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Ultrasound Accurately Predicts Difficult Intubations

Montreal—The use of ultrasound to measure airway dimensions in order to predict difficult intubations has been proven reliable, potentially opening the door for its use in such situations.

“The difficulty of airway management is one of the prime risks for mortality and morbidity in patients undergoing anesthesia,” said Faraj W. Abdallah, MD, assistant professor of anesthesia at the University of Toronto, in Ontario, Canada. “We routinely resort to visual cues—like visual measurement of the ratio of tongue-to-oral cavity volume—to predict difficult airway. However, the sensitivity and specificity of this measurement does not exceed 65%.

“Ultrasound is fairly noninvasive and permits us to visualize the oral cavity,” he said. “However, its applicability in identifying the difficult airway has not been exploited because of a lack of evidence supporting its validity and reliability in this specific area.”

With that in mind, the researchers enrolled 42 adults into the trial, each of whom was undergoing high-resolution computed tomography (CT) scan of the head and neck for management of lymphoma, breast, bladder or cervical cancers. As part of the CT scan, the tongue-to-oral cavity volume and tongue thickness-to-oral cavity height ratios were calculated. Two sonographers separately measured these same ratios using ultrasound, both before and after the CT scans. All patients had CT scans performed while in the supine position with their mouths open. They held 5 mL of water in their mouths (Figure).

“The open mouth simulated the actual clinical scenario, while the water served to fix the tongue in one position because it wiggles a fair bit in awake patients,” said Dr. Abdallah, who discussed the study at the 2014 annual meeting of the International Anesthesia Research Society (abstract S-21). “In addition, the water kicked out the air and got rid of the attenuation caused by the air, and allowed us to see the palate. Without it, you cannot see the palate to measure the height of the oral cavity.”

Data from 41 patients were analyzed. The investigators found that Cohen’s κ coefficient for comparing CT-measured values of the ratio of tongue thickness relative to oral cavity height, as well as the ratio of tongue volume to oral cavity volume, was 0.94, suggesting that the first ratio is a highly accurate approximation of the second. It was also found that the κ coefficient for the comparison of ultrasound-measured ratio of tongue thickness to oral cavity height—and the same ratio measured by CT scan—was 0.87, suggesting that ultrasound is a very accurate approximation of CT scan.

“We also found high inter- and intraoperator reliability [0.84 and 0.81, respectively], which means that different sonographers can replicate the same values and the same sonographer repeating the same test has the same measurement,” Dr. Abdallah explained.

“We concluded that ultrasound is a valid and reliable tool for measuring dimensions of the tongue and oral cavity, which are relevant to identifying the difficult airway,” he added. “This opens the way to further studies examining the utility of ultrasound in identifying difficult airway.”

Despite these findings, Dr. Abdallah recognized that the study is just a first step in what may be a lengthy research chain. “Finding a high tongue volume may well be a sign of a potential difficult intubation,” he said. “But we are not drawing conclusions about difficult intubation yet; we are simply validating the use of ultrasound. This study served as a precursor for other studies on the utility of ultrasound in predicting difficult airway.”

D. John Doyle, MD, PhD, professor of anesthesiology at the Lerner College of Medicine of Case Western Reserve University, in Cleveland, told Anesthesiology News that the study not only advances the field of airway management, but also validates the use of ultrasound in measuring the dimensions of the tongue and oral cavity. “However, I wonder if—like the Mallampati classification—the obtained measurements might ultimately be found to lack sufficient sensitivity and specificity to be useful on their own,” said Dr. Doyle, who is also a staff anesthesiologist at the Cleveland Clinic Foundation in Ohio.

“Another issue is whether the need for access to an ultrasound machine and its required training will present an obstacle to the technique, even if these measurements turn out to be valuable,” Dr. Doyle added. “Finally, one emerging school of thought holds that since difficult airway prediction remains fair at best, airway training efforts should instead emphasize means to handle unexpected difficult airways, such as the use of video laryngoscopy.”