Anaesthesia for the Bariatric Patient
MEASURING OBESITY
BMI has limitations and may not be representative in certain ethnic groups, or in those of athletic build. It cannot describe the distribution of weight, nor discriminate the nature of the excess tissue. However, calculation of BMI, from two ubiquitous measurements, requires the minimum equipment and expertise. Hence, it is likely to remain the measure of choice as a shorthand to express obesity (Table 25.1, Fig. 25.1).
TABLE 25.1
WHO Obesity Classes by BMI (Other Nomenclatures Included)
| Category | BMI (kg m–− 2) |
| Underweight | < 18.5 |
| Normal | 18.5–24.9 |
| Overweight (pre-obese) | 25–29.9 |
| Obese Class I | 30–34.9 |
| Obese Class II (severe to morbid) | 35–39.9 |
| Obese Class III (morbid to super) | 40 + |
| (Super obesity) | 45–50 + |
COMORBIDITY AND ANAESTHETIC MANAGEMENT
Airway
Anatomy: In the obese patient, the airway undergoes progressive adipose infiltration. This occurs at all levels from the oropharynx through to the glottis and vocal cords. Adipose infiltration causes progressive narrowing and reduction in airway diameter, which may reduce by 50% or more from the physiological male normal of about 20 mm in the hypopharynx.
Careful positioning is key to successful management of the bariatric airway. This can be achieved either using specifically designed equipment, or special modifications to normal equipment. In urgent situations a number of aids to achieving the position have been suggested including multiple towels, fluid bags or inflatables. The key is to ensure true neck flexion and AO extension. This is best achieved by ensuring that the patient posture, in particular neck/head position is viewed from the side (Fig. 25.2A and B).
FIGURE 25.2 (A) Lateral CT Scout (supine, female, BMI 59 kg m− 2). Note neck position, ‘free-floating’ head and the potential for neck, chest and breast adipose tissue to hinder airway management. (B) Sagittal cross-section (BMI 55 kg m− 2). Note depth of both anterior and posterior airway structures and the extent of airway soft tissue around the 7.5 tracheal tube.
Airway Adjuncts: A range of products to assist in the management of difficult airways exists. Simple adjuncts such as oral and nasopharyngeal airways, and the use of CPAP can help to splint the airway open. Laryngeal mask airway products retain their role in airway salvage. Their routine use in the morbidly obese patient remains controversial, focusing on concerns around pulmonary aspiration and optimization of pulmonary function.
Anatomy
Lung size is predicted by height or ideal body mass, rather than gross weight. The lung fields of obese patients often look small when assessed by chest radiography (Fig. 25.3). This is an artefact of accommodating the patient on the chest X-ray. Total lung capacity is usually nearly normal and it is functional spirometry which reveals the associated pathology (Fig. 25.4).
FIGURE 25.4 Spirometric changes with obesity. Note the significant reduction in FRC. TLC, total lung capacity; IC, inspiratory capacity; FRC, functional residual capacity.
Pathophysiology
Functional residual capacity decreases (Fig. 25.5) and closing volume increases. Resulting atelectatic shunt reduces PaO2. At higher levels of morbid obesity, tidal ventilation may impinge on closing volume even in the standing position.
FIGURE 25.5 Changes in functional residual capacity (FRC) with increasing body mass index (BMI). (Adapted from Pelosi et al (1998).)
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