A Randomized Study Comparing the Sniffing Position with Simple Head Extension for Glottis Visualization and Difficulty in Intubation during Direct Laryngoscopy.

BACKGROUND: Proper positioning of the head and neck is important for an optimal laryngeal visualization. Traditionally, sniffing position (SP) is recommended to provide a superior glottic visualization, during direct laryngoscopy, enhancing the ease of intubation. Various studies in the last decade of this belief have challenged the need for sniffing position during intubation. We conducted a prospective study comparing the sniffing head position with simple head extension to study the laryngoscopic view and intubation difficulty during direct laryngoscopy.
MATERIALS AND METHODS: Five-hundred patients were included in this study and randomly distributed to SP or simple head extension. In the sniffing group, an incompressible head ring was placed under the head to raise its height by 7 cm from the neutral plane followed by maximal extension of the head. In the simple extension group, no headrest was placed under the head; however, maximal head extension was given at the time of laryngoscopy. Various factors as ability to mask ventilate, laryngoscopic visualization, intubation difficulty, and posture of the anesthesiologist during laryngoscopy and tracheal intubation were noted. In the incidence of difficult laryngoscopy (Cormack Grade III and IV), Intubation Difficulty Scale (IDS score) was compared between the two groups.
RESULTS: There was no significant difference between two groups in Cormack grades. The IDS score differed significantly between sniffing group and simple extension group (P = 0.000) with an increased difficulty during intubation in the simple head extension. Patients with simple head extension needed more lifting force, increased use of external laryngeal manipulation, and an increased use of alternate techniques during intubation when compared to SP.
CONCLUSION: We conclude that compared to the simple head extension position, the SP should be used as a standard head position for intubation attempts under general anesthesia.

RCT Entities:   Population Interventions Outcomes

INTRODUCTION

Proper positioning of the head and neck is vital for an optimal laryngeal visualization during direct laryngoscopy. The ability to have a good visualization of the glottis is the major determinant of easy tracheal intubation. In 1936, Ivan Magill recommended sniffing position (SP) for the best laryngeal exposure.[1] Bannister and Macbeth proposed the three axes alignment theory (TAAT) advocating that SP causes alignment of laryngeal, pharyngeal, and oral axis causing line of vision to fall on the glottis.[2] Horton et al. gave further quantitative dimensions to this concept by defining the angle for neck flexion to be 35° and the plane of face extension with the horizontal at 15°.[3] Alternative theories to TAAT were propounded by Chou and Wu[4] and Isono.[5] Chou and Wu[4] provided an alternative to three axes alignment theory, pointing out that the airway is actually a three-dimensional space. They observed that in majority of the patients with slight head extension, the tongue could be easily displaced and laryngeal exposure was satisfactory. “Obstacle theory” proposed that SP is a “gold standard” for laryngoscopy as head elevation in SP moves the anterior obstacles upward, head extension moves posterior obstacle downward, and laryngoscopy moves anterior obstacle upward, thus allowing line of vision to pass through the created space.[5]

The basis of SP was challenged by Adnet et al. who could not find any axis alignment in a radiograph obtained during intubation in the SP, thus challenging the anatomical soundness of the SP.[6] However, these findings were criticized subsequently as it was noted that the neck flexion angle in the radiograph was barely 5°.[7] Benumof noticed that the inability by Adnet et al. to observe the alignment may have been a result of failure to position the patients head in a proper SP.[7] This criticism led to further studies by Adnet et al. and they found that the Intubation Difficulty Scale (IDS) was similar between the patients in SP and simple head extension.[8] However, laryngoscopy in these patients was done without administering a muscle relaxant. This could have led to suboptimal conditions for laryngoscopy. Most of the studies[91011] have studied the Cormack–Lehane grading or percentage of glottic opening scoring[12] as a predictor for the ease of intubation. However, in our study, we have incorporated the IDS in addition to Cormack–Lehane grading (after administration of neuromuscular blocking agents) to assess the ease of intubation during direct laryngoscopy. The use of IDS appears to be more scientific than the comparison of Cormack–Lehane grading as direct laryngoscopy is a dynamic act, and many factors as the type of laryngoscopy blade, its lifting force, and airway anatomy determine the ease of intubation. The aim of our study was to evaluate the differences, if any, between SP and simple head extension position, with regard to the incidence of difficult mask ventilation, difficult laryngoscopy, and difficult intubation in adult patients undergoing surgery under general anesthesia with the administration of muscle paralysis.

MATERIALS AND METHODS

This prospective, randomized study was undertaken at a tertiary care hospital in northern India. Five-hundred consecutive, American Society of Anesthesiologists (ASA) Physical Status I–III adult patients scheduled for elective surgical procedures under general anesthesia needing tracheal intubation were included in the study. All the patients were evaluated in the preanesthetic assessment a day before surgery by an anesthesiologist (Zulfiqar Ali). The following things were noted in the airway assessment: (1) Modified Mallampati Classification as described by Samsoon and Young,[13] (2) temporomandibular joint mobility, whether interincisor gap was ≤3 cm on mouth opening, (3) forward protrusion of the mandible which was assessed by the ability to move the lower teeth in front of the upper teeth, (4) thyromental distance and sternomental distance - measured from the thyroid notch and upper border of the manubrium sterni to the mentum, respectively, with the head in full extension and the mouth closed, (5) any abnormal dentition in the form of loose, protruding, or missing upper incisors or canine teeth, (6) neck length from the mastoid process to sternal head of clavicle with head in neutral position, (7) the maximum range of head and neck movement <80° or > 80° as described by Wilson et al.,[14] (8) body mass index in kg/m2, and (9) presence of short neck, beard, or cervical spondylosis. Patients with ASA Physical Status IV or any abnormal airway anatomy were excluded from the study.

All patients were randomized into sniffing group or simple head extension group by a computer-generated randomized table. In the sniffing group, an incompressible head ring was placed under the head to raise its height by 7 cm from the neutral plane. This was followed by maximal extension of the head at the atlanto-occipital joint at the time of laryngoscopy. In the simple extension group, no headrest was placed under the head; however, maximal head extension was given at the time of laryngoscopy. The height of the operating table was adjusted to place the patient's forehead at the level of xiphisternum of the attending anesthesiologist (Zulfiqar Ali). Face mask ventilation, laryngoscopy, glottis visualization, and intubation were done by a senior anesthesiologist (Zulfiqar Ali) with more than 10 years of experience.

Anesthesia was induced with fentanyl (2 μg/kg) and propofol (2 mg/kg) till loss of verbal commands by the patient. Atracurium (0.5 mg/kg) was used to facilitate the tracheal intubation. Ventilation was done using a mixture of 50% oxygen and 50% nitrous oxide. Mask ventilation was graded as easy, difficult, or impossible [Table 1] as described by Langeron et al.[15] During the induction of anesthesia, a fresh gas flow of oxygen was maintained at 10 L/min.

Table 1

Grading of mask ventilation

Laryngoscopy was performed by the same anesthesiologist (Zulfiqar Ali) using Macintosh size 3 blade. The laryngoscopic view was graded according to Cormack and Lehane grading scale[16] without optimal external laryngeal manipulation (ELM). Intubation was performed with tracheal tube size 7–7.5 mm in females and size 8 and 8.5 mm in males. Intubation difficulty was assessed by seven variables of the IDS [Table 2] as described by Adnet et al.[17] An anesthesiology assistant observed the posture of the anesthesiologist performing laryngoscopy and intubation.

Table 2

Intubation Difficulty Scale and its grading

Statistical analysis

Data collected intraoperatively were summarized and compiled. It was entered in a spreadsheet (Microsoft Excel) and then exported to data editor of SPSS Version 20.0 (SPSS Inc., Chicago, Illinois, USA) software. Based on the study by Adnet et al.,[8] it was found that the incidence of difficult laryngoscopy was 11.4% in SP and 10.7% in simple extension positions. Using this parameter as the primary outcome measure, an estimated sample size of 43,000 was required with alpha value of 0.05 and beta value of 0.10. However, due to the limitation of resources and the work of Prakash et al.,[18] a sample size of 500 patients was taken (250 patients in each group). Student's unpaired t-test was used for continuous variables and Chi-square test with Yates’ correction factor for categorical variables (to see any significant differences between the sniffing and the simple head extension groups). P < 0.05 was considered statistically significant.

RESULTS

A total of 500 patients were included in this study: 250 patients in the sniffing group and 250 patients in head extension group. The demographic profile of the patients is summarized in Table 3. The two groups were comparable in terms of age, sex, weight, and body mass index. There was no significant statistical difference between the two groups in the factors [Table 4] that may be predictive of difficult intubation. Hence, both the groups included demographic and anatomically homogenous population. In the sniffing group, there was a significant gas flow leak from the face mask in three patients and necessity to perform two-handed mask ventilation in two patients. Similarly, in the head extension group, three patients required an increase in oxygen gas flow to 15 L/min and one patient had a gas leak from the face mask. There was no difference in the visualization of the glottis [Table 5] as assessed by Cormack and Lehane grades with 27 (10.8%) patients having a difficult visualization in sniffing group and 30 (12.0%) patients in simple head extension group (P = 0.082).

Table 3

Demographic profile

Table 4

Comparison of patient characteristics predictive of difficult intubation

Table 5

Comparison of views obtained during laryngoscopy

The distribution of IDS scores for patients in the two groups is depicted in Table 6. In SP, 147 patients were easily intubated with no difficulty while as in simple head extension only 100 patients were intubated easily. In SP, 96 patients had moderate difficulty in intubation while as in simple extension 150 patients had moderate difficulty in intubation. In SP, seven patients had severe difficulty in intubation.

Table 6

Intubation Difficulty Scale score

The summary of variables of difficult intubation is presented in Table 7. It was seen that greater than one attempt at intubation, increased lifting force, an increased need for application of external laryngeal pressure and increased use of alternate techniques when the patient was not in SP. When the patient was not in SP, there was an increased use of stylet (72.72% in simple head extension and 64.70% in SP). There was a statistically significant difference between the two groups in the stance adopted by the anesthesiologists performing laryngoscopy and intubation; in 62% (155 patients) of cases in the simple extension group, the anesthesiologist bent at the knee, or stooped to bring the face closer to the patient during laryngoscopy and intubation to obtain the best laryngeal view compared with 12% (102 patients) in the SP (P < 0.001).

Table 7

Variables of the Intubation Difficulty Scale

DISCUSSION

Our results showed that the more patients had lower IDS (easier intubations) in SP when compared to simple head extension. The posture adopted by the anesthesiologists was upright in most instances when the patients were intubated in SP than the simple head extension group. We observed in our study that a greater number of patients intubated in the simple head extension position required more than one attempt at intubation, use of ELM, use of alternate techniques, and use of increased force during laryngoscopy compared to those intubated in the SP. Hence, it was easier to intubate in the SP than in the simple head extension. Our study was consistent with the results observed by Levitan et al.[19] who in their study showed that glottic opening scores measured from 31% in the head flat to 64% in mid-elevation to 87% with maximal elevation positions. Similarly, Park et al.[20] compared the laryngoscopic views in the neutral position and with different pillows of 3, 6, and 9 cm heights in adult patients. The laryngoscopic view was best with 9 cm elevation and the view improved with the addition of pillows in short-necked patients. They found a significant correlation between the neck length and the pillow height needed to provide the best view.

Schmitt and Mang[21] showed that head and neck extension beyond SP improved laryngeal view in those patients who had difficult in direct laryngoscopy. In a manikin study conducted by Brindley et al.,[22] “win with the chin” analogy resulted in adequate airway positioning in novices being trained for airway management. In a study conducted by Johnson and Goodman,[23] a lot of variabilities were seen by the anesthesiologists in mimicking SP; hence, there is a need to standardize the SP. The lack of standardization may lead to the findings observed by Adnet et al.[824] that SP did not really facilitate better glottis visualization and easier intubation.

Singhal et al.[10] and Sahay et al.[12] did a similar study comparing the IDS scores between the SP and simple head extension. Singhal et al.[10] used suxamethonium for muscle relaxation in a dose of 1.5 mg/kg while Sahay et al.[12] used vecuronium in contrast to our study. We observed similar results to Singhal et al.[10] with better glottis visualization and lower intubation difficulty scores with the use of atracurium (0.5 mg/kg) to facilitate tracheal intubation. Our study had the advantages over both these studies, in that we standardized the height of the operating table to place the patient's forehead at the level of xiphisternum of the attending anesthesiologist. Observations by a single anesthesiologist reduced the interobserver variability and reduced the observer bias between the two groups. In contrast to studies of Singhal et al.[10] and Sahay et al.,[12] our study additionally noted the lifting force of the laryngoscope and posture adopted by the anesthesiologists during intubation. A reduced lifting force on the laryngoscope and upright posture was noted when the patients were intubated in SP than the simple head extension group.

Recent studies by Greenland et al. based on magnetic resonance imaging of the airway proposed the two-curve theory – primary oropharyngeal curve and secondary pharyngo-glotto-tracheal curve instead of three axes alignment theory. The authors have shown the superiority of SP on the basis of magnetic resonance imaging study done in neutral, extension, head left, and SP in healthy adult volunteers.[25] They found a reduction in the area between the line of sight and the airway curve in the SP compared with the neutral position. Although the SP may not result in complete axis alignment on its own, it brings the axis as close as possible in preparation for complete alignment to be achieved by laryngoscopy blade.[26]

CONCLUSION

Direct laryngoscopy is a dynamic procedure, and multiple maneuvers are needed to improve the glottis view. Many factors such as type and size of the blade, laryngoscopic lifting force, operator experience, and patient airway anatomy affect the degree of glottis exposure and need to be individualized to avoid the bias arising from these factors. Large trials on different patient populations are needed. However, we conclude that with the available evidence so far, and the results from our study, SP is better for optimal laryngeal exposure and ease of intubation. Our study showed that SP conferred a distinct advantage for tracheal intubation as assessed by the intubation difficulty score. We conclude that, compared to the simple head extension position, the SP should be used as a standard head position for intubation attempts under general anesthesia till definitive results are available from large multicentric trials.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.