Definition: intubation difficulty may be defined as a measure of the degree of divergence from a predefined "ideal" intubation, i.e. one performed without effort, on the first attempt, practiced by one operator, using one technique, with full visualization of the laryngeal aperture and vocal cords abducted. Such an intubation is accorded an IDS value of 0. Impossible intubation is defined by infinity (1).
Symptoms (1): - The number of supplementary attempts, an attempt defined as one advancement of the tube in the direction of the glottis during direct laryngoscopy or one like advancement of the tube in the case of a blind intubation trial -> N1.
- The number of supplementary operators (represents the number of additional persons directly attempting intubation) -> N2.
- The number of alternative techniques used. For example, changing from an oral intubation to blind nasotracheal intubation or from a curved blade to straight blade increases N3 by 1 point. The various techniques used should be noted in chronological order, so that subsequent identification of techniques ineffective in a particular case (or series) may be undertaken -> N3.
- Glottic exposure as defined by the Cormack grade (2) minus one. Glottic exposure is evaluated during the first attempt by the first operator. In case of successful intubation after blind nasotracheal intubation, N4 = 0. If the blind attempt(s) fail, glottic exposure is evaluated during the first subsequent alternative visualized laryngoscopic attempt -> N4.
- Grade I (N4 = 0): complete visualization of the vocal cords.
- Grade II (N4 = 1): visualization of the inferior portion of the glottis.
- Grade III (N4 = 2): visualization of only the epiglottis.
- Grade IV (N4 = 3): nonvisualized epiglottis.
- The lifting force applied during laryngoscopy; N5 = 0 if little effort is necessary, and N5 = 1 if subjectively increased lifting force is necessary -> N5.
- The necessity of applied external laryngeal pressure for optimized glottic exposure; N6 = 0 if no external pressure is applied. N6 = 1 if external laryngeal pressure is necessary. Application of the Sellick Maneuver is intended to inhibit aspiration of gastric contents and does not alter the score -> N6.
- Position of vocal cords; N7 = 0 if vocal cords are in abduction. N7 = 1 if the vocal cords are in adduction, presenting an impediment to tube passage. If the vocal cords are not visualized, N7 = 0 by default -> N7.
Measurements (1): - IDS score 0: easy
- 0 < IDS =< 5: slight difficulty
- 5 < IDS: moderate to major difficulty
References: - The Intubation Difficulty Scale (IDS). Anesthesiology. 1997;87:1290-1297: pdf.
- Difficult tracheal intubation in obstetrics. Anaesthesia. 1984;39:1105-1111.
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