Anaesthesia for the Bariatric Patient


Anaesthesia for the Bariatric Patient




MEASURING OBESITY


A patient’s mass varies with size and shape. Absolute mass can be important when considering factors such as equipment safety limits. However, it is more important to reference a person’s expected mass to height. The most widely used technique is calculation of Body Mass Index (BMI).


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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).




The limitations of BMI may account for some of the variability in obesity research. Other difficult measures such as hip to abdominal girth ratio and skin fold thickness may be better at describing more dangerous patterns of fat distribution. Central obesity (‘apple-shaped’, predominantly male) carries more associated risks than peripheral obesity (gluteofemoral or ‘pear-shaped’) often seen in females.



OBESITY PATHOPHYSIOLOGY, COMORBIDITY AND THE METABOLIC SYNDROME


Obesity is a multi-system disorder. The aetiology is complex and has long been oversimplified. The network of contributing factors includes socio-economic, ethnic, societal, social and psychological. The underlying and acquired pathophysiology cross the traditional boundaries of medicine to include endocrine, cardiovascular, respiratory, GI tract, locomotor and psychiatric disorders.




The Adipose Organ


The traditional view of fat tissue has been as a metabolically inert triglyceride energy store, with insulation from physical and temperature stress as a useful secondary effect. However, fat tissue actually exists as a continuum. In particular, hepatic and intra-abdominal visceral fat tissue is much more active than previously thought. It is known to excrete over 20 mediators. The observed effects are pro-inflammatory (cytokines, adipsin), pro-coagulant (plasminogen activator inhibitor 1) and endocrine (leptin, resistin, adiponectin).


The underlying biochemical state is probably responsible for the common patterns of accelerated comorbidity observed in morbid obesity. The ‘metabolic syndrome’ is the name applied to the pattern of central obesity associated with at least two of the following:




COMORBIDITY AND ANAESTHETIC MANAGEMENT




Airway


The airway of the obese patient should be regarded with caution. Traditional teaching and audits of national practice predict that the management of the obese patient’s airway is likely to be difficult. However, a number of studies have demonstrated that, in experienced hands, certain aspects of airway management, in particular tracheal intubation, may be no more difficult than normal.



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.


The effect of adipose deposition on airway anatomy is not simply internal. External factors also need to be considered. The presence of a thoracic ‘hump’ can significantly affect supine posture, resulting in extension of the neck and flexion at the atlanto-occipital (AO) joint. Moreover, posterior adipose deposition between the occiput running inferior to the spine of T1, can hinder atlanto-occipital extension. Hence, in an unsupported supine position, the airway of the obese patient can easily become the opposite of ideal; neck extension and fixed AO flexion.


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).





Respiratory Pathophysiology



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).




In obesity, there is heavy adipose infiltration of the chest wall and breast tissue, which leads to decreased chest wall compliance and damping of natural recoil expansion of the chest wall. This is further exacerbated by abdominal wall infiltration and a raised intra- abdominal pressure. Additionally, there is peribronchial parenchymal fat infiltration. Respiratory muscles demonstrate fat infiltration and the effect of inflammatory mediators. Diminished muscle power and respiratory endurance result.



Pathophysiology


Total lung capacity and vital capacity reduce in a gentle, linear manner with rising weight. The spirometric observations reflect the change in the balance between chest wall and parenchymal forces with rising obesity.



image 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.


May 31, 2016 | Posted by in ANESTHESIA | Comments Off on Anaesthesia for the Bariatric Patient
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