Determining the amount of training needed for competency of anesthesia trainees in ultrasonographic identification of the cricothyroid membrane

Airway management is an important skill that anesthesiologists must become proficient in. In the worst clinical scenario of a ‘can’t intubate-can’t oxygenate’ airway crisis the anesthesiologist must perform a cricothyrotomy, an emergency procedure that is potentially life-saving [1, 2]. Success depends on understanding the correct neck landmarks [3, 4]. However, misidentification of the cricothyroid membrane is a major cause of tube misplacements leading to cricothyrotomy failures and serious complications [5, 6]. Evidence showed that palpation of the cricothyroid membrane by anesthesiologists is inaccurate, even under elective conditions [3, 7‐9]. When ultrasound identification of the cricothyroid membrane is compared to identification by palpation, the imaging modality provides superior success rate and decreased complications [10, 11].

Several airway guidelines for difficult airway management recommend ultrasound to be used to identify neck landmarks and the cricothyroid membrane before airway management in non-emergency situation [12, 13]. The Canadian Society of Anesthesiologists’ guidelines for difficult airway management mention ultrasound as a potentially useful tool to identify the cricothyroid membrane prior to a surgical airway, but outline the need of an experienced operator [12]. Moreover, the Difficult Airway Society guidelines recommend the identification of the cricothyroid membrane during the preoperative assessment and if this is not possible with palpation, ultrasonography should be used [13]. These guidelines recognize that airway evaluation using ultrasound is an invaluable skill for anesthesiologists and training in its use is recommended. Currently, the amount of training required to achieve competence in ultrasound-guided identification of the cricothyroid membrane is not known.

The aim of this study was to determine the amount of training an anesthesiologist would need to become competent in identifying the cricothyroid membrane using ultrasonography in a non-emergency situation.

A group of six anesthesia trainees completed the study, consisting of two anesthesia fellows, 2nd-year anaesthesia trainees and two first-year anaesthesia trainees. None of the trainees had previous experience in airway ultrasound. The group of volunteers consisted of 13 females and seven males, with a mean (SD) age of 41 (10) years, weight of 71 (14) kilograms, BMI 25 (4) kgm⁻² and neck circumference 35 (3.6) cm. The degree of neck landmarks difficulty of the volunteers was graded to be moderate to difficult.

Each trainee performed 20 consecutive neck ultrasound scans for a total of 120 assessments, while a learning curve was built for each trainee. Four out of the six trainees achieved competence in identifying the cricothyroid membrane (Fig. 1a-d). The mean [range] success rate among the four trainees who achieved competence was 94.0% [90–100%]. The median [range] number of attempts required to achieve 90% success rates was 14 [9-18]. Trainee E and F did not achieve competence by CUSUM analysis, but obtained a success rate of 80 and 75%, respectively (Fig. 1e, f). The overall (number of attempts) success rate for the entire group was 88.3% (106/120). The mean (SD) time to identify the midpoint of the cricothyroid membrane was 36.9 (9.0) sec.

Three months after training, all trainees were assessed to identify the cricothyroid membrane using ultrasound on five healthy volunteers. The overall (number of attempts) success rate was 86.7% (26/30) with two and four anesthesiologists had a success rate of 100% (5/5) and 80% (4/5), respectively. The mean (SD) time to identify the cricothyroid membrane was 47.7 (16.0) sec.

Our study determined competency using CUSUM analysis of six anesthesia trainees in ultrasound identification of the cricothyroid membrane of 20 healthy adult volunteers. Our findings showed that a median of 14 consecutive ultrasound scans were required to achieve 90% competence with a mean 94.0% success rate in four of the six trainees. Furthermore, this skill appeared to be maintained 3 months after training.

The total time of the didactic teaching and interactive session was 120 min, suggesting the training required to identify neck landmarks and the cricothyroid membrane with ultrasonography was reasonably short. Furthermore, competence can be achieved with less than 20 scans (range 9–18) and the mean time to perform the ultrasound scan was less than 60 s. These findings imply that ultrasound of neck landmarks and identification of the cricothyroid membrane can be easily learned. In addition, it suggests that pre-procedural use of the ultrasound is not likely to cause a substantial delay in proceeding with the planned airway management.

CUSUM analysis showed that four (Fig. 1a-d) out of six trainees became competent, while two trainees did not achieve competency. However, the success rate of ultrasound identification of the cricothyroid membrane by the non-competent trainees was 80 and 75% (Fig. 1e, f), which is higher than the reported success rate ranging from 34 to 72% by palpation only [7‐9].

Learning curves using CUSUM have been used to assess several procedural skills in anaesthesia [20, 21]. The CUSUM analysis is an effective mathematical model to define learning curves for technical skills [18, 22]. It has been employed to investigate competence acquisition in anesthesia technical skills, and more recently, to assess competency in ultrasound-guided procedures [20, 22, 23]. deOliveira Filho have described the learning curves of anesthesiologists for basic procedures such as intravenous cannulation, intubation, spinal and epidural anesthesia [18]. In our institution, we have assessed the proficiency in ultrasound assessment of gastric content and spinal anatomy using CUSUM [20, 23]. It represents a practical and useful tool for objectively measuring performance during the learning phase of basic procedures. When using the CUSUM method in the education context, it is important to understand that it is a statistical method that looks at the outcome rather than the process of performing procedural skills [18, 21].

Previous studies have used expert consensus and institutional results to define unacceptable failure rates [17, 18, 21, 22]. In this study, we defined the acceptable and unacceptable failure rates at 10 and 30%, respectively, based on the performance of previous research on ultrasound assessment [20, 23], and also considering the poor performance of 30% accuracy rate for the standard method of palpation to identify the cricothyroid membrane [7‐9].

Ultrasound can be an efficient, life-saving tool to improve the success of cricothyrotomy in patients identified as potentially difficult surgical airway. A cadaveric study by our group demonstrated that ultrasound identification of the cricothyroid membrane significantly improved the success of cricothyrotomy and decreased complications [10]. The American Society of Anesthesiologists [24] and Canadian Anesthesiolgists’ Society [12] airway guidelines reinforce the need to assess and to prepare for a difficult surgical airway scenario with a “double set-up airway intervention”, possibly identifying and marking the cricothyroid membrane before airway manipulation, while incorporating ultrasound and an operator familiar with the technique. The Difficult Airway Society guidelines recommend to identify the trachea and the cricothyroid membrane during the preoperative assessment, and training in the use of ultrasound to evaluate the airway if the landmarks are not clear [13]. Our study supports these recommendations in that competence in ultrasound of neck landmarks and identification of the cricothyroid membrane can be achieved with minimal training. However, there is no evidence for the use of ultrasound during an already established ‘can’t intubate-can’t oxygenate’ airway crisis since preparation and use of the ultrasound can delay the emergency surgical airway required to be performed expediently. A study in the pre-hospital setting showed that open surgical cricothyrotomy was successful in all 42 patients requiring a surgical airway [25]. Identifying and pre-marking the cricothyroid membrane with ultrasound prior to airway manipulation could synergize the open technique by limiting the size of the incision and minimizing complications, particularly, in patients with thick neck and difficult neck landmarks.

Our study has several limitations. First, the small sample size of six anesthesia trainees in our study limits generalizability to a larger population, particularly among practicing anesthesiologists. However, the performance of a diverse group of anesthesia trainees (2 year-1, 2 year-2 residents and two fellows) none with experience in airway ultrasound helps to reassure the feasibility of teaching this technique. Studies have shown that the accuracy to palpate the cricothyroid membrane was low and similar amongst junior and senior anesthesia residents, anesthesia fellows, and practicing anesthesiologists [8, 9]. Second, it is possible that some participants may have used ultrasound as part of their airway assessment during their clinical rotation prior to the 3-month period after training, leading to enhanced ultrasound skill that may potentially bias the assessment at the 3-month period. However, neck ultrasound is not routinely used to assess the airway and it is not currently taught as a mandatory skill required for airway management in our department at our institution (University of Toronto). Third, the anesthesia trainees did not perform the neck landmark palpation prior to scanning in order to avoid bias on the ultrasound examination. However, palpation is always performed in a clinical scenario, as it is still the standard technique to identify the cricothyroid membrane in an emergency situation. Although our volunteers presented with a wide range of anatomic characteristics, including moderate and difficult landmarks palpation, it does not completely represent the clinical patient of a difficult surgical airway with head and neck malformation, tumors and airway trauma. None of our volunteers had an impossible neck to palpate due to excessive tissue. However, excess tissue overlying the cricothyroid membrane could improve focus of the ultrasound beam to provide better image resolution [7, 11]. Nicholls demonstrated that excessive neck tissue in obese patients improves visualization of neck landmarks with ultrasonography and the time to identify the cricothyroid membrane was not affected [11]. In our study ultrasound scans were assessed using the longitudinal technique. Kristensen et al. demonstrated the transverse technique was significantly faster than the longitudinal technique to identify the cricothyroid membrane [26]. It is possible that learning curves might be affected by the different techniques. However, the same authors showed the success rate between techniques were similar [26]. Lastly, we also performed the ultrasound scan in a controlled non-stressful environment. We neither created the stressful environment of a difficult airway situation nor assess directly the cricothyrotomy procedure. However, we previously demonstrated that the context of a stressful simulated airway crisis scenario did not affect the cricothyrotomy procedure [27].