Identifying the cricothyroid membrane in healthy subjects: a comparison of three techniques to assess accuracy, confidence and speed

BRIEF NAME Identifying the location of the cricothyroid membrane (CTM) with three methods: palpation, laryngeal handshake and ultrasound. All work was undertaken at the School of Medicine, University of St Andrews, UK. FULL EXPLANATION Operator Recruitment and Training: Six operators were recruited to undergo training and evaluation in identifying the CTM on simulated patients (SPs). Three of the operators were anaesthetists from Ninewells Hospital, Dundee, and three were medical students from the School of Medicine, University of St Andrews. All anaesthetists (mean age 33 years, min-max range 30 - 34 years), were in an anaesthesia specialty training post recognised by the Royal College of Anaesthetists. Two were female, with seven and eight years of anaesthesia experience respectively. One was male, in his first year of anaesthesia training. All had previously undergone training in the use of ultrasound (although not relating to eFONA), and regularly use it to place venous central lines and perform regional anaesthesia techniques. All medical students (mean age 21 years, min-max range 20 - 23 years) were in their first three years of medical school, and none had any previous formal training or experience in using ultrasound. The operators attended a three hour educational tutorial on identifying the CTM, provided by a group of experienced anaesthetists (three consultants, two registrars in their fifth year of training). This began with a 30 minute presentation to summarise the aims of the study, the scenario of CICO and eFONA, and a review of the relevant anatomy using models and images. The ultrasound assessment using a 6 - 12 MHz linear array ultrasound probe (LOGIQ V2; GE Healthcare). Finally, all three techniques were demonstrated in real time on a member of the study investigator group. Operators were directed to learn them in this stepwise manner and maintain this approach throughout the study, using the following steps for each technique: - Palpation: identify sternal notch; identify laryngeal prominence; identify CTM; mark CTM - Laryngeal handshake: identify greater cornu of the hyoid (roll side-side); identify thyroid laminae; identify cricoid cartilage (middle finger and thumb); identify CTM (index finger); mark CTM - Ultrasound: identify sternal notch; place ultrasound probe transversely above the notch; slide the probe to the SP's right and rotate 90 degrees; identify 'string of pearls' (tracheal cartilages); move probe superiorly to identify CTM; mark CTM. For consistency, operators were instructed to carry out all techniques with their non-dominant hand (as the dominant hand would be ready to perform tracheal access when undertaking eFONA). During the subsequent two hours a mastery learning approach was used to provide the operators with extensive, structured iterative practice, identifying the trachea and CTM on other members of the tutorial group under the direct supervision of study investigators. They were each able to assess over 20 individuals (each other and other medical students from the University of St Andrews) for each technique. In the final 30 minutes of the tutorial, operators were able to practise and familiarise themselves with the technique to be used in the subsequent assessment phase - namely, using the three techniques to identify the CTM and placing markers on the skin in the manner required during the later assessment phase. SP Recruitment 12 SPs were recruited from the University of St Andrews, School of Medicine, simulated patient pool. As the traditional methods of palpation and laryngeal handshake are known to carry a lower success rate in females, all SPs were women. No SPs had any known pathology affecting their anterior neck. Operator Assessment of SP CTM: The SPs subsequently moved through a series of six stations where their neck was assessed by all operators trained in stage one (three anaesthetists and three medical students). The SP moved through this series of stations three times, so each operator assessed the neck of each SP three times (once with each technique: palpation, laryngeal handshake or ultrasound). The order of the techniques used was randomised, to minimise the effect of operator familiarity with any given technique or SP’s neck. For each assessment, the operators were allowed three minutes (180 seconds) to position the SP (in the extended eFONA position), assess the neck with one of the techniques and identify the central point of the CTM, then mark this point by applying the 5 mm sticker to the skin. The pre-marked transparent dressing was then reapplied to the front of the SP’s neck by an independent assessor (aligned with the identifier markings: see below - 'comparator/control treatment'), and the sticker position compared to the experts’ markings, with the operator blinded to the result. If the centre of the 5 mm sticker lay within the experts’ marks of the upper and lower boundaries of the CTM, this was categorised as successful. Similarly, if the central point of the sticker lay within 5 mm either side of the midline over the CTM, this was also deemed successful. In all cases, the distance from the centre of the sticker to the central point of the CTM (as identified by the experts and marked on the dressing) was measured in the axial (superior/inferior) and sagittal (lateral, either side of midline) planes, and recorded to the nearest millimetre. The operator was then asked to assess their confidence in each technique using a linear numerical scale (0 = no confidence, 10 = extremely confident). Each time the neck was assessed, the operator was timed from the point they first touched the SP’s neck (with their hand or the ultrasound probe) to the time they placed the sticker on the skin. The time was recorded to the nearest second (sec). Second Expert Assessment of SP CTM (Distance from Suprasternal Notch to Centre of CTM and Consistency of Mark Position with Changing Neck Position): Finally, the same experts repositioned each SP into the position optimal for eFONA once again, and reassessed the central point of the CTM using palpation and ultrasound. The point was marked on the skin with permanent marker pen and the distance from suprasternal notch was measured (to the nearest mm). The SP then moved off the table and sat whilst other SPs were assessed (to alter the position of the head and neck, and underlying laryngeal/pharyngeal structures). The SP was reassessed (for a final time), using the same method and experts, again in the extended position. The previous skin mark indicating the central point of the CTM was assessed in relation to the upper and lower boundaries of the CTM, to determine if it still lay within the boundaries of the CTM after changing position. The distance from the suprasternal notch to the centre of the CTM was again measured (to the nearest mm). The difference in these distances (before and after changing position and repositioning) was recorded.