What is the definition of obesity?

The World Health Organization classifies obesity based in the patient’s body mass index (BMI); their weight (in kilograms) divided by the square of their height (in metres):

What are the physiological consequences of obesity of relevance to anaesthetists?

There is increasing evidence that obesity stimulates the innate immune system to increase production of the proinflammatory mediators tumour necrosis factor-α (TNF-α) and interleukin-6 (IL-6). The resulting state of low-grade inflammation is thought to be at least partially responsible for many of the pathophysiological changes associated with obesity.

System by system, the key features are:

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  Respiratory:

  
  
  
  
  1. 
     

     – Airway: increased prevalence of upper airway collapse (see Chapter 6) and difficult laryngoscopy. Correct patient head positioning before induction is crucial: a head-elevating laryngoscopy pillow may be required.

     
     
  2. 
     

     – Lung volume: reduced functional residual capacity (FRC) due to weight of chest wall; closing capacity may exceed FRC, causing atelectasis and ventilation–perfusion (V̇/Q̇) mismatch; decreased thoracic compliance, leading to an increased work of breathing (see Chapter 21).

     
     
  3. 
     

     – Gas exchange: increased O₂ consumption and CO₂ production due to the increased tissue mass – minute ventilation, V̇_E, must increase to maintain normocapnoea.

     
     
  4. 
     

     – Obstructive sleep apnoea (OSA): obesity increases the risk of OSA, which manifests as repeated cycles of overnight hypoxaemia and hypercapnoea (see Chapter 6).

     
     
  5. 
     

     – Asthma: obese patients are twice as likely to develop asthma as the non-obese population. The cause is unclear, but may be related to the chronic inflammation associated with obesity or decreased airway calibre as a result of reduced lung volumes.

  
  

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  Overall: obese patients have the potential for rapid desaturation at induction of anaesthesia due to increased O₂ consumption combined with decreased FRC. Pre-oxygenation prior to induction of anaesthesia is of great importance and is perhaps best achieved using high-flow nasal oxygen therapy (see Chapter 6).

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  Cardiovascular system changes occur as a result of adaptation to excess body mass and increased metabolic demands:

  
  
  
  
  1. 
     

     – Left-sided circulation: excess body mass requires an increase in intravascular volume and cardiac output (mainly due to an increase in stroke volume, SV). Systemic hypertension is common in obesity and, together with the increased SV, results in left ventricular hypertrophy and an increase in left ventricular work.

     
     
  2. 
     

     – Coronary disease: increased incidence of ischaemic heart disease due to systemic hypertension, dyslipidaemia and diabetes mellitus. It is important to note that symptoms of significant coronary artery stenosis may be masked by physical inactivity.

     
     
  3. 
     

     – Right-sided circulation: the changes in the respiratory system outlined above stimulate hypoxic pulmonary vasoconstriction, which leads to pulmonary hypertension and increased right ventricular work. This can lead to right ventricular hypertrophy and failure (cor pulmonale).

     
     
  4. 
     

     – Venous system: increased risk of venous thromboembolism. Intermittent calf compressors should be used intra- and post-operatively until the patient is adequately mobile.

  
  
  
  
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  Gastrointestinal:

  
  
  
  
  1. 
     

     – Gastroesophageal reflux is common due to lower oesophageal incompetence and increased intragastric pressure. General anaesthesia is associated with an increased aspiration risk.

     
     
  2. 
     

     – Hepatobiliary: increased prevalence of gallstone disease and non-alcoholic steatohepatitis.

  
  
  
  
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  Endocrine: the state of low-grade chronic inflammation promotes insulin resistance and the development of type 2 diabetes mellitus and hyperlipidaemia.

  
  
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  Musculoskeletal: there is an increased prevalence of osteoarthritis (OA) and gout in the obese. As one would expect, there is a definite association between OA in weight-bearing joints and obesity. Interestingly, there is also an increased prevalence of OA in non-weight-bearing joints (e.g. those of the hand) in obese individuals, suggesting that the mechanism for the development of OA is not purely biomechanical, and possibly the low-grade state of inflammation plays a role.

  
  
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  Morphological:

  
  
  
  
  1. 
     

     – Practical aspects: obesity increases the difficulty of a number of practical procedures, including venous access, regional anaesthesia and surgical access. Limb tourniquets may not adequately fit.

     
     
  2. 
     

     – Monitoring: an upper arm non-invasive blood pressure cuff may have a poor fit and thus be unreliable. Instead, either the blood pressure cuff may be positioned at the forearm or calf or an invasive arterial line may be required.

     
     
  3. 
     

     – Positioning and lifting of an unconscious patient is more difficult, requiring a greater number of staff. Special equipment may be required, such as an intubation positioning aid, a hover mattress, a bariatric operating table and/or a bariatric bed.