CHAPTER 2 Respiratory system
Assessing The Airway
During routine anaesthesia, the incidence of difficult tracheal intubation (≥3 attempts at intubation or >10 min to accomplish it) has been estimated as 3–15%. Tracheal intubation is best achieved with the neck flexed and the atlantoaxial joint extended (‘sniffing the morning air’). Factors affecting this position may result in difficult intubation.
History and examination
Remember to check anaesthetic notes for previous difficulties and ask the patient if he or she is aware of any anaesthetic problems.
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.
Predictive tests
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):
Class I view is grade I intubation >99% of the time. Class IV view is grade III or IV intubation 100% of the time.
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:
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)
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.
Combined indicators
By combining prognostic indicators, a greater specificity for predicting difficult intubation may be achieved.
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
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.
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:
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.
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
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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.
Anaesthesia for chronic respiratory disease
Risk factors for postoperative pulmonary complications
American Society of Anesthesiologists (ASA) classification:
Anaesthetic techniques
Aims
Use either regional/local technique or maximal support approach with GA. Plan elective surgery for summer.
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.
Anaesthesia for asthmatics
Asthma affects 4–5% of the population. There are 1400 deaths p.a. in the UK. Prevalence and mortality are increasing. Avoid factors precipitating bronchospasm (differential diagnosis = aspiration, pulmonary oedema, endobronchial intubation, patient too light, mechanical obstruction of the tube).
Preoperative
Assess severity: severe if patient unable to speak sentences, pulse >120/min, pulsus paradoxus >20 mmHg, FEV1 <25%, ↑Paco2. Consider bronchodilators, hydration, antibiotics and steroids. Measure baseline arterial blood gases (ABGs). Percussive physiotherapy is contraindicated because it exhausts the patient further.
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