The present invention is directed to systems and methods for analyzing anatomical structures of a subject and, more particularly, to systems and methods for determining and predicting potentially difficult intubation of an anesthetized subject.
Subjects undergoing medical procedures that require general anesthesia are ventilated using endotracheal intubation and/or face mask techniques. Unfortunately, intubation difficulty varies between subjects, and multiple failed intubation attempts can result in hypoxia or even death. As such, subjects are pre-operatively examined for anatomic features that are believed to indicate difficult intubation. Typically, at least two examinations are used. One examination is the Mallampati or “MP” test, which involves an examination of oropharyngeal structures that are visible when the seated subject maximally opens the mouth and extends the tongue without phonation, such as described in Mallampati, S. R. et al. Can. Anaesth. Soc. J. 1985; 32: 429-34 and Samsoon, G. L. et al. Anaesthesia 1987; 42: 487-90, which is incorporated herein by reference. Specifically, the subject receives a grade of I through IV, corresponding to an “easy-to-intubate” through “difficult-to-intubate” indication, based on the visibility of some oropharyngeal structures, such as the uvula, compared to other structures, such as the hard palate. The other examination involves measuring the thyromental distance or “TMD” of the subject. The TMD is the distance between the top of thyroid cartilage and the inside of the tip of the mandible with the head in a neutral position, such as described by Frerk, C. M. Anaesthesia 1991; 46: 1005-8 and incorporated herein by reference. Generally, a subject having a TMD less than three fingerbreadths is considered difficult to intubate.
Unfortunately, both of the above tests perform only modestly, with a sensitivity of 30-60 percent, specificity of 60-80 percent, and positive predictive value of just 5-20 percent, as described by Shiga, T. et al. Anesthesiology 2005; 103: 429-37 and incorporated herein by reference. As such, other methods for predicting difficult intubation have been created. For example, Suzuki et al. described in Anesthesiology 2007; 106: 916-23 the use of photographs of subjects' faces to calculate five ratios and angles from measurements derived from placement of anatomic markers on the photographs. The “submandibular angle” was found to correlate with difficult intubation. As another example, Naguib et al. described in Can. J. Anaesth. 1999; 46: 748-59 the measurement of 22 indices from plain radiographs and eight indices from three-dimensional computed tomography scans of the head of subjects who were easy or difficult to intubate. This data was used to create a model including the bedside tests of MP evaluation, TMD evaluation, and thyrostemal distance measurement, and two radiographic features that accurately separated easy- and difficult-to-intubate cohorts.
Both of these previous investigations used a priori assumptions of which anatomic features might correlate with difficult intubation. As such, these investigations may ignore anatomic features that have a non-intuitive correlation with difficult intubation and can lead to faulty indications upon which medical decisions are based.
Therefore, it would be desirable to have methods and systems for predicting potentially difficult intubation that are based on an unbiased investigation to thereby provide a more accurate indication of the potential difficulty of intubating an individual and allow clinicians to make more accurate medical treatment decisions.