Comparing ease of intubation in obese and lean patients using intubation difficulty scale

Abstract

Background:

Difficult tracheal intubation contributes to significant morbidity and mortality during induction of anesthesia. There are divided opinions regarding ease of intubation in obese patients. Moreover, the definition of difficult intubation is not uniform; hence we have use the Intubation Difficulty Scale (IDS) to find the incidence of difficult intubation in obese patients.

Aims:

The primary aim of the following study is to find out the incidence of difficult intubation in obese and lean patients using IDS and secondary aim is to assess the performance of bedside screening tests to predict difficult intubation, mask ventilation and laryngoscopy in obese and lean patients.

Materials and Methods:

A prospective, observational cohort study of 200 patients requiring general anesthesia were categorized into 100 each based on body mass index (BMI) into lean (BMI <25 kg/m²) and obese (BMI ≥25 kg/m²) groups. IDS score ≥5 was termed as difficult intubation. Pre-operative airway assessment included Mallampati score, mouth opening, neck circumference (NC), upper lip bite test, thyromental distance, sternomental distance (SMD) and head neck mobility. Patients having difficulty in mask ventilation and laryngoscopy was recorded.

Results:

Over all in 200 patients the incidence of difficult intubation was 9%. Obese patients were slightly more difficult to intubate than lean (11% vs. 7%, P = 0.049). Age >40 years, NC >35 cm, SMD <12.5 cm and restricted head neck mobility were factors which were associated with IDS ≥5. Multivariate analysis revealed SMD <12.5 cm to predict difficult intubation in obese patients. Obese patients were difficult to mask ventilate (6% vs. 1%, P = 0.043). There was no difference regarding grading of laryngoscopy between the two groups.

Conclusion:

Obese patients are difficult to mask ventilate and intubate. During intubation of obese patients who is more than 40 years age and SMD <12.5 cm, it is preferable to have a second skilled anesthesiologist.

INTRODUCTION

Obesity is a growing problem globally and anesthesiologists frequently encounter such patients whose airway management is their major responsibility. Tracheal intubation of obese patients can be challenging due to impaired respiratory mechanics and increased sensitivity to adverse effects of apnea and hypoxia.[1] According to World Health Organization, overweight and obesity is defined as body mass index (BMI) ≥25 and ≥30, respectively.[2] However, obesity in Asian population is associated with more obesity related diseases at lower BMI than their Caucasian counterparts. Thus, the Health Ministry of India redefined overweight and obesity as BMI ≥23 and ≥25, respectively.[3]

Difficult tracheal intubation accounts for anesthesia related morbidity and mortality. Nearly 30% of anesthesia deaths can be attributed to a compromised airway.[4] The incidence of difficult intubation in normal patients is 6.2%,[5] whereas in obese patients, it varies between 10% and 15% respectively.[6,7] Meta-analysis was performed by Shiga stated that obese patients have 4% increased risk of difficult intubation and bedside screening tests have limited value to predict difficult intubation.[5] The predictors of difficult airway in obese patients are obstructive sleep apnea (OSA), high Mallampati score, increasing age, male sex, short neck, high Wilson score and increased neck circumference.[6,8] However, some authors argue that obesity is not a risk factor for tracheal intubation.[9,10]

The definition of difficult intubation is not uniform as it involves a complex interaction between the patient factors, clinical settings and skill of anesthesiologist. Benumof defined difficult endotracheal intubation as Cormack Lehane (CL) grade III with several attempts.[11] In 1993, American Society of Anesthesiologist (ASA) defined difficult intubation as three attempts at endotracheal intubation when an average laryngoscope is used or when endotracheal intubation takes 10 min or more.[12] In the year 2013, ASA redefined difficult intubation as tracheal intubation requiring multiple attempts, in the presence or absence of tracheal pathology.[13] In 1997, Adnet et al. introduced the Intubation Difficulty Scale (IDS) which includes seven objective parameters to assess intubation.[14]

IDS has been used as a validated difficulty score to define difficult intubation. Juvin assessed the performance of IDS to predict difficult intubation in France.[6] IDS has not been used in Indian population to predict difficult intubation. Hence, the present study is conducted to compare ease of intubation in obese and lean Indian patients using IDS with following objectives:

• To assess the incidence of difficult intubation using IDS scoring system in obese and lean Indian patients

• To assess performance of various bedside screening tests to predict difficult intubation (IDS ≥ 5) in obese and lean patients

• To identify predictors of difficult mask ventilation and difficult laryngoscopy.

MATERIALS AND METHODS

This was a prospective, observational, open-labeled, cohort study. Following approval of Institutional Ethics Committee, ASA Physical Status I and II patients of either sex, aged between 18 and 60 years posted for surgery requiring general anesthesia with an endotracheal tube were enrolled for the study which lasted for 6 months. Patients with upper airway pathology (tumors, fractures), cervical spine injury, neck swelling and pregnancy were excluded from the study. The purpose of airway assessment was explained to patients and their consent was obtained.

Patient's height in a standing position (m) and weight (kg) were measured and BMI was calculated. One-hundred patients with BMI ≥25 were assigned to obese group (O) and 100 patients with BMI <25 were assigned to lean group (L). History of OSA using STOP questionnaire (snoring, tiredness during daytime, observed apnea and high blood pressure)[15] and co-morbidities were noted down. As shown in Table 1 all pre-operative airway assessments were done by the same anesthesiologist. All patients received oral ranitidine 150 mg and diazepam 0.1 mg/kg on the night and morning of surgery. In the operation theatre, patient was connected to standard monitors. All intubations were done by an anesthesiologist with >2 years experience and experience of anesthesiologist was noted down. Patient was pre-oxygenated for 3 min with 100% oxygen following which anesthesia induced with 1.5 μg/kg fentanyl and 1.5 mg/kg propofol. The ability to mask ventilate was checked and paralyzed with suxamethonium 1.5 mg/kg. Patient was positioned in sniffing position, laryngoscopy and intubation was performed after complete muscle relaxation (no twitches on the ulnar nerve stimulation). Patient who required an additional anesthesiologist or an oral airway or other adjuncts to facilitate mask ventilation were recorded as difficult mask ventilation. For laryngoscopy, a Macintosh 3/4 size blade and for intubation portex endotracheal tube of internal diameter 7.5 mm/8.5 mm were used. Intubation time was noted as the time taken from the introduction of laryngoscope blade up to inflation of endotracheal tube cuff. During laryngoscopy and intubation, if pulsoximetry reading is <90% it was recorded as hypoxic episode. Patients with CL grade III or IV on laryngoscopy termed as difficult laryngoscopy. The whole intubation process was scored by using seven measurable variables of IDS [Table 2]. If total IDS was zero, it was considered as an easy endotracheal intubation; 1-4 points as slightly difficult intubation; and ≥ 5 as moderate to difficult intubation. However, we considered patient with IDS ≥5 as difficult intubation for interpretation purpose.

Table 1.

Parameters included in airway examination

Table 2.

Intubation difficulty scale

Statistical analysis

Based on previous study of Dhonneur, a power calculation of 17% difference in success rate with probability level α of 0.05 and power of 0.08 yielded a sample size of 57 patients for each group.[16] However, we included 100 patients in each group to match for drop outs. The analysis was carried out using Statistical Package for Social Science (SPSS version 16.0 IBM). BMI was divided into three subgroups as <25, 25-35, >35 and weight was divided into <40 years and >40 years for interpretation purpose. Univariate regression analysis was done to evaluate the association between covariates and difficult intubation. Subsequent multivariate regression analysis was carried out for the significant covariate from univariate analysis. Chi-square test or Fischer's test used wherever appropriate. P < 0.05 was considered to be statistically significant.

RESULTS

The demographic data and patient assessment are as shown in Table 3. There was a significant difference between obese and lean groups regarding upper lip bite test (ULBT), modified Mallampati classification (MMC), neck circumference (NC) and head and neck mobility. Obese group had a higher incidence of difficult mask ventilation and greater IDS score however, there was no difference in difficult laryngoscopy.

Table 3.

The characteristics and airway assessment in obese and lean patients

Difficult intubation was observed in 11% obese and 7% lean. We were unable to intubate one obese patient in whom the procedure was accomplished using fiberoptic device. The significant predictors of difficult intubation were NC >35 cm (P = 0.03), sternomental distance (SMD) <12.5 cm (P = 0.036) and head and neck mobility ≤90° (P = 0.031). Multivariate logistic regression analysis of these predictors showed length of SMD <12.5 cm to be the only significant predictor of difficult intubation (P = 0.038, odds ratio 3.195-1.251). In the obese group, age >40 years (P = 0.033) and SMD <12.5 cm (P = 0.029) were the only significant independent predictors of IDS ≥5.

Difficult mask ventilation was encountered in six obese and one lean patient (P = 0.043). Three patients had both difficult mask ventilation and intubation (P = 0.014). Weight >70 kg (P = 0.003), BMI >35 (P = 0.000), OSA (P = 0.005), NC >35 cm (P = 0.016), SMD <12.5 cm (P = 0.031), restricted neck mobility (P = 0.009) were significant predictors of difficult mask ventilation. Multivariate logistic regression did not show a correlation between these factors.

Difficult laryngoscopy was observed in 14 obese and 13 lean patients. It was more common in males (P = 0.032). Three patients had associated difficult mask ventilation (P = 0.021) and 15 patients had difficult intubation (P = 0.000). In the obese group, MMC III and IV was a mild predictor of difficult laryngoscopy (P = 0.046). In the lean group, male sex and OSA were predictors of difficult laryngoscopy (P = 0.011).

Patients with IDS <5 were compared with those of IDS ≥5 as shown in Table 4. Age of >40 years, SMD <12.5 cm, CL grade III/IV, duration of laryngoscopy and years of experience in anesthesia were factors significantly different between the easy and difficult intubation groups. Two out of nine patients with MMC III/IV had difficult intubation. All nine patients with ULBT-3 had easy intubation.

Table 4.

Demographics and airway assessment factors of patients with easy and difficult intubation

DISCUSSION

Airway management is the basic and essential skill of an anesthesiologist and identifying characteristics of difficult intubation and ventilation is the first step in airway management. The definitions of difficult endotracheal intubation by Benumof[11] and ASA[12,13] does not include factors such as change of operator, type and number of alternative technique used, requirement of additional force during laryngoscopy, influence of external pressure and relaxation status of vocal cords. Hence, Adnet et al.[14] had introduced the IDS to standardize the complexity of tracheal intubation.

Identification of risk factors for difficult intubation is important to distinguish between anticipated and unanticipated difficult airway and take precautions. Although recent advances in supraglottic airway devices, intubating stylettes, fiberoptic bronchoscopes and video-laryngoscopes have been included in difficult airway situations, they will not replace the traditional conventional laryngoscopy.[17,18]

In the present study, we observed that significantly higher number of patients in the obese group were >40 years and had a higher incidence of OSA, diabetes mellitus and hypertension. Obesity is a recognized risk factor for OSA and other comorbidities of diabetes mellitus and hypertension.[19] Obese people have higher grading of MMC due to increase in neck circumference and restricted neck mobility because of increased fatty tissue around the neck and in the peripharynygeal tissue.[20]

Obese patients were slightly more difficult to intubate compared to lean patients based on IDS. This is concurrent with studies by Juvin, et al.,[6] Kim, et al.[21] and Seo, et al.[22] who observed 15%, 13.8% and 11.8% incidence of difficult intubation in obese, respectively. Wong and Hung[23] stated that Asians were more difficult to intubate than Caucasians. Lavi et al.[24] in their study found that obese had higher IDS because of poor glottis exposure, need for increasing lifting force during laryngoscopy and need of laryngeal pressure to improve glottis exposure. A meta-analysis estimates overall prevalence of difficult intubation to be 4.5-7.5% respectively.[5] This falls within range of difficult intubation in lean patients of our study.

Present study population showed a significant correlation between NC >35 cm, SMD <12.5 cm and restricted neck mobility to predict IDS >5. This is consistent with other studies.[8,25,26] In the obese group of our study, age >40 years and SMD <12.5 cm were the only significant predictors of difficult intubation. Although obese are slightly more difficult to intubate than lean patients, the actual weight or BMI grading of obese people did not predict who will have IDS >5. Similar observation was made by Brodsky[7] and Neligan, et al.[26] who stated that grade of obesity had insignificant association with difficult intubation. Kheterpal, et al.[27] and Shiga, et al.[5] in their study observed patients with BMI >30 were difficult to intubate than lean patients. Moon, et al.[28] found that in young obese with bulky neck, the cervical joint rigidity decreases after administration of muscle relaxant. Similarly, we did not find any correlation of NC and neck mobility with IDS in the obese group. Age >40 years was a predictor of difficult intubation as also observed by Moon, et al.[28] and Rose and Cohen.[29] Ezri[30] stated that laryngoscopy and airway class increase with aging owing to changes in bone joints and dental condition. Our study has excluded patients with restricted mouth opening (MO) and since all patients had mean MO >4.5 cm, it had no influence on difficult intubation.

A meta-analysis revealed that MMC had a poor prognostic value to identify difficult intubation.[31] This is supported by our study, where only two had IDS >5 out of nine patients who had MMC III/IV. Although nine patients out of 200 had ULBT-3, all of them were easy to intubate. A study by Arné, et al.[32] stated that thyromental distance (TMD) <6 cm predict difficult intubation, the present study failed to show such correlation. Patients with small body frame compared to the large body frame have relatively short TMD.[28] Hence, in our patients we could not find any correlation with TMD and difficult intubation. Relatively few studies have addressed the importance of SMD which indirectly signifies the head and neck mobility.[31,33] Present study had significant correlation with decreased SMD and high IDS score similar to that observed by Shiga, et al.[5]

In our study, the mean duration of laryngoscopy and experience of anesthesiologist was significantly higher in difficult intubation group than in easy intubation group. Thus, presence of experienced anesthesiologist is vital when intubating an obese, >40 years with SMD <12.5 cm. Importance of experience of anesthesiologist was also stressed by Brodsky.[8]

The incidence of difficult mask ventilation is 0.9-12.82%. This wide variation in incidence is due to the discrepancy in the definition of difficult mask ventilation and these definitions are subjective and observer dependent.[34] In the present study, 6% obese and 1% in the lean group had difficult mask ventilation. Obese patients is a predictor of difficult mask ventilation as observed in the present study and also identified in other studies.[26,33,34] We found weight >70 kg, BMI >35, OSA, NC >35 cm, SMD <12.5 and restricted neck mobility to be predictors of difficult mask ventilation. Similar observations were made by Shah and Sundaram.[34] However, multivariate logistic regression did not show any correlation between these factors and difficult mask ventilation.

Patients with difficult mask ventilation have a higher incidence of difficult intubation.[34] 1.5% patients (2 obese and 1 lean) had difficulty in both mask ventilation and intubation. Incidence is similar to that observed by Shah and Sundaram[34] and Langeron, et al.[35] Conversely patients with difficult mask ventilation may be easy to intubate and those with easy mask ventilation may have higher CL grading.[27]

Nearly 14% obese and 13% lean had difficult laryngoscopy (CL III/IV). Similar incidence is stated by Shah and Sundaram[34] and Krobbuaban, et al.[36] It was more common in males, obese with MMC III and IV and with OSA. These three factors are predictors of difficult intubation as stated by Neligan, et al.[26] and Kheterpal, et al.[27] who have used criteria of laryngoscopy (CL >II) to define difficult intubation. However, the relationship between grading of laryngoscopy and difficult intubation is being questioned.[25] Kim, et al.[21] and Juvin, et al.[6] found that difficult laryngoscopy was similar in obese and non-obese patients similar to our results. The incidence of CL III varies between 5% and 8.3% and CL IV is 0-1% respectively.[37] In the present study, we observed a higher incidence of CL III, it is because it was recorded without external laryngeal pressure, application of pressure on larynx causes CL grading to decrease.

The limitations of the present study are that we were unable to do randomized controlled trial because the observer could not be blinded to do airway examination or intubation as obesity cannot be concealed. Although obese patients had higher IDS score the grade of obesity which influences difficult intubation could not be assessed due to a limited number of patients with BMI >35. A larger study with more number of patients and also higher degree of obesity it needed.

CONCLUSION

Incidence of difficult intubation in obese is 11% and 7% in lean. Obese patients are difficult to mask ventilate and slightly more difficult to intubate than lean patients but have no difference regarding laryngoscopy. Age >40 years and sternomental distance <12.5 cm are predictors of difficult intubation in obese. It is preferable to have another skilled anesthesiologist while intubating an obese patient.