Accuracy of cricothyroid membrane identification using ultrasound and palpation techniques in obese obstetric patients: an observational study

Highlights

• Cricothyroid membrane (CTM) identification in obese obstetric patients may be difficult.

• Ultrasound (US) improves CTM midpoint identification when compared with palpation.

• CTM identification takes significantly longer to achieve with US than palpation.

• There is no correlation between perceived ease of landmark technique and accuracy.

• No correlation between perceived ease of landmark technique and time to identification.

Abstract

Background

During performance of emergency front of neck access, the final step in management algorithms for the ‘can’t intubate, can’t oxygenate’ scenario, accurate identification of the cricothyroid membrane is crucial. Accurate identification using palpation techniques is low, with highest failure rates occurring in obese females.

Methods

This prospective observational study recruited 28 obese obstetric patients. The cricothyroid membrane was identified using ultrasound, marked with an ultraviolet pen and covered with a dressing. The candidate was asked to perform cricothyroid membrane identification using landmark technique (group L) followed by ultrasound (group U). The primary outcome was the distance between the actual and estimated cricothyroid membrane midpoint. Secondary outcomes were the proportion of accurate assessments, time taken, and subjective ease of identification using a visual analogue score.

Results

Distance from the cricothyroid membrane midpoint was shorter in group U than Group L (2.5 mm vs 5.5 mm, P=0.002). The proportion of correctly identified cricothyroid membranes was greater in group U than group L (71% vs 39%, P=0.015). Time required for identification was shorter in group L than group U (16.9 s vs 23.5 s, P=0.001). Visual analogue scores for ease of identification were lower in group U than group L (2.4 cm vs 4.2 cm, P=0.013).

Conclusions

Ultrasound-guided cricothyroid membrane localisation was significantly more accurate but slower than the landmark technique in obese obstetric patients. As such, we recommend the use of pre-procedural identification of the cricothyroid membrane in this patient population and formal training of anaesthetists in airway ultrasound.

Introduction

Emergency front of neck access (e-FONA) is the final step in the management algorithms for the ‘can’t intubate, can’t oxygenate’ (CICO) scenario in adult and obstetric populations.1, 2 This is a potentially life-saving and morbidity-reducing intervention. However, its accurate performance depends on the ability to palpate anatomical landmarks. Research shows that accuracy in identifying the cricothyroid membrane (CTM) using palpation techniques is low, even for skilled providers,4, 5, 6, 7, 8 and CTM identification failure occurs most frequently in obese females.4, 5, 6 The Fourth National Audit Project (NAP4), a UK study on airway management complications, showed that obese patients were twice as likely as non-obese patients to suffer serious airway problems during general anaesthesia.⁹ Severe obesity (body mass index (BMI) ≥40 kg/m²) is becoming more prevalent in obstetric populations.¹⁰ Obesity significantly increases the risk of emergency and elective caesarean section.¹¹ This patient cohort has a higher risk of difficult or failed intubation, and effective, evidence-based rescue strategies are of vital importance.¹²

The increasing availability of bedside ultrasound (US) has driven a rapid increase in its use and has led to its evolution from a research tool to a clinical tool.¹³ Several studies have looked at the use of US compared with palpation to identify the CTM in obese pregnant patients.4, 6 However, these studies had a variable skill-mix in terms of clinician participants which might potentially lead to inter-rater variability. The aim of the study was to compare CTM identification accuracy by an expert when using US compared with palpation in a group at high risk of CTM identification failure. We hypothesised that using US in this population would result in a shorter distance between the actual and estimated CTM midpoints when compared with palpation.