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GENERAL ARTICLES

The Predictive Value of the Height Ratio and Thyromental Distance: Four Predictive Tests for Difficult Laryngoscopy

Department of Anesthesiology, Chaiyaphum Hospital, Thailand

Address correspondence and reprint requests to Banjong Krobbuaban, MD, Department of Anesthesiology, Chaiyaphum Hospital, Bannakan Road, Muang Chaiyaphum, Thailand. Address e-mail to Albkb{at}diamond.mahidol.ac.th.

	Abstract

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Preoperative evaluation of anatomical landmarks and clinical factors help identify potentially difficult laryngoscopies; however, predictive reliability is unclear. Because the ratio of height to thyromental distance (RHTMD) has a demonstrably better predictive value than the thyromental distance (TMD), we evaluated the predictive value and odds ratios of RHTMD versus mouth opening, TMD, neck movement, and oropharyngeal view (modified Mallampati). We collected data on 550 consecutive patients scheduled for elective-surgery general anesthesia requiring endotracheal intubation and then assessed all five factors before surgery. An experienced anesthesiologist, not apprised of the recorded preoperative airway assessment, performed the laryngoscopy and grading (as per Cormack and Lehane’s classification). Difficult laryngoscopy (Grade 3 or 4) occurred in 69 patients (12.5%). RHTMD had a higher sensitivity, positive predictive value, and fewer false negatives than the other variables tested. In the multivariate analysis, three criteria were found independent for difficult laryngoscopy (neck movement 80 degrees; Mallampati Class 3 or 4, and RHTMD 23.5). The odds ratio (95% confidence interval) of the RHTMD, Mallampati class, and neck movement were 6.72 (3.29–13.72), 2.96 (1.63–5.35), and 2.73 (1.14–6.51), respectively. The odds ratio for RHTMD was the largest and thus may prove a useful screening test for difficult laryngoscopy.

	Introduction

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Failure in managing the airway is the most significant cause of morbidity and mortality in anesthetized patients (1). Difficult laryngoscopy (defined by poor glottic visualization) is synonymous with difficult intubation during surgery in most patients (2). Difficult intubation is reported in 1.5%–13% of patients (3–12). Preoperative evaluation is important for the risk of difficult airway management, but which anatomical landmarks and clinical factors are the best predictors is debated (3,5,13–16).

Several studies describe prediction schemes using a single risk factor or a multifactorial index (3,10,12,17). One test for difficult laryngoscopy is the thyromental distance (TMD), which varies with patient size (7). However, several studies question whether the TMD is either sensitive or specific enough to be used as the only predictor of difficult laryngoscopy (7,11,15,19). Although Schmitt et al. (18) found that the ratio of height to TMD [RHTMD = Height (cm)/TMD (cm)] had a better predictive value than the TMD, no published study has quantified its sensitivity, specificity, and positive predictive value (PPV) versus other bedside tests for assessing a patient’s airway for difficult laryngoscopy. We, therefore, conducted a prospective, blind study of the predictive value of the RHTMD versus four other methods of airway assessment for difficult laryngoscopy.

	Methods

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The protocol was approved by the Ethics Committee of our hospital before commencing this study, and all patients gave written, informed consent. We then studied 550 consecutive ASA physical status I–II adult patients scheduled to receive general anesthesia requiring endotracheal intubation for elective orthopedic, urologic, abdominal, and gynecologic surgery.

Patients younger than 18 yr of age, with obvious malformations of the airway, edentulous, or requiring a rapid sequence induction or awake intubation were excluded from the study to avoid the introduction of a variable that might independently affect predictability of difficult laryngoscopy. Preoperative airway assessment was performed for all patients by the same anesthesiologist to avoid interobserver variability. The tests used to predict difficult laryngoscopy were measurement of mouth opening, TMD, maximum range of head and neck movement, and assessment of the oropharyngeal view. The standard examination method was used for each test.

Mouth opening was assessed by measuring the interincisor gap. Each patient was asked to open his or her mouth as wide as possible, and the distance between the upper and lower incisors at the midline was measured (20). TMD was measured from the bony point of the mentum while the head was fully extended and the mouth closed (9).

The maximum range of head and neck movement was assessed using the method described by Wilson et al. (12). The patient was asked to extend his or her head and neck fully while a pencil was placed vertically on the forehead. Then, while the pencil was held firmly in position, the head and neck were flexed. The range of head and neck movement was classified as 1 80 degrees or 2 80 degrees.

The oropharyngeal view was assessed using a modified Mallampati classification (21). While seated, each patient was asked to open his or her mouth maximally and to protrude the tongue without phonation (22). The view was classed as (a) good visualization of the soft palate, fauces, uvula, and tonsillar pillars; (b) pillars obscured by the base of the tongue but the soft palate, fauces, and uvula visible; (c) soft palate and base of the uvula visible; and (d) soft palate not visible (21).

We also assessed height, RHTMD, body weight, and body mass index. Each patient was routinely monitored during the entire procedure by electrocardiography, pulse oximetry (for measurement of oxygen saturation), and a noninvasive arterial blood pressure monitor.

After the administration of oxygen, all patients were anesthetized using standard drugs including midazolam 0.03 mg/kg, fentanyl 1–2 µg/kg, and propofol 2.5 mg/kg and then paralyzed using neuromuscular blocking drugs to facilitate orotracheal intubation. The patients’ lungs were ventilated by mask with 100% oxygen. Laryngoscopy was performed after the loss of the fourth twitch in the train-of-four. The head of the patient was placed in the "sniffing" position, and laryngoscopy was performed, with a Macintosh Number 3 laryngoscope blade, by a nurse anesthesiologist with 7 yr experience.

Glottic visualization was assessed using a modified Cormack and Lehane (23) classification without external laryngeal manipulation. This classification involved four grades of glottic visualization: Grade 1 = complete of the vocal cords; Grade II = the inferior portion of the glottis; Grade III = only the epiglottis; and Grade IV = a nonvisualized epiglottis. External laryngeal pressure was permitted after evaluation of the insertion of the endotracheal tube. Difficult laryngoscopy in this study was set at Cormack and Lehane Grades 3 and 4. After evaluation, endotracheal intubation was performed after standard anesthetic management.

The preoperative assessment data and the laryngoscope findings were used to evaluate the predictive value of each test for difficult laryngoscopy. The sensitivity, specificity, PPV, and negative predictive value (NPV) of each test were calculated. In addition, receiver operating characteristic curves were used to identify the optimal predictive cutoff points for RHTMD measurement, interincisor gap, and TMD.

	Results

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A total of 550 patients were included. Laryngoscopies were possible for all of the patients. There were no failed tracheal intubations. Demographic data and the distribution of the Cormack and Lehane (23) grades are presented with the mean for the interincisor gap, TMD, and RHTMD (Table 1). The receiver operating characteristic curves of the interincisor gap, TMD, and RHTMD are presented in Figure 1.

View larger version (23K): [in this window] [in a new window]   Figure 1. Receiver operating characteristic curves for the ratio of height to thyromental distance (RHTMD), thyromental distance (TMD), and interincisor gap (IIG) in the prediction of difficult laryngoscopy.

The optimal cutoff point for the RHTMD, TMD, and interincisor gap for predicting difficult laryngoscopy was 23.5 (sensitivity, 77%; specificity, 66%), 6.5 cm (sensitivity, 52%; specificity, 71%), and 3.5 cm (sensitivity, 39%; specificity, 69%), respectively. An interincisor gap 3.5 cm, a TMD 6.5 cm, neck movement 80 degrees, and a Mallampati Class 3 or 4 were the factors selected predictive of difficult laryngoscopy. Difficult laryngoscopy (Cormack and Lehane Grade 3 or 4) was reported in 69 patients (12.5%; Table 2).

RHTMD had a higher sensitivity, PPV, and fewer false negatives than the other factors. The multivariate analysis odds ratios (95% confidence interval) of the RHTMD, Mallampati class, and neck movement were 6.72 (3.29–13.72), 2.96 (1.63–5.35), and 2.73 (1.14–6.51), respectively. The interincisor gap 3.5 cm and TMD 6.5 cm were not recognized as independent variables for difficult laryngoscopy.

	Discussion

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In our study, the incidence of difficult laryngoscopy was frequent (12.5%) compared with other studies (3–5). Our results might therefore favor a low NPV and a high PPV. RHTMD allows a higher PPV (24%) than previously reported (9%–12%) (3–5) but, nonetheless, implies that predicted difficult laryngoscopy is not observed in between 4 and 5 patients.

Ideally, any preoperative assessment scheme for difficult laryngoscopy should have a high sensitivity and specificity and produce few false positives and negatives. The advantage of RHTMD is its higher sensitivity than other tests, thus false-negative predictions are minimized. The consequence of a false-negative result may be deleterious and even life-threatening; therefore, decreasing false-negative prediction is far more important than falsely predicting difficult laryngoscopy in unaffected patients. Because difficult laryngoscopy is infrequent, the incidence of false negatives is small. Nevertheless, a test should be sufficiently sensitive to detect possible difficulties with laryngoscopy. Although all the tests in this series were not highly sensitive, RHTMD measurement resulted in the least amount of detection failure for difficult laryngoscopy of the other four tests. This is our most important finding.

Using a multivariate analysis, we found that the tests using neck movement 80 degrees, a Mallampati Class 3 or 4, and RHTMD 23.5 were the major factors for predicting difficult laryngoscopy. The RHTMD had the highest odds ratio for prediction of a difficult laryngoscopy. Interestingly, we did not find any association between difficult laryngoscopy and the interincisor gap 3.5 cm or TMD 6.5 cm. Our study agrees with reports from Randell (19) and Chou and Wu (24) who suggested that although TMD has been investigated, it has proven of little value in predicting difficult intubation. Furthermore, Savva (11) found no correlation between the interincisor gap and the view on laryngoscopy.

Schmitt et al. (18) found that RHTMD 25 can be used to predict difficult laryngoscopies for white men and women. They suggested that it might not apply to other races. In our study, a RHTMD 23.5 was a determining factor for predicting a poor laryngeal view among Thai patients. This difference is small, and further investigation is required to determine the significance of ethnicity.

This was a carefully conducted randomized design involving a single physician’s assessment of the airway and a single nurse anesthesiologist’s assessment of intubation difficulty. Whereas inter-observer variability was minimized, it leaves open the possibility of bias based on how representative these two individuals are of the population of anesthesiologists.

In conclusion, several studies have evaluated different clinical risk factors, alone or in combination, for a useful method to predict difficult intubation. Our results suggest that RHTMD may be a useful bedside screening test for preoperative prediction of difficult laryngoscopy.

This research was supported by Chaiyaphum Hospital under the Ministry of Health, Thailand. The authors thank Dr. Yongyut Gumpupong, Director of Chaiyaphum Hospital, for his support and guidance and Mr. Bryan Roderick Hamman at the Faculty of Medicine, Khon Kaen University, Thailand, for assistance with the English-language presentation.

	Footnotes

 
Accepted for publication May 23, 2005.

	References

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (6) Citing Articles via Google Scholar Google Scholar Articles by Krobbuaban, B. Articles by Tanomsat, M. Search for Related Content PubMed PubMed Citation Articles by Krobbuaban, B. Articles by Tanomsat, M. Related Collections Airway