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

Miller is a straight blade with a slight upward curve near the tip. Macintosh is a curved blade that extends up to 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 Macintosh blades among adult patients.

Review of literature

Nalini KB et al.¹ conducted a comparative crossover study where a total of 172 patients who were >18 years age, with American Society of Anaesthesiologists (ASA) grades I and II, 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%). BURP application helped improve the laryngoscopic views with Macintosh blade. 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 and1`` 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 grade), ease of intubation, number of attempts; need to change the blade and need for external laryngeal manipulation were 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 Cormack and Lehane 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.

Achen B et al.⁵ conducted a randomized control trial which included 161 patients scheduled for elective surgery under general anaesthesia. Patients were randomly assigned to one of the two groups (Miller vs. Macintosh). A standard general anaesthetic agent was administered. Comparisons were made of the percentage of the vocal cords visible at laryngoscopy and graded using the Cormack and Lehane scale. Laryngoscopy using the Miller blade allowed 100% of the vocal cords to be viewed in 78% of cases, whereas this was achievable in only 53% with the Macintosh blade. The Miller blade enabled greater than 25% of the vocal cords to be seen in 95% of the cases, whereas this was achievable in only 80% with the Macintosh laryngoscope. A grade I Cormack and Lehane view of the larynx was obtained in 96.5% of cases in the Miller group compared with 85% in the Macintosh group. They concluded that direct laryngoscopy using the Miller blade and paraglossal approach afforded a much-improved view of the larynx in the majority of cases.

Yadav P et al.⁶ 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). Haemodynamic changes and other intubation parameters were comparable among the groups. IDS was considered as primary outcome, and Cormack–Lehane grade and POGO score were taken as secondary outcome. 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 and heart rate, systolic and diastolic blood pressure at baseline, immediately before induction, following induction, and at 1, 3 and 10 minutes after intubation were noted. 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. Rise in heart rate following intubation was greatest with Miller blade followed by Macintosh and least with McCoy. Rise in both systolic and diastolic blood pressure following intubation was highest with the Miller blade followed by Macintosh and least with McCoy. They concluded that Miller blade provides best visualization of larynx out of the three blades but McCoy blade produced least hemodynamic response, hence the latter is preferable when less hemodynamic response is desired.

Soltani AE et al.⁷ conducted a comparative randomized control trial which aimed to compare Miller and Macintosh laryngoscopes. A total of 72 children with a score of I and II, according to the ASA physical status classification, who were candidates for elective surgery with general anesthesia and tracheal intubation were enrolled in the study. The children were divided into two equal groups (36 persons) according to used laryngoscope: Miller laryngoscope (group 1) and Macintosh laryngoscope (group 2). The number of endotracheal intubation attempts and complications were also recorded for both groups. Regarding the Cormack-Lehane classification system, 5 patients were classified as grade 1 (13.9%), 14 patients as grade 2 (38.9%), 15 patients as grade 3 (41.7%), and 2 patients as grade 4 (5.6%) in the Macintosh group. In contrast, in the Miller group, 5 patients were classified as grade 1 (13.9%), 27 patients as grade 2 (75%), and 4 patients as grade 3 (11.1%). The authors concluded that Cormack-Lehane system of the Miller blade was better than the Macintosh blade, giving a better la­ryngoscopic view.

 Objectives

Primary objectives

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

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

Secondary objectives

1.     To compare the haemodynamic stress response to laryngoscopy and tracheal intubation between Miller and Macintosh blades

 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: Randomised control trial

Study period: 12 months (August 2022 to July 2023)

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 in whom laryngoscopy will be planned with either Miller or Macintosh blade. 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. So study population will include a group of 83 who will undergo laryngoscopy and intubation with Miller blade and another group of 83 patients who will undergo laryngoscopy and intubation with Macintosh 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-anaesthetic 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 tonsilla  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 classes based on the intubation prediction score, patients in each class will be randomly allocated into either group MC (patients undergoing laryngoscpy with Macintosh blade) or group ML (patients undergoing laryngoscopy with Miller blade). The randomization for each class 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 monitor and parameters like ECG, SpO2, non-invasive blood pressure and heart rate will be monitored. Intravenous access will be obtained using 18G intravenous cannula. Anaesthetic technique will be standardized. Patients will be pre-oxygenated with 100% O₂ for 3 minutes using the circle system through face mask. Patient will be premedicated with inj.midazolam 0.01mg/kg, inj.fentanyl 2mcg/kg, inj.lignocaine hydrochloride (preservative free) 1.5mg/kg, followed by propofol 2 mg/kg over 30s given intravenously. After confirming the ease of mechanical ventilation using circle system through face mask, inj.vecuronium 0.1 mg/kg will be administered to facilitate endotracheal intubation. Mechanical ventilation with 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. Laryngoscopy will then be performed by an experienced anaesthesiologist (with either Miller or Macintosh laryngoscope blade) enabling a clear view of the vocal cords.

Macintosh laryngoscope blade will be gently introduced and the tip of the blade will be placed in the vallecula. By lifting the laryngoscope upward and forward, the epiglottis will be lifted indirectly exposing the larynx.

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.

The laryngeal view will be classified according to Cormack and Lehane.⁹ as follows:

·       Grade 1: full view of glottis

·       Grade 2: only posterior commissure 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

·       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) just before (0^th minute) and 1^st, 3^rd, 5^th, 10^th, 15^th, 20^th minute after laryngoscopy and intubation 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.

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.     Achen B, Terblanche OC, Finucane BT. View of the larynx obtained using the Miller blade and paraglossal approach, compared to that with the Macintosh blade. Anaesth Intensive Care 2008; 36: 717-21

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

7.     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.

8.     Soltani AE, Maleki A, Espahbodi E, Goudarzi M, Ariana P, Takzare A. Comparison of the Laryngoscopic View using Macintosh and Miller Blades in Children Less than Four Years Old. Journal of Medicine and Life 2020;13:431–4.

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