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Transtracheal Block
Transtracheal block provides topical anesthesia of small branches of the recurrent laryngeal nerve that lines the tracheal lumen below the vocal cords. Because transtracheal injection usually elicits a strong cough reflex, local anesthetic also will distribute over the walls of the larynx and pharynx in the territory of the superior laryngeal nerve. This injection can improve conditions for awake fiberoptic intubation and upper endoscopy.
The distance from the skin to the cricothyroid membrane is highly variable. Moreover, palpation assessment of the cricothyroid membrane is frequently inaccurate. Ultrasound guidance is particularly useful for transtracheal injections in obese subjects with poorly palpable landmarks.
Ultrasound can be used to refine transtracheal injection (performed through the cricothyroid membrane, not between the first and second tracheal rings, etc.). The goal is to guide needle placement through the cricothyroid membrane and not puncture the cricoid cartilage, adjacent tracheal rings, or esophagus. Ultrasound imaging can be used to verify that the thyroid gland isthmus and blood vessels are not within the field of view.
Identification of the Cricothyroid Membrane
The cricothyroid membrane forms a shallow flattened hyperechoic band. The cricoid cartilage is larger and shallower than the tracheal rings. The cricoid cartilage is 2 to 4 mm thick and is draped over the sides of the infraglottic air column.
There is discontinuity in the surface of the air column, with the cricothyroid membrane being the shallowest part of the air surface. The surface of the air column bends at the junction of the first tracheal ring and the cricoid cartilage (the point of inflection is best appreciated in longitudinal view).
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