Suboptimal management of the difficult airway is one of the main causes of anesthesia-related mortality.1 Thus, accurate prediction of difficult laryngoscopy (Cormack and Lehane [C–L] grades III and IV) is very important in anesthesia practice.2 The incidence of difficult laryngoscopy is reported in the range of 1.5% to 20%.3–7 Prediction of difficult laryngoscopy has been the subject of studies using single anatomical landmarks and multifactorial indexes.8–13 While multifactorial indexes slightly outperform single measures, no anatomical landmark alone has been reported to have acceptable accuracy for prediction of difficult laryngoscopy. Hence, identifying a single reliable predictor of difficult intubation would be valuable. We hypothesized that the height between the anterior borders of the mentum and thyroid cartilage, measured while the patient is lying supine with her/his mouth closed (which we termed “thyromental height test” [TMHT]), could be regarded as a predictor for difficult laryngoscopy. We assumed that it could be a surrogate for the following frequently cited anthropometric measures: (1) amount of mandibular protrusion; (2) dimensions of submandibular space; and (3) anterior position of the larynx. A pilot study (data not shown) suggested that there is a close association between small thyromental height and occurrence of difficult laryngoscopy. Thus, we designed this observational prospective study to determine whether we could rely on it as a predictor of difficult laryngoscopy.
After obtaining the Ethics Committee approval, 314 consecutive male and female patients aged ≥16 years requiring general anesthesia and elective direct laryngoscopy between September 2011 and February 2012 were invited to participate. Patients who needed awake intubation and patients who underwent emergency operations were excluded.
Considering all investigations and measurements were performed in a totally noninvasive manner, there was no need for informed written patient consent. Oral patient consent was obtained during routine preoperative physical examinations.
The following 4 predictive test measurements were obtained in the preoperative clinic by a member of the research team who was not involved in assessment of laryngoscopy:
- Modified Mallampati test (MMT): the oropharyngeal view was assessed using a modified Mallampati classification while patients were seated, with mouth maximally opened, tongue protruded, and without phonation.14 Classes 1 and 2 were considered predictive of easy laryngoscopy, and classes 3 and 4 were considered predictive of difficult laryngoscopy.
- Thyromental distance (TMD): the distance in centimeters between the thyroid prominence and the most anterior part of the mental prominence of the mandible, with the head fully extended, was measured with a ruler.7 A TMD ≤6.5 cm was considered predictive of difficult laryngoscopy.15
- Sternomental distance (SMD): the distance in centimeters between the superior border of the manubrium sterni and the bony point of the mentum, with the head in full extension and the mouth closed. An SMD ≤13.5 cm was considered predictive of difficult laryngoscopy.16
- The new clinical test (TMHT) was performed as follows: we measured the height between the anterior border of the thyroid cartilage (on the thyroid notch just between the 2 thyroid laminae) and the anterior border of the mentum (on the mental protuberance of the mandible), with the patient lying supine with her/his mouth closed (Fig. 1A). This height was measured with a depth gauge (INSIZE® Electronic Depth Gage, INSIZE Co LTD., Suzhou New District, China; Fig. 1B). We have termed this the TMHT. Furthermore, age, sex, height, body weight, body mass index, and ASA physical status of all patients were recorded.
General anesthesia was induced via a combination of midazolam (2–4 mg), fentanyl (2–3 μg/kg), and sodium thiopental (2–4 mg/kg). Muscle relaxation was achieved with atracurium (0.5 mg/kg).
The patient’s head was placed in a sniffing position, and laryngoscopy was performed 3 minutes after atracurium injection using a Macintosh #4 blade. If no laryngeal view was achieved, a second attempt was made using a Miller #4 blade combined with external, backward laryngeal pressure and, occasionally, adjustment of head position. All 3 intubation attempts were performed by the same resident in all 314 patients under the supervision of an attending anesthesiologist. The laryngoscopist was unaware of airway assessments. The best view that led to successful intubation was assigned a grade of I to IV according to C–L criteria.17 C–L grades I and II were classified as “easy,” whereas C–L grades III and IV were designated as “difficult.”
The primary end point was calculation of the prediction index (cutoff value) and validity indexes (sensitivity, specificity, accuracy, positive predictive value, negative predictive value, likelihood ratio, odds ratio, κ index) for the TMHT. Calculation of validity indexes for the 3 other methods of airway assessment was a secondary objective of this study.
We used preoperative assessment data and findings during intubation for evaluation of the above-mentioned tests in predicting difficult laryngoscopy. Data are presented as number/percent or mean ± SD. Area under the receiver operating characteristic (ROC) curve for the TMHT was used to show the predictive values of various thyromental height distances. Area under the curve and area under the ROC curve were used to calculate the ideal cutoff point for TMHT. In addition, the validity indexes (sensitivity, specificity, accuracy, positive and negative predictive values, odds ratio, likelihood ratios), κ index, and P values were calculated for each test. Estimating a 10% incidence of difficult laryngoscopy, a sample size of 180 was calculated to have at least 90% power to detect the agreement between the C–L test and the predictors. Since a dramatic imbalance in the number of patients with and without outcome was anticipated, power attenuation had been expected. Therefore, for power maintenance, we decided to increase sample size about 2-fold (314). Sensitivity, specificity, accuracy, positive predictive value, negative predictive value, and confidence interval (CI) were calculated based on normality assumption as the prerequisite condition (np and nq >5)* was met in most of the indexes. In cases that the prerequisites were not met, our emphasis was on point estimate rather than CI and significance. Therefore, we did not compute CI by bootstraps or other suitable statistical approaches. Normality was also applied to them roughly.
χ2 test (Fisher exact test) was used for statistical comparison, and κ measure was used for agreement. Through a pilot study, intraclass correlation coefficient was calculated by 2 examiners (blinded to each other’s measurements) and the same depth gauge. All statistical analyses were performed using SPSS version 16 (SPSS, Chicago, IL).
Three hundred fourteen ASA physical status І and II patients (149 men [47.5%] and 165 women [52.5%]) with a mean age of 44.5 ± 15 years participated in the study. Two patients were excluded from the study because of compromised airway diagnosis in the preoperative clinic. Twenty-three patients (7.3%) had a C–L grade III or IV laryngeal view. There were no failed intubations. Descriptive and quantitative data of the patients and the airway assessment methods are shown in Tables 1 and 2. According to the ROC curve, the optimal sensitivity and specificity values were in the range of 47.46 to 51.02 mm and are shown in Table 3. To facilitate clinical application, a cutoff value equal to 50 mm was chosen. True-positive, false-positive, true-negative, and false-negative (FN) results together with sensitivity, specificity, positive predictive value, negative predictive value, accuracy, odds ratio, likelihood ratio, κ index, and P values calculated for MMT, TMD, SMD, and TMHT are shown in Tables 2 and 4. κ index was significant for the TMHT (0.853 [0.74–0.95], P < 0.0001). However, it was not significant for TMD, MMT, and SMD tests. In a pilot study (15 eligible subjects) performed by 2 blinded examiners, the means of thyromental height measurements were 59.17 ± 10.71 mm and 58.98 ±10.94 mm (intraclass correlation coefficient = 0.93).
The TMHT was found to be a more accurate predictor of difficult laryngoscopy than other single anatomical measures. MMT is one of the most widely reported methods used for prediction of difficult laryngoscopy. Although when used alone this method has a poor predictive value, it may be valuable as part of a multivariate model for prediction of a difficult laryngoscopy.18–20 Schmitt et al.21 made a modification in the TMD test: “ratio of patient’s height to thyromental distance” (RHTMD). The sensitivity of the TMD and RHTMD test was the same, while the RHTMD test showed a little better specificity.21 Al Ramadhani et al.16 found that the SMD test by itself may not be an adequate sole predictor of subsequent difficult laryngoscopy. They suggested that the SMD test is better when used as part of a series of airway assessment tests.16
The consequence of an FN prediction (i.e., laryngoscopy and intubation would be easy) may prove to be catastrophic. Thus, decreasing FN predictions (increased sensitivity) is more important than falsely predicting difficulty (false positive) for patients in whom laryngoscopy and intubation are accomplished easily.15,22 The results of the current study support previous studies regarding poor sensitivity and positive predictive values of MMT, TMD, and SMD tests. Also, specificity and negative predictive values for the 3 above-mentioned tests in the current study are comparable with that of the previous studies.16,18–24 The typical advantage of the TMHT is high sensitivity and positive predictive values in comparison with the 3 other methods of airway assessment. Naguib et al.11 reported the sensitivity of 3 multivariate clinical models (Wilson, Arné, and Naguib) as 40.2%, 54.6 %, and 81.4%, respectively. The sensitivity of the TMHT, 82.6% (CI, 74%–88%), is approximately equal to Naguib et al.’s multivariate clinical model. Also, the specificity value of the TMHT test, 99.31% (CI, 96%–99.98%), is comparable with that of the above-mentioned multivariate clinical tests.
In an analytical observational study that focused exclusively on TMD, Qudaisat and Al-Ghanem24 found that the TMD method is a surrogate for inadequate head extension rather than dimensions of submandibular space. The calculated sensitivity of TMD (21.7%) in the current study is close to that of Qudaisat and Al-Ghanem’s study (19%). The TMHT on its own is not dependent on active head extension. However, it must be noted that patients lying supine inevitably would place 98% of them in a relative head extension.25 Both SMD and TMD tests must be measured in full head extension; thus, they are dependent on the patient’s cooperation, adequate cervical spine mobility, and having no contraindication for full head extension.
A more caudal or anterior larynx is associated with difficult laryngoscopy,26 and it can be expected to correlate with a shorter thyromental height. To compensate for this, backward, upward, and rightward pressure can be used to improve the laryngoscopic view.5,27–29 This posterior displacement increases the TMD, effectively increasing the thyromental height. One advantage of the TMHT is the use of an inexpensive, easily applicable instrument for measurement of an objective quantity (distance between anterior surfaces of the mentum and thyroid cartilage). It must be mentioned that the anatomical difference and measurement errors may affect the test results. For example, the assurance of a proper horizontal alignment may require the use of a bubble device such as those on carpentry levels. However, this study found a small interobserver variability for TMHT measurement (this was performed on a small subgroup of patients using only 2 observers). In contrast, large interobserver variability is a major problem with the MMT.4
Our study has some limitations. First, the study population was limited to patients scheduled for elective, nonemergent surgeries including orthopedic (trauma, spine), thoracic, abdominal, and vascular (central, peripheral). Thus, the results are only applicable to this group of patients. Second, determination of the best cutoff point for use of the TMHT, as a difficult laryngoscopy predictor, and its analysis, as a measure of prediction, have both been performed on the same population; this may explain the good statistical results calculated for the TMHT in comparison with the other tests. Third, we calculated the ideal cutoff value for the TMHT from the data obtained in the study, whereas the cutoff value for the other tests was obtained from the literature. Although this statistical approach may lead to a weaker or a better result for the competing parameters, it should be remembered that the primary end point of this study was the TMHT, not an evaluation of existing methods. In addition, the validity indexes calculated for the competing tests were in concordance with many other studies.
In summary, the TMHT appears promising as a single anatomical measure to predict the risk of difficult laryngoscopy, but validation will require further studies in more diverse patient populations.
Name: Farhad Etezadi, MD.
Contribution: This author helped design the study, analyze the data, and write the manuscript.
Attestation: Farhad Etezadi has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.
Name: Aylar Ahangari.
Contribution: This author helped conduct the study.
Attestation: Aylar Ahangari has seen the original study data and approved the final manuscript.
Name: Hajar Shokri.
Contribution: This author helped conduct the study.
Attestation: Hajar Shokri has seen the original study data and approved the final manuscript.
Name: Atabak Najafi, MD.
Contribution: This author helped design the study.
Attestation: Atabak Najafi has seen the original study data and approved the final manuscript.
Name: Mohammad Reza Khajavi, MD.
Contribution: This author helped conduct the study and analyze the data.
Attestation: Mohammad Reza Khajavi has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Name: Mahtab Daghigh, MA.
Contribution: This author helped write the manuscript.
Attestation: Mahtab Daghigh has seen the original study data and approved the final manuscript.
Name: Reza Shariat Moharari, MD.
Contribution: This author helped conduct the study and write the manuscript.
Attestation: Reza Shariat Moharari has seen the original study data and approved the final manuscript.
This manuscript was handled by: Sorin J. Brull, MD, FCARCSI (Hon).
The authors thank the Research Promotion Center of Sina Hospital for technical assistance and Dr. Mehrdad Pahlevani for his friendly cooperation.
* n= (number, sample size), p= prevalence, q= (1- prevalence)
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