A substudy of a large randomised controlled trial comparing etomidate with ketamine for RSI in the pre-hospital environment, emergency department, and intensive care unit examined immediate complication rates in relation to the intubation difficulty scale score (IDS).
They used the 7-criteria IDS previously developed and evaluated. The variables included in the IDS are as follows:

1. the number of attempts excluding the first;
2. the number of extra operators;
3. the number of additional techniques utilised;
4. the Cormack grade (0–3 points, grade 1 giving no IDS points);
5. the intensity of lifting force required (0 points if normal, 1 point if increased);
6. the need to apply external laryngeal pressure (0 or 1 point, application of cricoid pressure (Sellick manoeuvre) does not alter the score)
7. vocal cord position (abduction, 0 points; adduction, 1 point). Each criterion was scored and recorded by the physician who performed the procedure.

The sum gives the IDS score, and a score of 0 indicates an easy tracheal intubation at the first attempt by a single operator using a single technique, with a good view of the glottis and abducted vocal cords. Intubation was considered difficult if the score was greater than 5.
There was a positive linear relationship between IDS score and complication rate, and difficult intubation appeared to be a significant independent predictor of death.

OBJECTIVES: To evaluate the association between emergency tracheal intubation difficulty and the occurrence of immediate complications and mortality, when standardised airway management is performed by emergency physicians.

METHODS: The present study was a substudy of the KETAmine SEDation (KETASED) trial, which compared morbidity and mortality after randomisation to one of two techniques for rapid sequence intubation in an emergency setting. Intubation difficulty was measured using the intubation difficulty scale (IDS) score. Complications recognised within 5min of endotracheal intubation were recorded. We used multivariate logistic regression analysis to determine the factors associated with the occurrence of complications. Finally, a Cox proportional hazards regression model was used to examine the association of difficult intubation with survival until 28 days.

RESULTS: A total of 650 patients were included, with mean age of 55±19 years. Difficult intubation (IDS >5) was recorded in 73 (11%) patients and a total of 248 complications occurred in 192 patients (30%). Patients with at least one complication had a significantly higher median IDS score than those without any complications. The occurrence of a complication was independently associated with intubation difficulty (odds ratio 5.9; 95% confidence interval (CI) [3.5;10.1], p<0.0001) after adjustment on other significant factors. There was a positive linear relationship between IDS score and complication rate (R(2)=0.83; p<0.001). The Cox model for 28-day mortality indicated that difficult intubation (hazard ratio 1.59; 95%CI [1.04;2.42], p=0.03) was a significant independent predictor of death.

CONCLUSION: Difficult intubation, measured by the IDS score, is associated with increased morbidity and mortality in patients managed under emergent conditions.

Morbidity related to emergency endotracheal intubation—A substudy of the KETAmine SEDation trial
Resuscitation. 2011 May;82(5):517-22