Facial beard. Presence of beard was categorised as ‘Yes’or ‘No’. Moustache, goatee or beard stubbles were categorised as ‘No’.
Snoring. History of snoring was categorised as ‘Yes’or ‘No’.
History of sleep apnoea. History of obstructive sleep apnoea that requires continuous positive airway pressure (CPAP), bi-level positive airway pressure (BiPAP) or surgery. The risk factor was categorised as ‘Yes’or ‘No’.
Neck radiation changes. The risk factor was categorised as ‘Yes’or ‘No’.
Mouth opening. In patients with incisors the distance between the teeth was measured at maximum mouth opening. In edentulous patients the intergingivale distance was measured at maximum mouth opening. The distance was measured and recorded in centimetres. The distance was categorised as ‘< 4 cm’ or ‘≥ 4 cm’.
Thyromental distance. The distance was measured along a straight line from the “Prominentia Laryngea of ‘Cartilago Thyroidea’ to the notch of ‘Mentum Mandibulae’ with maximum head extension. The distance was measured and recorded in centimetres. The distance was categorised as ‘< 6 cm’ or ‘6.0-6.5 cm’ or ‘> 6.5 cm’.
Modified Mallampati classification. The visibility of the oropharyngeal structures are assessed on the patient sitting in neutral position with maximum mouth opening and tongue protrusion without phonation. The classification was categorised into Class I: Soft palate, fauces, uvula and faucial pillars visible; Class II: Soft palate, fauces and uvula visible; Class III: Soft palate and base of uvula visible; Class IV: Soft palate not visible.
Neck movement. The range of motion from full extension through full flexion was categorised as ‘< 80 °’, ‘80°- 90 °’ or ‘> 90 °’. The range was assessed by asking the patient to do a full extension of the neck. Then, the anaesthetist places, and fixates, a specially designed card in the patient’s temporal region in a way that the longer side of the card aligns a vertical line e.g. in a window frame. The position of the card in relation to the head is held fixed while the patient does a maximum neck flexion. Subsequently, the position of the longer side of the card is compared with a horizontal line in the room, for example the window frame.
Ability to extend lower jaw. The capacity to bring the lower incisors in front of the upper incisors was categorised as ‘Yes’or ‘No’. Edentulous patients are categorised as ‘Yes’.
Body mass index. Based on medical records or the patient’s own information, the weight (kilograms) and height (centimetres) was recorded. Body mass index was calculated as BMI = weight / (height x height). In the original cluster randomised trial, the anaesthesiologists were informed that a BMI above 30 kg/m^2 might be considered a risk factor. In present paper we categorised ‘BMI < 25’; ‘25 ≤ BMI < 35’; ‘35 ≤ BMI’.
History of difficult intubation, was categorised as ‘Definite’, ‘Questionable’ or ‘None’.
Age in years. In the original cluster randomised trial, the anaesthesiologists were informed that age above 56 yr. might be considered a risk factor. In present paper we categorised age as: ‘15 ≤ Age < 45’; ‘45 ≤ Age ≤ 60’; ‘60 ≤ Age ≤ 80’; ‘80 ≤ Age’.