Laryngoscopic View after Application of Manual In-Line Stabilization - A Comparison with Early Morning Sniffing Position in the Same Patient.

Background: The application of manual in-line stabilization (MILS) for minimizing spinal cord injury is known to increase difficulty in airway management. Aim: The study aims to assess the change in Modified Cormack-Lehane (CL) laryngoscopic view with the application of MILS from the early morning sniffing position (EMSP) in adult patients. Setting and Design: This was a prospective, interventional, self-controlled study conducted on 220 patients aged 18-65 years, belonging to the American Society of Anesthesiologists Physical Status Class I or II, having a normal airway, and scheduled for elective surgery under general anesthesia. Materials and
Methods: After inducing general anesthesia, MILS was applied to the patient's neck, and a Modified CL view of the vocal cords was recorded under direct laryngoscopy. The view was again noted after applying backward-upward-rightward pressure (BURP). MILS and BURP were released. The view was obtained again with and without BURP in EMSP. Statistical Analysis: Normality of data was tested by Kolmogorov-Smirnov test. Wilcoxon ranked-sum test for quantitative variables and Chi-square test for qualitative variables were used.
Results: On application of MILS, the majority of patients had Modified CL Grade 3a (121 patients) and 3b (53 patients) views. The majority of patients had Modified CL Grade 1 (114 patients) and 2a (71 patients) views on placing in EMSP. These findings were statistically significant (P < 0.0001). Consequent to the placement of BURP upon MILS, patients with lower CL Grade views (2b: 101 patients) were significantly higher (P < 0.0001) in comparison with MILS alone.
Conclusion: In patients with a normal airway, MILS leads to a significantly greater incidence of higher grades of laryngoscopic views in comparison to EMSP. Use of BURP after MILS causes significantly less incidence of higher grades of laryngoscopic view when compared with MILS alone. All patients requiring MILS should be considered to be a difficult airway, and hence, preparation should be done accordingly. Copyright:

INTRODUCTION

“Manual in-line stabilization” (MILS) is a standard of care for the airway management of any patient with a suspected or known injury to the cervical spine. MILS is done in all patients with depressed consciousness, cervical pain, posterior midline cervical tenderness, extremity paresthesia, focal neurological deficit, or when the pain from other injuries is likely to mask the neck pains.[1] MILS helps avoid inadvertent movements of the cervical spine or head during laryngoscopy and intubation, thereby preventing any exaggeration of the existing cervical cord injury.

Although the application of MILS significantly reduces secondary neurological damage associated with trauma to the cervical vertebra,[2] there is impairment in the visualization of the larynx, as consistently shown by reduced incidence of Cormack and Lehane (CL) Grade 1 visualization and increased incidence of Grades 2, 3, and 4.[34] It is also known to prolong the duration of laryngoscopy and intubation, increase the requirement for various intubation aids, and also increase the rate of intubation failure.[56]

Studies have compared the difficulty of intubation in patients subjected to the application of MILS with patients not subjected to MILS (intubation done in the conventional early morning sniffing position [EMSP]).[56] Since airway parameters vary from patient to patient and multiple factors determine the laryngoscopic view, we hypothesized that the same patient will serve as his or her own best control for assessing the effect of MILS on the laryngoscopic view. We, therefore, conducted this study with the aim of finding change in the laryngoscopic view (using a McIntosh blade) with the application of MILS as compared to the view obtained in the EMSP in the same patient. Thus, the same patient served as his/her own control eliminating any intersubject variation in airway parameters that would be expected to affect the laryngoscopic view.

MATERIALS AND METHODS

This prospective, interventional, self-controlled study was conducted from October 2017 to October 2018 in a tertiary care institute after seeking clearance from the Institutional Ethics Committee and obtaining written informed consent from the enrolled patients. The procedures followed the guidelines laid down in the Declaration of Helsinki (2013). The trial was registered with ctri.nic.in (Clinical Trial Number: CTRI/2018/02/011906) and adheres to the applicable CONSORT guidelines [Figure 1].

Figure 1

Consort flow diagram

According to the results of a study conducted by Thiboutot et al., the incidence of CL grade laryngoscopic views with the application of MILS was as follows: 7.4% Grade 1, 34% Grade 2, 46.8% Grade 3, and 11.7% Grade 4; however, in the control group (MILS not applied), the incidence was 73.3% Grade 1, 21.9% Grade 2, 4.8% Grade 3, and 0% Grade 4.[5] Taking these values as a reference, the minimum required sample size with 80% power of study and 5% level of significance was 212 patients. The total sample size taken was 220.

Patients between the ages of 18 and 65 years, belonging to the American Society of Anesthesiologists Physical Status (ASA PS) Class I or II, and scheduled for elective surgery under general anesthesia were included in the study. Pregnant patients and patients with cervical spine pathology were excluded from the study. Patients with anticipated difficult airway as assessed by standard airway parameters including modified Mallampati class (MMPC) IV, retrognathism and upper lip bite test class III, thyromental distance <6 cm, mouth opening <2 fingers of the patient, obesity with body mass index >30 kg.m−2, and neck circumference >44 cm were also excluded from the study.

All patients followed the standard fasting guidelines. Premedication with tablet alprazolam 0.25 mg was administered orally the night before and 2 h before the surgery for anxiolysis. In the operation theater, monitors were attached, baseline vitals were noted, and an intravenous (i.v.) line was secured. Following preoxygenation with 100% oxygen for 3 min, general anesthesia was induced with i.v. injections of fentanyl (2 μg.kg−1), propofol (2 mg.kg−1), and vecuronium bromide (0.1 mg.kg−1). This was followed by bag-and-mask ventilation for 3 min with 100% oxygen and isoflurane (0.8–1.2 minimum alveolar concentration). Ventilation was done using 100% oxygen in anticipation of the prolonged laryngoscopy duration required for this study. MILS was then applied before intubation as described. An assistant (anesthesiologist who had past experience of applying more than 50 MILS) stood by the side of the patient facing the observer anesthesiologist with forearms resting on the patient's chest or table. The assistant's hands grasped the patient's mastoid processes and applied sufficient force to limit as much movement of the head and cervical spine as possible during laryngoscopy. Direct laryngoscope with McIntosh blade size 3 was then introduced by the observer anesthesiologist (anesthesiologist who had successfully completed more than 50 intubations using McIntosh laryngoscope in the presence of MILS). During the entire period of laryngoscopy, the patient received apneic oxygenation with nasal prongs at 12 L.min−1. The laryngoscopic view was recorded according to the Modified CL Grading.[7] Next, the backward-upward-rightward pressure (BURP) maneuver was applied by an assistant, and the laryngoscopic view was recorded again according to the Modified CL Grading. MILS was then released. BURP was also released. The patient's head was then placed in the EMSP and the laryngoscopic view was recorded according to the Modified CL Grading. Hence, the patient in whom MILS had been applied served as his or her own control when put in the EMSP. BURP maneuver was then applied if the Modified CL Grade was recorded to be more than Grade 1. The laryngoscopic view was recorded again according to the Modified CL Grading. Following this, the airway was secured using either an endotracheal tube or supraglottic device of appropriate size and ventilation continued with intermittent positive pressure ventilation. The hemodynamic parameters (blood pressure, heart rate) and SpO2 of the patient were recorded at baseline, during laryngoscopy with MILS, and during laryngoscopy with EMSP.

The primary outcome of this study was to determine the change in laryngoscopic view with MILS using Modified CL Grading. The secondary outcome of this clinical research study was to correlate the MMPC with the laryngoscopic view during MILS and EMSP.

Statistical analysis

Categorical variables were presented in number and percentage (%), and continuous variables were presented as mean ± standard deviation and median. Normality of data was tested by Kolmogorov–Smirnov test. If the normality was rejected, then a nonparametric test was used. Quantitative variables were compared using Wilcoxon ranked-sum test (as the data sets were not normally distributed) between baseline and during MILS and EMSP. Qualitative variables were correlated using the Chi-square test. P < 0.05 was considered statistically significant. The data were entered in the MS EXCEL spreadsheet and analysis was done using Statistical Package for the Social Sciences (SPSS) Statistics for Windows, Version 21.0. (IBM Corporation, Armonk, NY, USA).

RESULTS

A total of 234 patients were enrolled in the study, of whom 14 patients were excluded based on the exclusion criteria. The remaining 220 patients consented to be enrolled in the study [Figure 1]. The mean value of thyromental distance was 7.66 ± 0.63 cm with a median (interquartile range) of 7.5 (7–8) cm, sternomental distance was 15.55 ± 1.29 cm with a median (interquartile range) of 15.5 (14.5–16) cm, and neck circumference was 33.51 ± 2.71 cm with median (interquartile range) of 33 (32–35) cm. The interincisor gap was more than three fingers in 217 patients (98.64%) and more than two and a half fingers in the remaining 3 patients (1.36%). Of the 220 subjects, 70 patients (31.82%) had MMPC I, 118 patients (53.64%) had MMPC II, and 32 patients (14.55%) had MMPC III. 42 patients (19.09%) had the upper lip bite test result of Grade 1, and for the remaining 178 (80.91%) patients, it was Grade 2. The demographic and airway parameters of the study population are depicted in Tables 1 and 2, respectively.

Table 1

Demographic parameters of study population (n=220)

	Mean±SD	Median (IQR)
Age (years)	36.85±11.64	35 (27.500–45)
Weight (kg)	57.92±11.86	57 (50–66)
Height (cm)	163.94±6.5	162 (160–167)
BMI (kg.m−2)	21.4±3.21	21.3 (19.050–23.500)
	Frequency (%)
Sex
Female	151 (68.64)
Male	69 (31.36)
ASA PS class
I	178 (80.91)
II	42 (19.09)

ASA PS=American Society of Anesthesiologists physical status , BMI=Body mass index, IQR=Interquartile range, SD=Standard deviation

Table 2

Airway parameters of study population (n=220)

Airway parameters	Mean±SD	Median (IQR)
Thyromental distance (cm)	7.66±0.63	7.5 (7–8)
Sternomental distance (cm)	15.55±1.29	15.5 (14.500–16)
Neck circumference (cm)	33.51±2.71	33 (32–35)
Airway parameters	Frequency (%)
Interincisor gap
~2.5 fingers	3 (1.36)
>3 fingers	217 (98.64)
Upper lip bite test
Grade I	42 (19.09)
Grade II	178 (80.91)
MMPC
Class I	70 (31.82)
Class II	118 (53.64)
Class III	32 (14.54)

IQR=Interquartile range, MMPC=Modified Mallampati class, SD=Standard deviation

On application of MILS, the incidence of patients with Modified CL Grade 3a was the highest (121 patients, 55%) followed by Grade 3b (63 patients, 28.64%). Twenty-one patients had Grade 2 view (2a in 4 patients [1.82%] and 2b in 17 patients [7.73%]). Fifteen patients (6.82%) had a Modified CL Grade 4 view, whereas none of the patients had a Grade 1 view. On further application of BURP maneuver in the presence of MILS, the number of patients with Modified CL Grade 4 view decreased to eight patients (3.64%). The incidence of Grades 3a and 3b also reduced (47 patients, 21.36% and 11 patients, 5%, respectively). There was an increase in the incidence of patients with Grade 2 view (149 patients, 67.73%) of whom 48 patients (21.82%) had Grade 2a view and 101 patients (45.91%) had Grade 2b view. The remaining five patients (2.27%) now had Modified CL Grade 1 view. These findings are depicted in Table 3 and Figure 2.

Table 3

Laryngoscopic views with different maneuvers (n=220)

Modified CL grade	MILS (%)	MILS with BURP (%)	EMSP (%)	EMSP with BURP (%)	P
1	0	5 (2.27)	114 (51.82)	183 (83.18)	MILS and EMSP (<0.0001)
2a	4 (1.82)	48 (21.82)	71 (32.27)	30 (13.64)
2b	17 (7.73)	101 (45.91)	27 (12.27)	7 (3.18)	MILS and MILS with BURP (<0.0001)
3a	121 (55)	47 (21.36)	8 (3.64)	0
3b	63 (28.64)	11 (5)	0	0	MILS with BURP and EMSP (<0.0001)
4	15 (6.82)	8 (3.64)	0	0

BURP=Backward-upward-rightward pressure, EMSP=Early morning sniffing position, CL=Cormack–Lehane, MILS=Manual in-line stabilization

Figure 2

Incidence of Modified Cormack–Lehane grades with different maneuvers. BURP = Backward-upward-rightward pressure, EMSP = Early morning sniffing position, MILS = Manual in-line stabilization

Following the above, BURP and MILS were removed and the patient's head was placed in the EMSP. With the conventional EMSP for laryngoscopy, it was found that the majority of patients (114 patients, 51.82%) had Modified CL Grade 1 followed by Grade 2a (71 patients, 32.27%) and Grade 2b (27 patients, 12.27%). The remaining eight patients (3.64%) had a Grade 3a Modified CL view and none of the patients were found to have Modified CL Grades 3b and 4 when laryngoscopy was performed in EMSP. On application of BURP with the patient's head in EMSP, the laryngoscopic views were found to improve further. Now, 183 patients (83.18%) had Modified CL Grade 1 view and 37 patients (16.82%) had Modified CL Grade 2 view (2a in 30 patients [13.64%] and 2b in 7 patients [3.18%]), whereas none of the patients had either Grade 3 or Grade 4 view. These findings are depicted in Table 3 and Figure 2.

Significant association [Table 4] was seen between Modified CL Grade under EMSP and MMPC (P < 0.0001). Majority of the 70 patients with MMPC I (51 patients, 72.86%) had Modified CL Grade 1 when laryngoscopy was done in EMSP. The majority of the 118 patients with MMPC II had Modified CL Grade 1 (62 patients, 52.54%), whereas the majority of patients with MMPC III had Modified CL Grade 2a (15 patients, 46.88%) followed by 2b (11 patients, 34.38%).

Table 4

Association of modified Cormack–Lehane grade with Early morning sniffing position and Modified Mallampati class

Modified CL grade with EMSP	MMPC (%)			Total (%)	P
	I	II	III
1	51 (72.86)	62 (52.54)	1 (3.13)	114 (51.82)	<0.0001
2a	15 (21.43)	41 (34.75)	15 (46.88)	71 (32.27)
2b	4 (5.71)	12 (10.17)	11 (34.38)	27 (12.27)
3a	0	3 (2.54)	5 (15.63)	8 (3.64)
3b	0	0	0	0
4	0	0	0	0
Total	70 (100)	118 (100)	32 (100)	220 (100)

CL=Cormack–Lehane, EMSP=Early morning sniffing position, MMPC=Modified Mallampati class

Significant association [Table 5] was seen between MMPC and Modified CL Grade with MILS (P < 0.0001). When MILS was applied to patients who had MMPC I (70 patients), it was found that the majority of patients (40 patients, 57.14%) had Modified CL Grade 3a view. Application of MILS on the 118 patients with MMPC II resulted in a predominance of modified CL Grade 3a (74 patients, 62.71%). The majority of the 32 patients with MMPC III revealed modified CL Grade 3b (16 patients, 50%).

Table 5

Association of modified Cormack–Lehane grade with manual in-line stabilization and Modified Mallampati class

Modified CL grade with MILS	MMPC (%)			Total (%)	P
	I	II	III
1	0	0	0	0	<0.0001
2a	4 (5.71)	0	0	4 (1.82)
2b	15 (21.43)	1 (0.85)	1 (3.13)	17 (7.73)
3a	40 (57.14)	74 (62.71)	7 (21.88)	121 (55)
3b	9 (12.86)	38 (32.20)	16 (50)	63 (28.64)
4	2 (2.86)	5 (4.24)	8 (25)	15 (6.82)
Total	70 (100)	118 (100)	32 (100)	220 (100)

CL=Cormack–Lehane, MILS=Manual in-line stabilization, MMPC=Modified Mallampati class

There was no episode of desaturation or any other adverse events throughout the procedure.

DISCUSSION

In the current study, the impact of MILS on laryngoscopy was studied by comparing the laryngoscopic view obtained after MILS with the laryngoscopic view under conventional EMSP in the same patient. Hence, intersubject variations of airway parameters that would be expected to affect the laryngoscopic view were eliminated. There was a significantly higher incidence of a more difficult laryngoscopic view on the application of MILS compared with the view obtained under laryngoscopy with EMSP alone (P < 0.0001). The majority of patients upon applying MILS had Modified CL Grade 3a (55%) followed by 3b (28%), whereas the majority of patients when placed in EMSP had Modified CL Grade 1 (51.82%) followed by 2a (32.27%). Around 51.36% of the patients with MILS had three levels of improvement of Modified CL Grade with EMSP, and 37.27% of the patients with MILS had two levels of improvement of Modified CL Grade with EMSP. Our findings were similar to studies done by Thiboutot et al. and Adesida et al. where laryngoscopic Grades 3 and 4 were more frequently observed in the MILS group.[56] However, these studies were done taking two separate groups of patients, while our study had the same patient as their own control. In another study done by Heath, there was a worsening in CL Grade by one or two levels with MILS in the majority of patients.[4]

We also found that on the application of BURP to MILS, there was at least one-grade improvement in Modified CL Grade in the majority of patients (P < 0.0001). The majority of patients with MILS had Modified CL Grade 3a (55%) followed by 3b (28%), whereas the majority of patients after application of BURP with MILS had Modified CL Grade 2b (45.91%) followed by 2a (21.82%). Studies have been done by Knill and Takahata et al. show that BURP improves the laryngoscopic view in difficult laryngoscopy.[89] It can thus be inferred that the application of BURP will be useful while securing the airway of patients who require MILS.

Although there was an improvement in Modified CL Grade after the application of BURP along with MILS, the incidence of worse views was significantly higher compared with that in EMSP (P < 0.0001). The majority of patients even upon application of BURP with MILS had Modified CL Grade 2b (45.91%) followed by 2a (21.82%), whereas the majority of patients in EMSP had Modified CL Grade 1 (51.82%) followed by 2a (32.27%). Around 44.54% of the patients with MILS and BURP had one level worse Modified CL Grade than that with EMSP. Around 41.37% of the patients with MILS and BURP had two levels worse Modified CL Grade than those with EMSP. In a study done by Nolan and Wilson, the laryngoscopic view worsened by at least one grade in 45% of patients when the sniffing position was changed to MILS with cricoid pressure.[3]

Both in MILS and EMSP, a significant association was seen between MMPC and Modified CL Grade (P < 0.0001). In EMSP, higher MMPC was associated with higher Modified CL grade and lower MMPC was associated with lower Modified CL Grade. With MILS, the majority of patients with lower MMPC such as MMPC I and MMPC II were also associated with higher modified CL Grades (3a and 3b).

No episodes of desaturation were recorded. This could be because of continuous nasal insufflation of oxygen at 12 L.min−1 throughout the procedure. No significant hemodynamic changes were recorded. Inclusion of only ASA PS Class I and II patients with normal airway examination findings and performance of direct laryngoscopic evaluation by an experienced anesthesiologist could have helped avoid significant hemodynamic changes.

The knowledge that the application of MILS can deteriorate Modified CL view, which can further increase intubation difficulty, is very important for the anesthesiologist. In addition, it is important to know that once MILS is applied, not only MMPC 3 and 4 but even patients with MMPC 1 and 2 may present with a poor CL view. The anesthesiologist should apply MILS in patients undergoing elective cervical spine surgery. Preparedness with a difficult airway cart, checking the availability of video laryngoscopes or flexible fiberoptic bronchoscope, adequate preoxygenation, and above all presence of an experienced anesthesiologist are a must to enhance intubation success. It can also be emphasized that routine use of BURP in such patients may help improve CL view.

The situation becomes more challenging during acute airway management of polytrauma patients who may already be hypoxic, hypotensive, and give no time for airway assessment. The presence of blood, vomitus, and secretions in the oral cavity and nasopharynx may further increase intubation difficulty. It cannot be overemphasized that if MILS is being applied, these patients may always be deemed as difficult airway patients.

The main strength of our study was that the laryngoscopic views were observed with the application of MILS and EMSP in the same patient. Hence, all the airway examination parameters remained constant and the same patient served as his or her own control. However, this study had certain limitations. It was conducted on subjects with a normal airway and not in patients with an anticipated difficult airway. In addition, intubation ease and duration were not studied since it was not feasible to intubate the same patient's trachea several times. Only the laryngoscopic views were recorded.

CONCLUSION

The laryngoscopic view as assessed by Modified CL Grading increases significantly (by 2–3 grades) during the application of MILS with the neck in a neutral position compared with laryngoscopic views assessed in EMSP with the neck in extension. Application of BURP in the presence of MILS improves the laryngoscopic view. Hence, BURP should be applied in all laryngoscopy performed under MILS. Although the application of BURP with MILS does improve the laryngoscopic view by 1 or 2 grades, it does not restore views obtained in EMSP. We, therefore, recommend all patients mandating MILS while securing the airway to be considered challenging with readiness for all measures for difficult intubation.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.