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:

• Thyromental distance >6.5 cm = easy intubation

• Thyromental distance <6.0 cm = difficult intubation.

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

Parameter	Risk level
Weight	0–2 (e.g. >90 kg = 1; >110 kg = 2)
Head and neck movement	0–2
Jaw movement	0–2
Receding mandible	0–2
Buck teeth	0–2
Maximum	10 points

Radiographic predictors of difficult intubation

These have the disadvantage of X-ray exposure and thus cannot be performed as routine tests.

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)

• Incisor tooth to posterior border of ramus

• Alveolar margin to lower border of the mandible

• Angle of the mandible.

White and Kander (1975)

• Increased length of mandible

• Increased depth of mandible

• Reduced distance between occiput and spinous process of C₁ or reduced distance between C₁–C₂ interspinous gap.

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.

Combined indicators

By combining prognostic indicators, a greater specificity for predicting difficult intubation may be achieved.

• Freck (1991) found that a thyromental distance of 7 cm in patients with Mallampati class III/IV predicts a grade IV intubation. The test has high sensitivity and specificity.

• Benumof (1991) suggested that a combination of relative tongue/pharyngeal size, atlantoaxial joint extension and anterior mandibular space provides a good predictor of difficult intubation and that the tests are quick and easy to perform.

• Best multifactorial index scores the following criteria to give sensitivity and specificity >90% in prediction of difficult intubation (Arné et al 1998):

– Previous history of difficult intubation

– Pathologies associated with difficult intubation

– Clinical symptoms of a pathological airway

– Inter-incisor gap

– Thyromental distance

– Head and neck movement

– Mallampati’s modified test.

Difficult Airway Society Guidelines for Management of the Unanticipated Difficult Intubation 2004

Problems with tracheal intubation are the most frequent cause of anaesthetic death in the analyses of records of the UK medical defence societies. The Difficult Airway Society (DAS) has developed guidelines for management of the unanticipated difficult intubation in an adult non-obstetric patient, see http://www.das.uk.com/home

Figure 2.4 Simple composite chart.

(Difficult Airway Society, 2004)

Figure 2.5 Unanticipated difficult tracheal intubation.

(Difficult Airway Society, 2004)

Figure 2.6 Failed intubation.

(Difficult Airway Society, 2004)

Figure 2.7 Management of unanticipated difficult tracheal intubation.

(Difficult Airway Society, 2004)

Bibliography

Arné J., Descoins P., Ingrand P., et al. Preoperative assessment for difficult intubation in general and ENT surgery: predictive value of a clinical multivariate risk index. Br J Anaesth. 1998;80:140-146.

Benumof J.L. Management of the difficult airway. Anesthesiology. 1991;75:1087-1110.

Cass N.M., James N.R., Lines V. Difficult laryngoscopy complicating intubation for anaesthesia. BMJ. 1956;1:488-489.

Charters P. What future is there for predicting difficult intubation? Br J Anaesth. 1996;77:309-311.

Chou H.C., Wu T.L. Mandibulohyoid distance in difficult laryngoscopy. Br J Anaesth. 1993;71:335-339.

Cormack R.S., Lehane. Difficult tracheal intubation in obstetrics. Anaesthesia. 1984;39:1105-1111. Difficult Airway Society, 2004, British Airway Society Guidelines Flow-chart 2004, http://www.das.uk.com/files/rsi-Jul04-A4.pdf.

Freck C.M. Predicting difficult intubation. Anaesthesia. 1991;46:1005-1008.

Henderson J.J., Popat M.T., Latto I.P., et al. Difficult Airway Society guidelines for management of the unanticipated difficult intubation. Anaesthesia. 2004;59:675-694.

Lavery G.G., McCloskey B.V. The difficult airway in adult critical care. Crit Care Med. 2008;36:2163a-2173a.

Lee A., Fan L.T.Y., Gin T., et al. A systematic review (meta-analysis) of the accuracy of the Mallampati tests to predict the difficult airway. Anesth Analg. 2006;102:1867-1878.

Mallampati S.R., Gatt S.P., Gugino L.D., et al. A clinical sign to predict difficult intubation: a prospective study. Can J Anaesth. 1985;32:429-434.

Nichol H.L., Zuck D. Difficult laryngoscopy – the ‘anterior’ larynx and the atlanto-occipital gap. Br J Anaesth. 1983;55:141-143.

Patil V.U., Stehling L.C., Zaunder H.L. Fibreoptic endoscopy in anaesthesia. Chicago: Year Book Medical Publishers, 1983.

Popat M. The airway. Anaesthesia. 2003;58:1166-1170.

Savva D. Prediction of difficult tracheal intubation. Br J Anaesth. 1994;73:149-153.

Vaughan R.S. Predicting difficult airways. BJA CEPD Reviews. 2001;1:44-47.

White A., Kander P.L. Anatomical factors in difficult direct laryngoscopy. Br J Anaesth. 1975;47:468-474.

Wilson M.E. Predicting difficult intubation. Br J Anaesth. 1993;71:333-334.

Anaesthesia And Respiratory Disease

Smoking

Postoperative pulmonary complications occur in 22% of current smokers but only 5% of those who have never smoked.

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:

• current smokers – 33% incidence

• ex-smokers <8 weeks – 57% incidence

• ex-smokers >8 weeks – 15% incidence.

Box 2.1 Effects of cessation of smoking

12–24 h	Clearance of carbon monoxide
2–10 days	Decreased upper airway reactivity
1–2 months	Increase in postoperative pulmonary complications (Bluman et al 1998)
6 months	Decrease in postoperative pulmonary complications
Years	Reduced risk of COPD, ischaemic heart disease, lung cancer and cerebrovascular disease

Chronic bronchitis

Chronic bronchitis is defined as productive cough >3 months of the year for ≥2 years.

Postoperative pneumonia

Predisposed by:

• decreased ciliary activity

• sputum retention

• poor cough

• lack of humidification.

Viral upper respiratory tract infection (URTI)

Common in paediatric ENT patients. Causes worsening of asthma and ↑ bronchial reactivity for up to 6 weeks following resolution of symptoms. May be due to viral-mediated damage to vagus releasing acetylcholine and potentiating bronchoconstriction.

In children, there is a 2–7-fold increase in respiratory related adverse events, if intubated, an 11-fold increase in adverse events, and an increased risk of transient hypoxaemia in the postoperative period. In adults with URTI, upper airway reactivity is also increased by an amount related to severity of symptoms.

Therefore, postpone routine surgery for at least 2–3 weeks. If anaesthesia is given during this period, consider topical anaesthesia to larynx to reduce vagally mediated reflexes and consider the use of atropine to block hyperreactivity. Monitor O₂ saturation postoperatively.

Effects of general anaesthesia

• Central respiratory depression

• Reduced compliance

• Cranial shift of diaphragm, atelectasis, reduced tone of chest wall and reduced thoracic blood volume cause ↓ FRC (17%, 500 mL)

• FRC approaches and may exceed closing capacity (CC)

• Impaired O₂ and CO₂ exchange

• Increased spread of V/

ratios. Increased shunting (shunt fraction during GA: 11% during spontaneous breathing, 14% during IPPV)

• Hypoxic pulmonary vasoconstriction inhibited by up to 25% with volatiles but not by i.v. agents.

All these factors result in an increased A–aO₂ (difference in alveolar and arterial oxygen tensions), which persists for at least 1–2 h postoperatively. Diaphragm may recover from neuromuscular blockade prior to muscles involved in coughing and swallowing. Postoperative wound pain, abdominal distension, pulmonary venous congestion and a supine posture all increase CC–FRC and result in further alveolar collapse.