Ultrasound assessment of cricothyroid membrane anatomy before and after labor: a prospective observational study

Pregnant patients are at increased risk of airway management difficulties. Ultrasound is a valuable tool for predicting difficult intubation and improving cricothyrotomy success in simulation models. We investigated whether labor and delivery alter the anatomy of the cricothyroid membrane.

In this prospective observational study, we enrolled parturients in early active labor. Ultrasound was used to measure cricothyroid membrane depth (primary outcome), and cricothyroid membrane height and neck circumference (secondary outcomes). Measurements at enrollment and post-delivery were compared using the Wilcoxon signed-rank test.

Fifty patients were included in the final analysis. At enrollment, the median [interquartile range] cricothyroid membrane depth was 0.80 [0.63-0.96] cm, height 1.03 [0.80-1.16] cm, and neck circumference 37.0 [34.5-38.5] cm. Cricothyroid membrane depth and neck circumference were significantly greater after labor (median difference 0.03 cm, 95% CI 0.004 to 0.09, P = 0.034; and 1.0 cm, 95% CI 0.50 to 1.50, P < 0.001, respectively), while cricothyroid membrane height remained unchanged (median difference 0.01 cm, 95% CI -0.06 to 0.10, P = 0.85). Body mass index ≥35 and total oxytocin dose were associated with increased neck circumference; no other labor-related factors were significantly associated with cricothyroid membrane measurements.

cricothyroid membrane depth increased slightly during labor, but the change fell within expected ultrasound variability and is unlikely to be clinically significant. Neck circumference increased and was associated with body mass index ≥35 and oxytocin dose. These findings require validation in larger studies.