Anatomic Measurements Can Predict Difficult Airways, but BMI Less Useful Independently

Increasing age, male gender and a history of obstructive sleep apnea (OSA) were predictive of difficult endotracheal intubation. But a recent prospective study found that, surprisingly, body mass index (BMI) as an independent variable was not predictive.

A number of other bedside measurements were suggestive of difficult intubation. The investigators reported that a Mallampati score of class III/IV, an upper lip bite test of grade 3, a large ratio of neck circumference to sternomental distance, smaller interincisor distance and larger neck circumference increased the incidence of difficult intubation (Table).

One of the most important aspects of proper airway management is the ability to predict which patients will require advanced techniques to achieve a definitive airway. Although a number of previous studies have sought to determine anatomic parameters that may predict difficult intubation, larger prospective studies were needed, Dr. Moon said.

Measuring for Difficult Airways

Dr. Moon and her colleagues at UT Southwestern Medical Center, a tertiary care teaching hospital, enrolled 1,351 patients for prospective analysis over a nine-month period. Individuals who were known to have difficult airways from previous medical care or had obvious anatomic abnormalities were excluded from the study. The researchers used the previously validated Intubation Difficulty Scale (IDS), a seven-item questionnaire, and defined a difficult endotracheal intubation as a score of 6 or more variances from the norm for the following measures:

• neck range of motion
• neck circumference
• thyromental distance
• sternomental distance
• interincisor distance
• upper lip bite test
• Mallampati score

The findings were reported by Dr. Moon at the International Anesthesia Research Society 2017 annual meeting (abstract 1350).

“Difficult intubation among the morbidly obese is a controversial topic, and has yielded conflicting results in prior studies,” Dr. Moon said. “Our analysis showed that even patients with very high BMIs were not more difficult to intubate, as long as their other medical and anatomical variables were favorable.

“Both thyromental and sternomental distance by themselves were not predictive, but neck circumference to sternomental distance ratio was a very predictive measure,” Dr. Moon explained. “The higher that number gets—indicating a thick, short neck—the greater the likelihood of difficult intubation.

“A smaller mouth opening was also associated with difficult intubation with standard laryngoscopy,” said Dr. Moon, perhaps as a consequence of difficulty inserting the blade into a smaller mouth.

Finally, an upper lip bite test grade of 3—indicating that the patient cannot bite the upper lip with the bottom teeth—had the highest odds ratio for difficult intubation (odds ratio, 6.239; 95% CI, 1.693-22.991; P<0.05).

“The upper lip bite test, which tests for jaw subluxation or mandibular protrusion of the bottom teeth above the upper lip, was quite predictive,” said Dr. Moon, who noted that very few people were unable to do the (grade 3) task. “A grade 1 or 2 upper lip bite test is not predictive, but a grade 3 is strongly predictive of a difficult intubation.”

Implementing the Measurements Test

The moderator of the session, Irina Gasanova, MD, PhD, associate professor of anesthesiology at UT Southwestern Medical Center, asked whether there were any unexpected difficult intubations.

“The incidence of difficult intubation was about 4.5%, but everyone was successfully intubated,” Dr. Moon said. “Our institution has a large percentage of residents, so you might expect that their IDS scores would be 1 to 2 points higher than someone who has been practicing for 10 years. However, when we analyzed the data to see if the level of the person intubating affected the IDS score, it was all the same, whether the provider was a certified registered nurse anesthetist; a resident in the first, second or third clinical years; or faculty.”

“How long did it take your researcher to perform these measurements, and how will this study impact your clinical practice?” Dr. Gasanova asked.

“The assessment is very quick,” Dr. Moon said. “We were measuring multiple items, and it took less than two minutes to complete. Some of these measures didn’t pan out as much as I’d hoped they would, but the Mallampati score, which we already use clinically, is definitely helpful. The upper lip bit test is useful and easy to do as well.

“Patient characteristics can be a constructive guide,” Dr. Moon added. “I’m much more concerned about an older male patient with OSA who has a short, fat neck, for example, than a young female patient who has a BMI of 61 [kg/m²] but looks totally fine from the neck up.”