack RS, Lehane J. Difficult tracheal intubation in obstetrics. Anaesthesia 1984;39:1105–11.