Abstract
Obesity is a risk factor for increased difficulty in most modalities of airway management. It decreases ease and effectiveness of face mask ventilation, supraglottic airway device use and front of neck airway techniques and probably makes laryngoscopy more difficult. When difficulty occurs, airway rescue techniques are more likely to fail in the obese patient. Obesity also increases the risk of aspiration and difficulty in lung ventilation, both of which may necessitate changes in anaesthetic technique. Most importantly, obesity reduces the time available for airway management before hypoxia supervenes. To worsen matters, obesity reduces the efficacy of pre-oxygenation and safe apnoea time is less prolonged with apnoeic oxygenation techniques than in the non-obese population. To compound these factors obesity is associated with obesity-specific (e.g. obstructive sleep apnoea, obesity hypoventilation syndrome) and non-specific co-morbidities (diabetes, asthma, hypertension). With increasing numbers of obese patients and increasing degrees of obesity in the surgical population it is essential that all anaesthetists are familiar with the potential complications of airway management in the obese and the techniques that may mitigate or manage risk.
Introduction
The World Health Organization (WHO) defines obesity as an abnormal/excessive accumulation of fat presenting a risk to health. Obesity is categorised according to body mass index (BMI), i.e. weight (kg) divided by the square of height (m) (kg m−2), and has been divided into three classes.
| Class 1 | BMI 30 to < 35 kg m−2 |
| Class 2 | BMI 35 to < 40 kg m−2 |
| Class 3 | BMI ≥ 40 kg m−2 |
| Class 3 is also termed ‘morbid’, ‘extreme’ or ‘severe’ obesity. | |
Anaesthetists are encountering increasing numbers of obese patients and airway management is a major concern in these patients. This chapter provides key points on safe airway management of this clinically demanding group.
Pulmonary Pathophysiology
Functional residual capacity (FRC) decreases as BMI increases, mostly due to a fall in expiratory reserve volume (ERV). Breathing at low lung volume promotes airway closure in dependent lung zones, causing decreased ventilation in the lung bases (atelectasis) with hypoventilation and ventilation–perfusion mismatch (shunt). The diminished lung volume also reduces airway calibre. Obesity is a pro-inflammatory condition causing airway hyper-reactivity. Obese patients have a high resting metabolic rate with increased oxygen demand. Oxygen consumption is increased because of the increased work of breathing. All these factors increase risk of hypoxaemia.
Preoperative Airway Assessment
Sleep-Disordered Breathing
Sleep-disordered breathing, typically obstructive sleep apnoea (OSA), occurs in 10–20% of Class 2 and Class 3 patients, and is undiagnosed in the majority of patients. OSA is associated with difficult mask ventilation, difficult direct laryngoscopy and upper airway obstruction following minimal sedation. Patients with OSA are prone to very rapid arterial oxygen desaturation during and immediately after induction of general anaesthesia and in the post-operative period. Untreated OSA may progress to obesity hypoventilation syndrome (OHS), a triad of severe obesity, daytime hypoventilation with hypercapnia, and sleep-disordered breathing. Chronic hypoxaemia and hypercapnia cause increased sensitivity to the effects of anaesthetic agents and opioids, which can progress to hypoventilation and respiratory arrest in the early post-operative period.
The American Society of Anesthesiologists and the Society of Anesthesia and Sleep Medicine recommend preoperative screening of surgical patients for OSA, and treatment with continuous positive airway pressure (CPAP) during the perioperative period when significant OSA is present. Overnight polysomnography is necessary to confirm an OSA diagnosis but is expensive and often impractical. The STOP-Bang Questionnaire (Table 24.1) is a useful OSA screening tool. Pulse oximetry readings of < 95% in room air, expiratory reserve volume < 0.5 L and a serum bicarbonate concentration > 28 mmol L−1 each suggest OSA.
Table 24.1 The STOP-BANG screening questionnaire for obstructive sleep apnoea. One point is scored for each positive feature; a score ≥ 5 is a significant risk.
OSA risk is associated with body shape, rather than absolute BMI. Men usually exhibit central or visceral obesity (‘apple shape’) with abdominal, neck and airway adipose distribution, while women have a predominantly peripheral fat distribution (‘pear shape’) and their airways are less commonly affected. Central (male-type) obesity is significantly associated with severity of OSA.
Airway Risk Assessment
Difficult face mask ventilation is common in obese patients. Independent predictors for difficult or impossible mask ventilation are described in the mnemonic OBESE (Table 24.2). Additional factors include modified Mallampati class III–IV and neck circumference > 50 cm. Mask ventilation in the obese patient often requires two operators, one to hold the face mask and another to squeeze the reservoir bag.
Table 24.2 Five independent risk factors (OBESE) for difficult mask ventilation
| OBESE | |
|---|---|
| O | Obese |
| B | Beard |
| E | Edentulous |
| S | Snoring (OSA) |
| E | Elderly (> 55 years) |
A meta-analysis reported a threefold higher incidence of difficult intubation in obese patients. The Intubation Difficulty Score (IDS) (Table 24.3) can be used to measure airway difficulty. An IDS > 5 was reported in 15.5% of obese patients compared with 2.3% of non-obese patients. Nevertheless, the relationship between obesity and difficult intubation is more complex as 378 of the 379 obese and morbidly obese patients in the four cited studies in that review were successfully intubated with direct laryngoscopy. Direct laryngoscopy is successful in most morbidly obese patients, but risk factors for increased difficulty include male sex, Mallampati class III/IV and neck circumference > 60 cm.
Table 24.3 Intubation Difficulty Score. IDS > 5 = moderate-major difficulty
| Variable | Score | Rules |
|---|---|---|
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Preparation for and Induction of General Anaesthesia
Position
Pulmonary mechanics are markedly altered in supine patients because the increased intra-abdominal pressure causes diaphragmatic impedance, which reduces FRC and total lung capacity. These effects impair the capacity of obese patients to tolerate apnoeic episodes. Their safe apnoea time, i.e. the time between apnoea onset and desaturation (SpO2 ≤ 90%), is very short. Positioning the operating room table in a 30° reverse Trendelenburg position increases safe apnoea time.
Direct laryngoscopy is conventionally performed on a supine patient in the ‘sniffing’ (flextension) position with a single pillow under the head. The laryngeal view is significantly improved in obese patients when their head, shoulders and upper body are ‘ramped’, such that their ear level and sternal notch are aligned. This head-elevated laryngoscopy position (HELP) can be achieved by placing multiple folded blankets, pre-manufactured foam pillows or inflatable pillows under the upper body, head and neck.
The combination of the HELP and reverse Trendelenburg position decreases dependent atelectasis by reducing mass-loading on the chest, increases the safe apnoea time and improves the laryngoscopic view by aligning the oral, pharyngeal and laryngeal axes (Figure 24.1) (see also Chapter 14).
