Visual inspection

The following features suggest a difficult intubation: obesity, pregnancy, large breasts, short muscular neck, full dentition, limited neck flexion or head extension, receding jaw, prominent upper incisors, limited mouth opening, high arched palate.

Cormack and Lehane grading (1984)

Used to grade the view at laryngoscopy (Fig. 2.1):

Figure 2.1 Cormack & Lehane grading.

(Adapted from Cormack & Lehane 1984.)

Recent reclassification has subdivided grade II view further:

Mallampati classification (Mallampati et al 1985)

The patient sits upright with the head in the neutral position and the mouth open as wide as possible, with the tongue extended to maximum. The following structures are visible (Fig. 2.2):

Figure 2.2 Mallampati classification.

(Reproduced from Mallampati et al 1985.)

This classification may fail to predict >50% of difficult intubations.

Thyromental distance (Patil et al 1983)

Measure from the upper edge of the thyroid cartilage to tip of the jaw with the head fully extended (Fig. 2.3). A short thyromental distance equates with an anterior larynx which is at a more acute angle and also results in less space for the tongue to be compressed into by the laryngoscope blade. This is a relatively unreliable test unless combined with other tests:

Figure 2.3 (A) Thyromental and (B) sternomental distance.

Sternomental distance (Savva 1994)

Measure from the sternum to the tip of the mandible with the head extended (Fig. 2.3). A sternomental distance of ≤12.5 cm predicts difficult intubation.

Horizontal length of mandible

Horizontal mandibular length >9 cm is suggestive of a good laryngoscopic view.

Cervical spine movements

The effect of mobility of the atlanto-occipital and atlantoaxial joints on ease of intubation is probably underestimated. It may be best assessed by asking patients to extend their head while their neck is in full flexion. Extension of the head with atlantoaxial joint immobility results in greater cervical spine convexity which pushes the larynx anteriorly and impairs the laryngoscopic view.

Prayer sign

The inability to place both palms flat together suggests difficult intubation. It is probably a reflection of generalized joint and cartilage immobility limiting atlantoaxial and cervical extension. It may be particularly common in diabetics.

Wilson risk score (Wilson 1993; see Table 2.1)

A total of 3 predicts 75% of difficult intubations, while a total of 4 predicts 90%. However, the test has a poor specificity and may fail to predict >50% of difficult intubations. Many of the measurements are subjective.

Table 2.1 Wilson risk score

Mandibulohyoid distance (Chou and Wu 1993)

This is the vertical distance between the anterior edge of the hyoid and the mandible vertically above measured by lateral X-ray. A distance >6 cm is suggestive of difficult intubation. A longer distance results in more of the tongue being present in the hypopharynx, which must be displaced to view the vocal cords.

Cass et al (1956) (absolute measurements not given)

White and Kander (1975)

Nichol and Zuck (1983)

These authors stressed the importance of a reduced atlanto-occipital distance as a predictor of ability to extend the head during laryngoscopy.

Effects of smoking

Preoperative cessation of smoking (Box 2.1)

A study in patients undergoing CABG (Warner 2006) showed that in those stopping smoking, the incidence of postoperative pulmonary complications did not fall until at least 8 weeks had elapsed:

Risk factors for postoperative pulmonary complications

American Society of Anesthesiologists (ASA) classification:

Respiratory function tests predicting increased postoperative morbidity with chronic respiratory disease

• <50% = high risk (obstructive pattern)

(FEV₁ = forced expiratory volume in 1 s, FVC = forced vital capacity)

• <50% = high risk (restrictive pattern)

(RV = residual volume, TLC = total lung capacity)

FEV₁ <1000 mL or <50% predicted

FVC <1500 mL or <50% predicted

Aims

Regional technique

Requires the patient to be able to lie flat for the duration of surgery and not cough. Avoid sedation which may precipitate respiratory failure. Can be combined with a light GA and spontaneous ventilation, but reactive airways are irritated by an endotracheal tube which may cause severe bronchospasm. Avoided by using local/regional technique.

General anaesthetic

The technique of choice for patients with respiratory failure or upper abdominal/thoracic surgery. Light/no premedication. Use a heat and moisture exchanger or humidifier. Use drugs allowing rapid recovery. IPPV allows control of O₂ and CO₂ and enables airway suctioning and may need to be continued postoperatively. Use slow inspiratory flow to allow equilibration of fast and slow alveoli. A long expiratory time reduces air trapping. Avoid PEEP in patients with COPD. If there are high-frequency bullae, avoid N₂O and consider double-lumen tube. Spontaneous respiration or jet ventilation (HFJV) may be appropriate with bullae.

At the end of surgery, ensure bowel is decompressed and drain any peritoneal air. Sit patient up postoperatively, which increases FRC. Epidurals also increase FRC (bupivacaine > morphine). Prior to extubation, ensure cardiovascular stability and fluid balance and ensure no residual effects of anaesthetic or neuromuscular blockade.

Preoperative

Assess severity: severe if patient unable to speak sentences, pulse >120/min, pulsus paradoxus >20 mmHg, FEV₁ <25%, ↑P_aco₂. Consider bronchodilators, hydration, antibiotics and steroids. Measure baseline arterial blood gases (ABGs). Percussive physiotherapy is contraindicated because it exhausts the patient further.