A 32-year-old woman with a BMI of 38 kg/m² presents to the emergency department with acute appendicitis requiring urgent surgery. During pre-operative assessment, the anesthesiologist notes difficult airway features: short neck, limited neck extension, and Mallampati Grade IV. Which investigation is most appropriate to predict the ease of intubation and guide the choice of RSI technique?

Explanation

## Airway Assessment in Rapid Sequence Induction **Key Point:** Thyromental distance (TMD) is a simple, bedside, non-invasive measurement that directly predicts laryngeal visibility and intubation difficulty. It is the gold standard screening test in RSI pre-operative assessment. ### Thyromental Distance: Clinical Utility TMD is measured from the thyroid notch to the mentum with the neck fully extended and mouth closed. | TMD Measurement | Intubation Difficulty | Clinical Implication | |---|---|---| | > 6.5 cm | Easy | Favorable for RSI | | 6–6.5 cm | Moderate | Caution; consider fiberoptic | | < 6 cm | Difficult | Difficult airway; avoid RSI | **High-Yield:** TMD < 6 cm has **sensitivity 80–90%** for predicting difficult intubation and is the single most reliable bedside predictor in the emergency setting. ### Why TMD in RSI Context? 1. **Rapid assessment** — no equipment needed; done in seconds 2. **Non-invasive** — safe in emergency, hemodynamically unstable patients 3. **Guides technique** — if TMD < 6 cm, modify RSI (awake fiberoptic, video laryngoscope, or surgical airway planning) 4. **Combines with other tests** — Mallampati, inter-incisor distance, neck circumference to build composite risk **Clinical Pearl:** In this obese patient with Mallampati IV + short neck + limited extension, TMD measurement is the **single most important next step** to decide whether RSI is safe or if awake fiberoptic intubation should be performed instead. **Mnemonic:** **LEMON** assessment for difficult airway: - **L**ook externally (face, neck, teeth) - **E**valuate 3-3-2 rule (inter-incisor distance, hyoid-to-floor distance, thyroid-to-floor distance) - **M**allampati score - **O**pen mouth (dentition, tongue size) - **N**eck mobility (extension, flexion) TMD is part of the **E** component and is the most predictive single measure. [cite:Miller's Anesthesia 8e Ch 16]