The ratio of a patient's height to the thyromental distance is a bedside measure predictive of difficult intubation. This compensates for variation in the thyromental distance related to body size.

Method for measuring the thyromental distance:

(1) full extend the head and close the mouth

(2) measure the distance between the thyroid notch and the lower border of the mental prominence (chin) with a rigid ruler

(3) round the distance to the nearest 0.5 cm



(1) height of the patient in cm

(2) thyromental distance in cm


ratio of height to thyromental distance =

= (height) / (thyromental distance)



• A ratio >= 25 is associated with difficult laryngoscopy. Other cutoffs (>= 23.5, 29.3) have been reported, which may be related to different ethnic groups.

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