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Review

Airway management and morbid obesity

Kristensen, Michael S

Author Information
European Journal of Anaesthesiology: November 2010 - Volume 27 - Issue 11 - p 923-927
doi: 10.1097/EJA.0b013e32833d91aa

Abstract

Introduction

Morbidly obese patients present with excess fatty tissue externally on the breast, neck, thoracic wall and abdomen and internally in the mouth, pharynx and abdomen. During intubation and tracheostomy, this excess tissue can impede access to and patency of the upper airway. During ventilation, decreased residual capacity and worsening of ventilation perfusion mismatch1 impair lung function when compared with patients with normal BMI. In recent large studies of mixed patient populations, high BMI was found to be a weak but statistically significant predictor of difficult or failed tracheal intubation2 and of difficult but not failed mask ventilation.3,4 However, those predicted to have a really difficult airway and those scheduled for elective awake fiberoptic airway management were excluded from these studies, leaving the true incidence of difficult or failed tracheal intubation or mask ventilation unknown.

Airway management in morbid obesity is not necessarily difficult, especially when surgery is elective, the patients are healthy in other respects and anaesthesia is performed or supervised by anaesthetists with experience with this category of patients.5 In contrast, when the morbidly obese patients present to emergency treatment or have severe co-existing medical conditions, airway management can be challenging. When time for preparation and positioning is sparse, for example in the emergency department or during cardiac arrest, adverse outcomes have been reported.6 Anaesthesiologists who are less frequently exposed to the morbidly obese patients and those who have to treat morbidly obese patients with co-morbidity, perhaps in emergency settings, have much to learn from the experts' experience from elective settings. The goal of this review is to consider the lessons that might be of value to anaesthesiologists who occasionally have to tackle morbidly obese patients (Table 1).

Table 1
Table 1:
Definitions

Preanaesthetic evaluation

Weight or BMI is just one of several factors to consider during a thorough airway evaluation.7 A useful addition to normal practice is measuring neck circumference at the level of the thyroid cartilage. A circumference greater than 43 cm is associated with an increased risk of difficult intubation.8 The decision as to whether to perform awake intubation, rapid sequence induction or conventional induction and by which means to secure the airway should depend on evaluation of the whole patient9 and not just on the presence of morbid obesity. When other predictors of intubation difficulties such as Mallampati classification, thyromental distance and range of movement in the neck for example, are normal, despite high BMI, then the risk of difficult laryngoscopy is low.7

Preinduction

Positioning the patient

Placing multiple folded blankets under the upper body, head and neck until the external auditory meatus and the sternal notch are horizontally aligned (Fig. 1) is called ‘ramping’. The ‘ramped’ position offers improved intubation conditions in the morbidly obese patients compared with the ‘sniff’ position (Fig. 2).10 In 180 prospective consecutive patients with a mean BMI of 49 kg m−2 presenting for bariatric surgery, only six (3.3%) had a difficult intubation, defined as three or more attempts, and 8.3% had a difficult laryngoscopy, defined as a Cormack and Lehane Grade 3 or 4 view,5 but all were successfully intubated by anaesthesiology residents using a conventional approach. The authors attribute this low rate of difficulty to systematic use of the ‘ramped’ position.11 A linear relationship appears to exist between BMI and the benefits of the ‘ramped’ position. When using patients as their own controls and comparing laryngoscopy in both ‘ramped’ and ‘sniffing’ position, the former provides improved or unchanged views of the larynx in 88% of patients with BMI less than 25 kg m−2, in 91% in patients with BMI higher than 30 kg m−2 and in 100% of patients with BMI higher than 45 kg m−2.12 The use of a premanufactured elevation pillow,13 or an inflatable pillow,14 instead of blankets can improve both view of the larynx and mask ventilation. During surgery, the inflatable pillow can be deflated, bringing the patient back to the supine position. If the operating room table allows it, the shape of the table can be configured, so that it is flexed at the thigh and raised at the trunk, providing some of the benefits otherwise obtained by ‘ramping’.15

Fig. 1
Fig. 1
Fig. 2
Fig. 2

Preoxygenation

In morbidly obese patients, oxygen saturation following preoxygenation falls more rapidly during apnoea than in those with normal BMI. This can be partially prevented by keeping the patient in a 25° head-up position during preoxygenation.16 When such a position was used during preoxygenation, the time from induction to the SaO2 reading of 92% was 201 s, as opposed to 155 s in the supine group. Another way to delay apnoeic desaturation is to passively insufflate oxygen at 5 l min−1 during laryngoscopy via a 10 Fr catheter in the nasopharynx.17 The combination of preoxygenation in the 25° head-up position and nasopharyngeal oxygen insufflation allowed 16/17 patients to maintain SaO2 at 100% for 4 min, whereas those without nasal oxygen had a fall in SaO2 to 95% after a mean time of 145 s. Positive end-expiratory pressure (PEEP) of 10 cmH2O applied during preoxygenation and induction can also increase the duration of nonhypoxic apnoea in morbid obesity18 and noninvasive bilevel positive airway pressure has given promising results.19

Preoxygenation increases the rate of atelectasis formation,20 but this can be countered by a recruitment manoeuvre that involves keeping the inspiratory pressure at 55 cmH2O for 10 s,20 immediately followed by PEEP of 10 cmH2O. After the airway is secured, the inspired oxygen fraction should be reduced because there is no benefit to be derived from a high perioperative fraction,21 and maintaining an inspired oxygen fraction of 0.4 instead of 0.8 during anaesthesia resulted in better lung function for 24 h after extubation in moderately obese patients.22 Despite all caveats, preoxygenation remains mandatory in the morbidly obese patients.

Inducing anaesthesia and securing the airway

In a series of 150 consecutive morbidly and super obese patients, awake fiberoptic intubation was used in only 6–7% of patients considered to be at high risk of difficult tracheal intubation. The remaining 93–94% of patients were induced with rapid sequence induction (RSI) with propofol 2–3 mg kg−1 ideal body weight (IBW) or with thiopentone 3–5 mg kg−1 total body weight (TBW), followed by succinylcholine 1.5 mg kg−1 TBW. Intubation in the ‘ramped’ position was straightforward.23 A comprehensive review24 concluded that RSI remains important in the morbidly obese patients with gastrooesophageal reflux, diabetes mellitus, pregnancy, gastrointestinal disorders and before emergency surgery, all of which predispose to pulmonary aspiration. However, the application of RSI is probably not necessary in fasted patients with no risk factors other then obesity.

If it is decided to anaesthetise the patient before securing the airway, it is important to ensure sufficient depth, because instrumentation of the pharynx in the presence of light anaesthesia, with or without prolonged attempts at intubation,24 is an important cause of aspiration.25

If facemask ventilation is difficult, a supraglottic airway should be available. The LMA Supreme performs satisfactory in those morbidly obese patients with factors predictive of difficult mask ventilation.26 The laryngeal mask airway ProSeal was successfully used for ventilation before intubation in 60 obese patients, providing an effective airway in all patients within one or two attempts, after a mean duration of 15 s. Oropharyngeal leak pressure was 32 (12–40) cmH2O.27 The intubating laryngeal mask airway, the LMA Fastrach, is more efficient in the morbidly obese patients than in normal patients, with fewer airway adjustments needed for establishing ventilation.28 It is well suited as a rescue device for both failed ventilation and failed intubation.

Alternatives to the Macintosh laryngoscope for intubation

Laryngoscopy may prove difficult in morbid obesity because thoracic fat pads and large breasts can interfere with a standard laryngoscope handle. This can be overcome by using a short handled laryngoscope,29 but use of the ‘ramped’ position improves access and will reduce the need for it.

Videolaryngoscopic guided intubation with the Glidescope, Storz V-Mac or McGrath systems has a high success rate in the morbidly obese patients with a difficult airway30 and can be performed without excessive force on the maxillary incisors.31 Both the LMA Ctrach and the Airtraq perform better than a standard Macintosh laryngoscope in these patients.32 A video-assisted Macintosh laryngoscope, however, was successful at the first attempt in an obese patient after three failed attempts at conventional intubation.31 Videolaryngoscopes generally perform well in this patient category, suggesting that one of these devices should ideally be available when intubating anaesthetised morbidly obese patient.

Dosage of induction agents and muscle relaxants

Compared with normal BMI, the distribution of drugs, their protein binding and elimination are different in obese patients.33 A dosage regimen of intravenous anaesthetics based on TBW may be too large and result in circulatory depression.34 The net pharmacokinetic effect in any specific individual is uncertain, and often an appropriate dosage of anaesthetic drugs will be somewhere between IBW and TBW. Good practice will involve careful titration using end-points such as heart rate, arterial blood pressure and depth of sedation or anaesthesia to guide dose,35 a lesson that is more important than learning empirical drug regimens based on published data.34 Most studies are limited to maintenance dosage and the few that investigate induction agents generally involve the concurrent administration of several drugs, making any recommendation for a single drug difficult. A high level of clinical judgement and vigilance is required.

Propofol

In a blinded pilot study an induction dose of 200 mg of propofol infused over 60 s was insufficient in 60% of morbidly obese patients, and it was concluded that 2.5 mg kg−1 TBW propofol with a dose cap of 350 mg, in conjunction with fentanyl 250 μg, was an appropriate induction dose.36 To avoid undesirable cardiovascular effects from this dose, careful monitoring is needed. For target-controlled systems, it is recommended that infusion is also based on the morbidly obese TBW without adjustment and that an EEG-based device should be used to monitor depth of anaesthesia during the predicted plasma propofol concentrations.37

Succinylcholine

A dose of 1 mg kg−1 TBW of succinylcholine is needed to ensure good or excellent intubation conditions. Lower doses are associated with poorer intubation conditions but without sufficient recovery of spontaneous ventilation, should ventilation or intubation fail.38 Regarding other drugs relevant to anaesthesia induction and their doses, readers are referred to the thorough review by Ogunnaike et al.39

Maintenance

Atelectasis increases from 1 to 11% of total lung volume in the morbidly obese patients, following induction of anaesthesia, and end-expiratory lung volume decreases to half the preinduction value. A useful recruitment manoeuvre to counteract these effects is to keep the inspiratory pressure at 55 cmH2O, followed by PEEP of 10 cmH2O.20 Neither PEEP alone nor recruitment alone had a lasting beneficial effect on the atelectasis. Pressure-controlled ventilation improves oxygenation compared with volume-control during obesity surgery.40 An infusion of dexmedetomidine during desflurane anaesthesia decreases fentanyl use, the need for antiemetic therapy and the postanaesthesia care unit stay, but does not improve the overall quality of recovery.41

Emergence and extubation

Morbidly obese patients are at higher risk for respiratory obstruction soon after extubation.42 It is essential that they are fully awake, responding to commands and performing purposeful movements, before extubation. Reflex movement such as reaching for an endotracheal tube, should not be mistaken as a purposeful movement. Extubation over an airway exchange catheter should be strongly considered if a possible re-intubation is likely to be difficult.42 It should follow a planned strategy and logically should be performed in the same ‘ramped’ position used for intubation. This is not easy to do in an anaesthetised patient without the inflatable pillow,14 or an operating table that can be suitably configured. Increasing the inspired oxygen fraction prior to extubation increases the time before saturation falls to a critical level, but 100% oxygen should probably be reserved for those with additional risk factors for a difficult airway. Evidence from the moderately obese patients indicates that 80% represents a practical standard, as this lower oxygen fraction results in less impact on postextubation pulmonary impairment.22

Postanaesthesia

Morbidly obese patients experience frequent periods of oxygen desaturation during the 24 h following surgery despite supplemental oxygen 3 l min−1 by nasal cannula.43 The recovery of lung function in the postanaesthesia care unit after minor surgery can be aided by short-term physiotherapy and incentive spirometry. This recovery lasts for at least 24 h.44 A simple CPAP system can also improve oxygenation postoperatively,45 as can noninvasive ventilation.46 Morbidly obese patients are at risk of respiratory arrest after narcotic or sedative medication42 and the use of intravenous keterolac after laparoscopic gastric bypass surgery resulted in earlier discharge from the postanaesthesia care unit and a better outcome.47 These findings encourage the use of nonopioid analgesia whenever possible, and opioids can be supplemented when necessary to maintain effective pain control.

Tracheostomy and percutaneous dilatation tracheostomy

Tracheostomy has an increased risk of complication in morbidly obese patients48. A videolaryngoscope can be beneficial in diagnosing and correcting puncture of the tracheal tube cuff49 and ultrasonography is useful in guiding percutaneous dilatation tracheostomy.50

Emergency settings

In a retrospective review of problems in the emergency setting, BMI was found to be a risk factor for early pulmonary, but not airway, complications. In obese and morbidly obese patients, the success rate for intubation in the field was 70–80%, no different from that found in normal patients.51 In contrast, the 20% of severely obese patients requiring prehospital tracheal intubation were difficult to manage52 and obesity was a risk factor for difficult intubation in this setting.53 One example of very complex airway management in this patient category was a 22-year-old, 445 kg hypercarbic man with a BMI of 163 kg m−2 who had unsuccessful attempts at awake fiberoptic intubation via the mouth and the nose. Eventually a #5 LMA ProSeal was inserted with the patient awake, and following anaesthesia, an Aintree catheter was placed over a flexible fiberoptic scope. Following removal of the LMA and the fiberscope, a Parker tube was railroaded over the Aintree catheter into the trachea.54 Other success stories include the tracheal-oesophageal Combitube which was used in a cannot ventilate-cannot-intubate situation,55 the Bullard laryngoscope in an easy awake intubation in a 240 kg patient with difficult conventional laryngoscopy56 and the Levitan optical stylet was used in a patient with respiratory failure, a BMI of 57 kg m−2 and a Cormack Lehane grade 3 view.6

Conclusion

The airway management plan, including a possible decision on awake intubation, should be based on a thorough airway examination and not on solely whether morbid obesity is present or not. Preoxygenation is mandatory and is more efficient in a 25° head-up position, by applying PEEP and by insufflating oxygen via the nasopharynx during intubation attempts. It should be followed by measures designed to limit atelectasis. During manipulation of the airway, the patient should be placed in a ‘ramped’ position with the external auditory meatus and the sternal notch aligned horizontally. An intubating laryngeal mask airway, the LMA Fastrach, is especially suited as a rescue device if ventilation or intubation becomes difficult. Extubation should be according to a plan, and performed in a position that will allow re-intubation.

Acknowledgements

The work was not supported financially and the author has no conflicts of interest.

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Keywords:

airway management; difficult airway; morbid obesity; obesity; preoxyenation; ramping

© 2010 European Society of Anaesthesiology