Facial Analysis to Classify Difficult Intubation

For any practitioner of airway management, predicting a difficult intubation in advance, reliably and with confidence, has obvious benefits. Know it in advance and you can plan for it. You can work out your plan A, as well as your plan B, plan C and plan D, refresh your memory of the appropriate algorithms, check the appropriate equipment is available and consult with colleagues. All in advance. All relatively stress free.

There are many predictive tests for difficult intubation, including thyromental distance, the mallampati classification and sternomental distance, as well as mnemonic difficult airway identifiers, such as LEMON (Look externally, Evaluate, Mallampati, Obstruction, Neck mobility) for difficult direct laryngoscopy, and MOANS (Mask seal, Obstruction, Age, No teeth, Stiff lungs) for difficult mask ventilation. For anticipation of difficult supraglottic/extraglottic device use, there is RODS (Restricted mouth opening, Obstruction of the upper airway, Distorted airway, Stiff lungs) and for difficult surgical cricothyrotomy, there is SHORT (previous neck Surgery, Haematoma, Obese, previous Radiation therapy, Tumour).

For tests such as thyromental distance, sternomental distance and the mallampati classification, the sensitivity and specificity vary, but range from poor to fair. Combinations of these, such as thyromental distance and mallampati, can be a useful predictor of difficult intubation, but still have their limitations, particularly since there can be variation in measurement conditions. These issues are explored in the meta-analysis Predicting Difficult Intubation in Apparently Normal Patients by Shiga et al, published in Anesthesiology. The full text is free to download.

Last month, an exciting project to develop a computer algorithm to accurately predict how difficult (or easy) it would be to intubate a patient using digital images was revealed by Tufts Medical Center in Boston, USA. This project follows on from a previous study at Tufts, which used computerised facial structure analysis, combined with thyromental distance, to produce a model for predicting difficult intubation. Their model accurately classified 70/80 airways, compared with 47/80 for mallampati + thyromental distance. The computing power needed might currently exceed the capability of today’s mobile phones, but the objective is to produce a computer algorithm that could utilise high speed computers, perhaps over a network, and the digital camera in a mobile phone for the image/photographic input. The dream scenario though has to be – data input and computation of the data in a totally self contained handheld device. Of course, even if this becomes possible, the algorithm will need to demonstrate a clear superiority to current techniques to be clinically useful. All very exciting!

You can keep up to date with progress of this study, ‘Facial Analysis to Classify Difficult Intubation’ at ClinicalTrials.gov.

Cricoid pressure – new data from the USA

As a postscript to my previous post on the use of cricoid pressure during RSI, I have just seen this abstract of a survey conducted in the United States regarding a modified RSII, entitled, Modified Rapid Sequence Induction and Intubation: A Survey of United States Current Practice. I have not seen the complete article yet, but the abstract seems to confirm cricoid pressure as one of three key components of a modified RSII, along with oxygen administration before induction and an attempt to ventilate the patient’s lungs before securing the airway. As the authors comment themselves, ‘Although this definition seems intuitively obvious, no previous work has tested whether it is commonly accepted’.

490 surveys were received from 58 institutions. 93% of respondents reported using a modified RSII. A majority of respondents (71%, CL: 63%-77%) reported administering oxygen before anesthesia induction, applying cricoid pressure, and attempting to ventilate the lungs via a facemask before securing the airway.

Staying with the United States, albeit via a UK report, it was noted in the UK 4th National Audit Project (NAP4) that the American Society of Anesthesiologists (ASA) Closed Claims Practice Group reported that cricoid force was ‘used’ in half of claims relating to aspiration. Claims for aspiration in which cricoid pressure was applied were settled for lower awards than those where it was omitted.

Interestingly, one of the recommendations in NAP4 related to RSI was as follows, ‘On balance, rapid sequence induction should continue to be taught as a standard technique for protection of the airway. Further focused research might usefully be performed to explore its efficacy, limitations and also explore the consequences of its omission.’

No doubt the debate about RSI and cricoid pressure will continue…