Abstract

Background

Most of the studies on difficult intubation and laryngoscopy focused on American and European populations. However, Indians have distinct anthropometric characteristics compared to these populations. This study aims to determine the gender difference in inter-incisor distance (IID) cut-off marks to assess the ease of intubation in the Indian population.

Methods

A prospective observational study was conducted on 516 patients scheduled for elective surgery under general anesthesia with endotracheal intubation. Preoperative assessments included IID, thyromental distance, neck circumference, sternomental distance (SMD), thyrohyoid distance, mentohyoid distance, and modified mallampatti grading. Intubation difficulty scores (IDSs) were assessed during intubation. The optimal cut-off values of inter incisor gap and difficult intubation were analyzed by receiver operating characteristic curve analysis.

Results

The total cohort was grouped into males (Group 1 [n = 258]) and females (Group 2 [n = 258]). Males had a significantly higher mean age ( P = 0.04) and weight ( P = 0.00) than females. Difficult intubation incidence was 16.3% of males and 6.2% of females. Increased IID ( P = 0.039), decreased SMD ( P = 0.046), and males ( P = 0.002) are identified as predictors of difficult intubation.IID and IDS showed greater specificity than sensitivity, with significant cut-off values of 4.25 and 1.50, respectively.

Conclusion

The cut-off value of IID did not show a significant difference in gender variation. Despite the low IID in females, intubation difficulties were not observed compared to males.

Keywords

American Society of Anesthesiologists, endotracheal tube, general anesthesia, inter-incisor distance, intubation difficulty score

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Introduction

Endotracheal intubation is often required under general anesthesia and is usually performed by direct laryngoscopy (DL). Identifying patients with difficult airways during preoperative assessments is essential because DL may be difficult or even impossible in some patients, potentially resulting in significant morbidity and mortality. ¹ The prevalence of difficult
laryngoscopic intubation ranged from 1.5% to 13.0%. ²

Various methods exist to evaluate difficult intubation, but none is a gold standard. ³ Several research has primarily focused on American and European populations, which differ anthropometrically from Indians. ⁴ The common preoperative predictive tests include inter-incisor distance (IID), mandibular protrusion (MP), thyromental distance (TMD), sternomental distance (SMD), modified Mallampati grade (MPG), and Cormack & Lehane laryngoscopic grade. These tests are cost-effective and simple for any level of anesthesia professional to employ in clinical practice. ⁵

Wilson et al. ⁶ studied the inter-incisor gap with the mouth fully opened and concluded that an inter-incisor gap of less than 5 cm (three finger breadths) and limited forward protrusion of the jaw were deemed to be at higher risk for difficult intubation. Narkhede et al. ⁷ , in their study, tried to find predictors of difficult intubation in Indian patients using inter-incisor gap and sternocricoid distance. The inter-incisor gap was 4.2 ± 0.9 cm in the easy group and 3.5 ± 1.3 cm in the difficult group, which is statistically significant. The prediction value of the inter-incisor was approximately 80% for easy intubation, and the sensitivity was high. Balakrishnan and Chockalingam ⁸ conducted a study to undertake several upper airway examinations to establish standards of normal measures in a South Indian population and analyze the data to look for indicators of difficult intubation. The study concluded that the cut-off value for the ratio of height to TMD to predict difficult laryngoscopy in the South Indian population was 17.1.

The present study hypothesized that there will be gender differentiation in the cut-off value of IID, and gender differentiation is considered a predictor of difficulty in intubation.

Methods

This prospective observational study was conducted after we obtained approval from the Institutional Ethical Committee, and written informed consent was obtained from the participating patients. A total of 516 patients belonging to American Society of Anesthesiologists (ASA) physical status I, II, and aged more than 18 years of either gender posted for elective surgery under general anesthesia with endotracheal intubation were included in the study. Patients with abnormalities of the airway, pregnancy, obesity with body mass index of > 30, tumors or mass in the neck, restricted neck mobility, and unwilling patients were excluded from the study. The patients were divided into two groups, Group 1(Male) and Group 2 (Female).

One investigator-assessed preoperative airway characteristics to reduce inter-observer variability: MPG, IID, neck extension (NE), neck circumference (NC), hyomental distance (HMD), TMD, SMD, and thyrohyoid distance (THD). The Mallampati grading was performed in the sitting position, with the mouth open and tongue protruding without phonation. Patients were graded as class 1 if the soft palate, faucial pillars, and uvula were visible; class 2 if the soft palate or/and uvula were visible; class 3 if only the soft palate and the base of the uvula were visible; and class 4 if the soft palate was not visible and only hard palate was visible. IID is measured as the distance between the upper and lower teeth of the open mouth of a patient in a neutral head position.

All patients were pre-medicated with oral alprazolam 0.5 mg and tablet ranitidine 150 mg on the night before surgery and the morning of surgery. After securing intravenous access, connecting monitor and baseline vitals were recorded on the operating table. All patients were anesthetized by using standard anesthesia technique, pre-medication with ondansetron 0.10 mg/kg and fentanyl 2.00 μg/kg. Following pre-oxygenation with 100% oxygen for 3 minutes, induction with thiopentone 5 mg/kg, sevoflurane 2%, and relaxed with atracurium 0.50 mg/kg. Intubation was done with McIntosh laryngoscope with the appropriate blade size and tube size and was maintained with O ₂ , air, sevoflurane, and atracurium with controlled ventilation and further anesthetic management continued. An intubation difficulty score (IDS) was noted, which included the direct-view Cormack grade, adduction of the vocal cord, and whether increased lifting strength was required for intubation. The assistants recorded and measured other variables including the number of additional intubation attempts required, the number of additional operators required, and the number of alternative intubation techniques used. The glottic view was defined using the Cormack grade (grade 1 = 0; grade 2 = 1; grade 3 = 2; grade 4 = 3), ⁹ lifting force required during laryngoscopy (1, if increased force was applied; 0, if no increased force was applied), whether cricoids compression was applied or not, and the vocal cords mobility during intubation (1 if adducted; 0, if abducted) was also recorded. The sum of the scores for the seven above-mentioned variables represented the total IDS score. The grading of IDS was assigned for that score, ¹⁰ where 0, 1 to 5, and > 5, indicate easy, slight difficulty, and moderate to major difficulty, respectively. ¹¹

Sample size calculation: Based on a previous research ⁸ , the sample size was estimated, considering 0.11% of difficulty in females (P1) and 0.23% in males (P2) with a level of significance at α = 5% and with the power of the test (1 – β) = 95%, with 252 patients in each group. However, the current study recruited 258 patients in each group during the study period.

Data were analyzed using Statistical Package for Social Sciences (SPSS) software version 24 (IBM Statistics, Chicago, USA) and Microsoft Office 2007. Descriptive statistics in the form of mean and standard deviation for interval variables, and frequency and percentage for categorical variables were performed. Student’s t -test was performed to determine significant differences between males and females for the interval variables. Chi-square tests were used to evaluate the association between the two groups for categorical variables. Stepwise logistic regression analysis was performed to identify important risk factors for gender and difficulty in intubation. Receiver operating characteristic (ROC) curve analysis was used to determine the cut-off values of the IID, difficult intubation, and other airway parameters. A P value of less than 0.05 (two-tailed) was considered statistically significant.

Results

A total of 516 patients were included in the study, and the total cohort was grouped into males (Group 1 [n = 258]) and females (Group 2 [n = 258]). Demographic variables were compared between the two groups (Table 1). The mean age ( P = 0.04) and weight ( P = 0.00) of the patients were higher in males compared to females and found significant differences.

The mean IID ( P = 0.00), NC ( P = 0.00), SMD ( P = 0.00), HMD ( P = 0.00), TMD ( P = 0.00), and intubation difficulty scale (IDS) were significantly higher in males; however, the mean THD ( P = 0.00) was higher in females (Table 2).

Most patients were in grades 1 and 2 for both Mallampati grading ( P = 0.66) and NE ( P = 0.47) and were comparable between males and females.

Among 516 patients, the incidence of intubation difficulty was 11.24%. In males, the incidence of difficult intubation was 16.3%, and that of females was 6.2%. Logistic regression identified that IID ( P = 0.000), NC ( P = 0.000), SMD ( P = 0.000), IDS ( P = 0.000), MP grade 3 ( P = 0.045), and MP grade 4 ( P = 0.018) were independent predictors of gender (Table 3). Increased IID ( P = 0.039), decreased SMD ( P = 0.046), and male gender ( P = 0.002) were identified as the predictors of difficult intubation (Table 4).

IID and IDS showed greater specificity than sensitivity and are statistically significant with cut-off values of 4.25 and 1.50 for IID and IDS, respectively (Table 5). The ROC curve showed that IID and IDS were above the reference line with the area under the curve of 0.666 and 0.608, respectively (Figure 1).

Sensitivity and specificity were identified for the other airway predictors as follows: SMD (54.65%, 72.87%), HMD (59.30%, 62.50%), MPG (18.60%, 84.88%) showed poor sensitivity and better specificity (Table 6). However, NC (72.5%, 64.5%) and TMD (79.1%, 44.5%) showed good sensitivity and poor specificity with a significantly good AUC (TMD: 0.643; NC: 0.728) (Figure 2). Gender differences for IID showed a poor AUC (males:0.474 and females: 0.416) and were not statistically significant with no difference in the cut-off values (Table 7 and Figure 3). Although males had higher means of IID, higher means of IDS were observed. However, in females, despite low IID, intubation difficulty was not observed when compared to that in males (Figure 4). There were no failed intubation or intubation-related complications in our study.

Discussion

The maintenance of a patent airway is the prime responsibility of an anesthesiologist. Failure to maintain adequate gas exchange, even for a few minutes, leading to hypoxia can result in catastrophic outcomes such as brain damage and death. ¹²

Advances in various airway rescue devices have decreased the incidence of major airway complications, but recent reports have shown that life-threatening airway complications still occur in patients undergoing anesthesia. ¹³

To decrease airway-related complications, the prediction of possible difficult intubation becomes essential. The incidence of difficult intubation in the Indian population may be as high as 30%. ¹⁴ Our incidence of intubation difficulty was 11.24%. In males, the incidence of difficulty intubation was 16.3%, and in females, 6.2%. The incidence in our study is consistent with the Bilgin and Ozyurt ¹⁵ , Karkouti et al. ¹⁶ , Patel et al. ¹⁷ , and Tse et al. ¹⁸ who reported a higher incidence of difficult intubation (8.00%, 8.24%, 8.10%, and 13.10%, respectively). However, in contrast to these results, a lower incidence was reported by Savva ¹⁹ , Khan et al. ²⁰ , and Yildiz et al. ²¹ reported as 4.90%, 5.00%, and 4.80%, respectively. This may be due to differences in the population characteristics and differences in the definition of difficult intubation. Several studies show that the male gender predicts difficult intubation. ⁸ Balakrishnan and Chockalingam ⁸ stated that more males have difficult laryngoscopy. They reported that a total of 429 males with 22.80% (n = 98) have difficulty with laryngoscopy compared to 1,575 females with 10.09% (n = 159) difficulty. Han et al. ²² in a study of 213 patients comprising 141 men (66.2%) and 72 women (33.8%), the overall incidence of difficult laryngoscopy was 16.4% (35/213).

In a cohort study from the Danish Anaesthesia Database of 91,332 consecutive patients, by examining the relative impact of weight and height, only weight was found as an independent risk factor for difficult tracheal intubation. ²³ A binary logistic regression identified that Mallampati class and TMD as independent predictors of difficult laryngoscopy with a P value of 0.000 and 0.017, respectively. The current study also identified identified IID ( P = 0.000), NC ( P = 0.000), SMD ( P = 0.000), IDS ( P = 0.000), MP grade 3 ( P = 0.045), and grade 4 ( P = 0.018) as independent predictors of difficult intubation.

This study is comparable with the study conducted by Prakash et al. ⁴ The mean age of their study was 37.8 ± 13.5, which is comparable with our study (38.450 ± 14.869). They identified MPG III and IV and IID ≤ 3.5 cm as independent predictors of difficult laryngoscopy. According to their study, increasing age and male sex have an association with difficult laryngoscopy. The present study also showed difficult intubation in males with high IID. Similarly, the study conducted by Yildiz et al. ²¹ in Turkish patients showed an incidence of difficult intubation as 4.8% and increased with age ( P < 0.05). They found that the incidence of difficult intubation was significantly higher in patients who had an MPG III or IV score, a decreased average thyromental and SMD, decreased mouth opening, or decreased protrusion of the mandible ( P < 0.05). The current study revealed that increased SMD ( P = 0.00), TMD ( P = 0.00), NC ( P = 0.01), increased weight ( P = 0.02), and decreased HMD ( P = 0.00), less height ( P = 0.00), low IDS ( P = 0.00), and male gender ( P = 0.00) are significantly effecting on the IDD.

Tamire et al. ⁵ reported that the gender of the patient does not show an association with difficult laryngoscopy and intubation. This finding was in line with the study by Khan et al. ²⁰ , and there was no significant difference regarding difficult intubation based on gender, whereas there were significant differences between the older tests and laryngeal view ( P < 0.05). However, in contrast to the reported studies, the current study observed that gender differences decreased SMD and increased IID, which was significantly difficult with intubation.

The current study showed good specificity than sensitivity for IID (specificity: 68.60%; sensitivity: 54.70%) and IDS (specificity: 67.06%; sensitivity: 55.04%) and are statistically significant with cut-off values of 4.25 and 1.50 for IID and IDS, respectively. The ROC curve showed the IID and IDS above the reference line with the area under the curve of 0.666 and 0.608, respectively. The present study is in line with Tamire et al. ⁵ who showed poor sensitivities but good specificity for both difficult laryngoscopy and intubation. The sensitivity, specificity, and values of IID for difficult laryngoscopy were (51.5%, 81.3%). Similarly, the sensitivity and specificity values of IID for difficult intubation were (66.7%, 79.1%), which was comparable to Shiga et al. and most of the available literatures. ^21,24-26

Similarly, Merah et al. ²⁵ found the sensitivity, specificity, and positive predictive values for the five airway predictors as follows: Modified Mallampati
test (61.5%, 98.4%, and 57.1%), TMD (15.4%, 98.1%, and 22.2%), SMD (0%, 100%, and 0%), horizontal length of the mandible (30.8%, 76.0%, and 4.3%), and inter-incisor gap (30.8%, 97.3%, and 28.6%), which shows poor sensitivity and positive predictive values but better specificity. According to Shiga et al. ²⁶ , screening tests included were Mallampati classification, TMD, SMD, mouth opening, and Wilson risk score. Each test yielded poor to moderate sensitivity (20%–62%) and moderate to fair specificity (82%–97%). The current study also evaluated the sensitivity, specificity, and cut-off values for the airway parameters as follows: SMD (54.65%, 72.87%, and 14.75), HMD (59.30%, 62.50%, and 4.75), MPG (18.60%, 84.88%, and 2.50) showed poor sensitivity and better specificity. However, the parameters NC (72.50%, 64.50%, and 34.50) and TMD (79.10%, 44.50%, and 5.75) showed good sensitivity and poor specificity with significantly good AUC (TMD: 0.643, NC: 0.728). This study is comparable with the study conducted by Iohom et al. ²⁴ , where they found poor sensitivity values for MC, IID, and SMD.

Future directions of research include the need for the development of tests for gender variations with high sensitivities to make them useful screening tests not only in anesthesia but also in prehospital, emergency department, and intensive care settings.

Conclusion

The study found an 11.24% incidence of intubation difficulty, 16.30% and 6.20% in males and females, respectively. IID, NC, SMD, HMD, TMD, and IDS were significantly high in males. Increased IID, decreased SMD, and males are identified as predictors of difficult intubation. The cut-off value of IID did not show any significant difference for gender variation. However, despite low IID in females, difficulty in intubation was not observed when compared to males.

Acknowledgments

None.

Conflict of Interest

There are no conflicts of interest.

References

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