Identifying and optimising co-morbidities is the most effective way to reduce perioperative risk.
Obese patients with obstructive sleep apnoea require perioperative CPAP therapy and appropriate post-operative monitoring.
Ambulatory surgery is best reserved for healthy obese patients with a BMI <40 kg/m2.
Anaesthetic pre-assessment is an opportunity to promote a healthy lifestyle for obese patients.
Obesity is a global epidemic, affecting more than 600 million adults. In the United Kingdom, the prevalence of obesity has doubled in the past 10 years, with one in four adults now classified as obese. In many Western countries, obesity now exceeds smoking as the leading preventable cause of death (WHO, 2015).
As the prevalence of obesity continues to rise, there will be a proliferation of obesity-related disease (Table 18.1). For obese patients requiring surgical care, these co-morbidities contribute to a greater risk of perioperative morbidity and mortality.
Obesity is a condition of excessive fat accumulation caused by caloric intake exceeding energy expenditure. The most widely used measure of obesity in adults is the Body Mass Index (BMI) (Figure 18.1). Obesity is defined as a BMI 30 kg/m2 or greater (Table 18.2).
Legend: BMI=body mass index, kg=kilogram, m2= metre square
Obesity and Perioperative Risk
Provided the patient is healthy, obesity alone does not increase the risk of complications or death from elective surgery (Dindo et al., 2003; Mullen, Moorman and Davenport, 2009). In fact, obesity has been shown to confer some survival advantage in both surgical and critical care settings (Akinnusi, Pineda and Sohl, 2008). This observation has been termed the Obesity Paradox (Mullen et al., 2009).
What does increase perioperative morbidity and mortality, however, is the presence of obesity-related disease. This is especially true of patients with features of metabolic syndrome – central obesity, hyperglycaemia, hypertension and dyslipidaemia (Glance et al., 2010).
For this subset of metabolically obese patients, the risk of perioperative complications and death is significantly higher than normal-weight patients or the healthy obese (Table 18.3).
|Patient population||Reported risk||Author|
|310,208 patients undergoing general surgery, vascular surgery or orthopaedic surgery||2–2.5-fold increased risk of cardiac adverse events||(Glance et al., 2010)|
|1.5–3-fold increase in pulmonary complications|
|3–7-fold increase in acute kidney injury|
|2-fold increased mortality for patients with BMI >50kg/m2|
|5,304 patients undergoing coronary artery bypass grafting (retrospective)||2.5-fold increased mortality||(Echahidi, Pibarot, et al., 2007)|
|5,085 patients undergoing coronary artery bypass grafting (retrospective)||2-fold increased risk of post-operative atrial fibrillation||(Echahidi, Mohty, et al., 2007)|
|921 carotid patients undergoing carotid endarterectomy||4-fold increased risk of stroke||(Protack et al., 2009)|
|60 patients undergoing non-cardiac surgery patients||2-fold increased risk of post-operative cognitive dysfunction (POCD)||(Hudetz et al., 2011)|
Legend: BMI=body mass index, kg=kilogram, m2= metre square
Preoperative Assessment of the Obese Surgical Patient
Anaesthetists tend to overestimate the perioperative risk of obese surgical patients who are otherwise healthy, while underestimating the risk faced by obese patients with co-morbidities, especially the metabolic kind. Identifying and optimising the co-morbidities of obese patients is the best way to reduce perioperative complications.
Obese patients often report breathlessness, and it can be difficult to determine whether symptoms are due to the mechanical effects of obesity or intrinsic respiratory disease. Obesity causes reduced lung volumes and is associated with adult-onset asthma, obstructive sleep apnoea (OSA) and obesity hypoventilation syndrome (OHS).
Independent risk factors for perioperative pulmonary complications in bariatric surgery cohorts include: increasing age, BMI, ASA status, OSA, asthma, congestive heart failure, metabolic syndrome, surgical duration and open surgery (Schumann et al., 2015). The risk appears greatest for patients with a BMI >40 kg/m2 undergoing major cavity surgery. For these patients, post-operative atelectasis or pneumonia occurs in up to 30 per cent of cases (Flier and Knape, 2006).
Reduced Lung Volumes
As fat accumulates in the chest and abdomen, the chest wall becomes stiffer, diaphragmatic movement is impaired and pulmonary compliance falls. This causes an exponential decline in functional residual capacity (FRC) and expiratory reserve volume as BMI increases (Jones, 2006).
As FRC falls, small airway closure during tidal breathing leads to alveolar collapse and atelectasis. The result is a mismatch of lung ventilation and perfusion and a tendency to hypoxemia. The problem is compounded by general anaesthesia (Figure 18.2) (Adams and Murphy, 2000).
Decreased pulmonary compliance also increases the work of breathing. A significant proportion of obese patients have reduced respiratory muscle endurance compared to normal-weight subjects when measured on pulmonary function tests (PFTs) (Sebastian, 2013). Muscle fatigue may contribute to an increased risk of respiratory failure after major abdominal surgery.
Weight gain and elevated BMI are associated with an increased risk of developing adult-onset asthma. The risk appears greater for obese women than for obese men (Sebastian, 2013).
Obese patients reporting episodic dyspnoea, wheeze or cough should undergo pulmonary function testing (PFTs), including bronchodilator challenge. Consider cardiac disease and gastro-oesophageal reflux disease (GORD) as differential diagnosis in obese patients with asthma-like symptoms and normal PFTs.
Obstructive Sleep Apnoea
Obstructive sleep apnoea (OSA) is characterised by recurrent episodes of upper airway obstruction during sleep. Obesity increases susceptibility to OSA by causing physical narrowing and impaired neuromuscular function of the pharynx (Sebastian, 2013). The severity of OSA is based upon the frequency of apnoea and hypopnoea per hour of sleep – Apnoea Hypopnoea Index (AHI).
The prevalence of OSA is estimated to be 40 per cent for obese women and 50 per cent for obese men (Chung, Yang and Liao, 2013). Most will be undiagnosed when they attend the pre-assessment clinic, so making the diagnosis and implementing CPAP therapy can have a positive impact on patients’ perioperative and long-term health.
Chronic untreated OSA causes systemic hypertension and has been associated with higher rates of cardiac arrhythmias, myocardial infarction, heart failure, stroke and diabetes mellitus (Jordan, McSharry, and Malhotra, 2014). Surgical patients with untreated OSA have higher rates of post-operative desaturation, respiratory failure, post-operative cardiac events, ICU transfers (Kaw et al., 2012) and increased length of stay (Gupta et al., 2001) compared to non-OSA patients. There are a large number of case reports describing perioperative complications in patients with OSA, including respiratory arrest, cardiac arrest and death (Chung, Yuan and Chung, 2008).
The easiest way to identify patients at risk of OSA in the pre-assessment clinic is to perform a screening questionnaire. The STOP-Bang questionnaire (Table 18.4) has the benefit of being easy to remember and has been validated for use in obese surgical patients (Chung et al., 2013). STOP-Bang has a sensitivity and negative predictive value approaching 100 per cent, meaning a score < 3 safely excludes the possibility the patient before you has moderate-to-severe OSA (Chung and Elsaid, 2009).
Fewer than three questions positive = low risk of OSA; ≥ three questions positive = high risk of OSA; five to eight questions positive = high probability of moderate-to-severe OSA.
Legend: BMI=body mass index, kg=kilogram, m2= metre square, cm=centimetre, OSA obstructive sleep apnoea
Patients with a STOP-Bang score 5–8 are at high risk of moderate-to-severe OSA, and will benefit most from diagnostic testing. The intermediate group (STOP-Bang score 3–4) should be referred for diagnostic testing if undergoing major surgery or surgery with high post-operative analgesic requirements.
The gold standard diagnostic test for OSA is overnight laboratory polysomnography. The result provides an accurate AHI index, allowing grading of OSA severity; however, it requires expensive equipment and trained staff and can be inconvenient for patients. Simplified diagnostics including overnight oximetry and home polysomnography are viable alternatives for most patients (Jordan et al., 2014).
Continuous positive airway pressure (CPAP) is effective in relieving the symptoms of OSA and reducing long-term respiratory and cardiovascular complications. Data on the efficacy of perioperative CPAP are poor. Preoperative CPAP therapy is effective in reducing the AHI, and there is a trend towards shorter length of stay post-operatively (Nagappa et al., 2015). Post-operative CPAP has been shown to reduce post-operative atelectasis, pneumonia and re-intubation after major abdominal surgery (Ireland et al., 2014).
We recommend a minimum 4-week trial of preoperative CPAP for patients with moderate to severe OSA. This allows time to optimise CPAP fitting and ventilation parameters. All patients on CPAP are required to bring their own machines and facemasks to hospital on the day of surgery.
Perioperative management of obese patients with OSA depends on the severity of disease, CPAP compliance, type of surgery and anticipated analgesic requirements. Table 18.5 describes one approach, with guidelines for the appropriateness of ambulatory surgery and post-operative care requirements for patients with OSA.
|Low-risk surgery||Intermediate-risk surgery||High-risk surgery|
|Mild OSA||Suitable for DSS||May be suitable for DSS||Not suitable for DSS|
|(AHI 5-15)||Post-op ward bed||Post-op ward bed||Post-op ward bed + oximetry*|
|Moderate OSA||Suitable for DSS||Not Suitable for DSS||Not Suitable for DSS|
|(AHI 16-30)||Ward bed||Post-op ward Bed + oximetry*||Post-op ward bed + oximetry*|
|Severe OSA||Not suitable for DSS (unless LA only)||Not Suitable for DSS||Not suitable for DSS|
|(AHI > 30)||Post-op ward bed + oximetry*||Post-op ward bed + oximetry*||Post-op HDU or ICU|
|±HDU referral||Notify NIV Service|
|Notify NIV service|
* Ward Bed + oximetry requires continuous pulse oximetry with audible alarms and nursing care trained and experienced in the use of CPAP. HDU = unit with 2:1 nursing; ICU = unit with 1:1 nursing; Low-Risk Surgery = superficial, endoscopic, ophthalmic, minor breast, minor gynaecological and minor urological procedures and transurethral resection of the prostate (TURP) under regional anaesthesia. Intermediate Surgery = peripheral surgery with moderate analgesic requirements, intra-cavity surgery, head and neck (but not airway) surgery, TURP under general anaesthetic. High Risk Surgery = emergency surgery, major surgery, airway surgery and patients with high analgesic requirements regardless of type of surgical procedure.
Legend: OSA = obstructive sleep apnoea, DSS = Day-Stay Surgery, AHI = Apnoea Hypopnoea Index, LA-local anaesthetic, NIV = non-invasive ventilation, HDU = high dependency unit, ICU = intensive care unit
This guide relies on a coordinated approach to care by anaesthetists, surgeons, physicians and nursing staff. It should be modified to suit local conditions and resources.
Obesity Hypoventilation Syndrome
This is a sleep disorder characterised by obesity, daytime hypercapnia and hypoxemia. Although affecting only 0.15–0.03 per cent of the general population, the prevalence of OHS among obese patients with OSA is as high as 10 per cent (Chau et al., 2012).
Patients with OHS have a reduced central respiratory drive. This places them at higher risk of post-operative respiratory failure; a problem compounded by general anaesthesia and sedating analgesics. Chronic OHS also contributes to higher rates of pulmonary hypertension, coronary artery disease and heart failure (Berg et al., 2001).
The diagnosis should be considered in obese patients who report sleep disturbance and have low oxygen saturations on room air (SpO2 ≤94%). Serum bicarbonate (HCO3) is an easily performed and sensitive screening test for chronic hypercapnia. If HCO3 is elevated, a resting arterial blood gas should be performed to quantify the degree of hypercapnia and hypoxemia.
It is appropriate to refer these complex patients to a sleep physician for assessment and treatment. They will require PFTs, polysomnography and titration of CPAP or bi-level positive airway pressure (BiPAP) preoperatively.
Respiratory Investigations and Optimisation
All obese elective surgical patients should have resting respiratory rate and room air pulse oximetry recorded at pre-assessment. Serum bicarbonate and arterial blood gas measurement should be performed in patients with low oxygen saturations as described previously.
Preoperative PFTs in obese patients without symptoms or signs of underlying lung disease are costly and are not beneficial. The same is true of routine chest radiographs, peek expiratory flow measurements and preoperative incentive spirometry (Cattano et al., 2010). PFTs are indicated for patients with unexplained dyspnoea at rest, symptoms of obstructive lung disease and obesity hypoventilation syndrome (OHS).
The principles of pulmonary optimisation in obese patients should include:
Minimum 6 weeks smoking cessation programme, including nicotine replacement therapy (Lee et al., 2015);
Inhaled corticosteroids, leukotriene inhibitors and long-acting beta agonists for treatment of asthma;
Antibiotics for patients with infected sputum;
Minimum 4 weeks titration of CPAP (or BiPAP) therapy for OSA and OHS;
Proton pump inhibitors for symptomatic GORD;
Obese individuals are at greater risk of hypertension, coronary artery disease (CAD), heart failure, cardiac arrhythmias and thromboembolic disease. Many of the signs of cardiovascular disease may be masked by obesity, making patient assessment more difficult. The challenge is to detect underlying disease and identify patients at increased risk of perioperative cardiac morbidity or death.
Coronary Artery Disease
Obesity, particularly central obesity, is strongly associated with coronary artery disease. In a large meta-analysis of 302,296 participants, obese subjects were 80 per cent more likely than normal-weight subjects to have a coronary event during follow-up (adjusted relative-risk 1.81, 95% CI 1.56–2.10) (Bogers et al., 2007). Many obese patients will have occult CAD.
Measuring waist and hip circumference in the pre-assessment clinic is useful, as a low waist-to-hip ratio predicts myocardial infarction risk and related mortality better than BMI (Chrostowska et al., 2013). A 12-lead ECG is recommended for patients with at least one risk factor for CAD or poor exercise tolerance (Poirier et al., 2009). Left bundle branch block and S-T and T wave abnormalities may be indicative of underlying CAD.
The AHA/ACC recommends non-invasive cardiac testing for patients with ≥2 revised cardiac risk indices and poor functional status (Fleisher et al., 2014). There are no specific recommendations for obese patients who score poorly on measures of function because of physical limitations. A recommended approach is to perform stress testing for patients with a BMI >40 kg/m2 with at least two or more CAD risk factors who cannot exercise or have unknown exercise tolerance (Poirier et al., 2009). The choice of stress test – exercise, pharmacological or combined – depends on the patient’s mobility and the facilities available.
Cardiopulmonary exercise (CPET) testing has been used to predict morbidity and length of stay after gastric bypass surgery (Hennis et al., 2012). In general, obese individuals have an anaerobic threshold only slightly less than normal-weight controls (Serés et al., 2003). If available, CPET testing can be recommended for obese patients undergoing high-risk procedures where functional status is unknown (Fleisher et al., 2014).
In addition to the most common causes of heart failure – CAD, systemic hypertension and heart valve disease – prolonged obesity is associated with the development of a reversible obesity cardiomyopathy. This is initially characterised by diastolic dysfunction, but may progress to both diastolic and systolic dysfunction (Poirier et al., 2009) (Figure 18.3).
A history of dyspnoea, orthopnoea and peripheral oedema are non-specific in obese patients. On examination, heart sounds may be muffled and jugular venous pulses obscured. Right axis deviation and right bundle branch on ECG suggests right ventricular hypertrophy. This may be an important sign of pulmonary hypertension and deserves further cardiac investigation.
Transthoracic echocardiography windows may be poor quality in very obese patients. Alternative cardiac imaging includes nuclear imaging, magnetic resonance imaging and computed tomography (Poirier et al., 2009). MRI has the advantage of no radiation exposure; however, specialised large-bore MRI is required for very obese patients. The preferred modality will depend on local availability and expertise.
The Framingham Heart Study observed that obese subjects were twice as likely to develop systemic hypertension than the non-obese. The mechanisms are unclear, but have been shown to be reversible with weight loss (Wilson et al., 2002). Obese patients with hypertension are also more likely to be resistant to antihypertensive therapy (Chrostowska et al., 2013). Although grade 1 and 2 hypertension per se does not increase clinically significant perioperative complications, it is an important marker of end-organ damage (Howell et al., 1996).
Manual or automated blood pressure measurements should be taken with the cuff bladder encircling 80 per cent or more of the patient’s upper arm circumference. Undersized cuffs tend to overestimate blood pressure. Ideally two measurements should be taken at least 1 minute apart, with the average recorded.
Patients with average systolic pressures >140mmHg or diastolic pressures >90mmHg should be referred to their primary physician and managed according to primary care guidelines (Krause et al., 2011). First-line therapy is usually an angiotensin-converting enzyme inhibitor (ACE-I) or angiotensin II receptor blocker (ARB).
Obesity is associated with diastolic dysfunction and atrial enlargement; both are risk factors for developing atrial fibrillation. In population-based cohort studies, obese individuals are at almost 50 per cent increased risk of developing AF than non-obese individuals (RR 1.49; 95% CI 1.36–1.64)(Wanahita et al., 2008). The likelihood of other cardiac arrhythmias and sudden death is also higher.
If OSA is present, the severity is an independent predictor of the incidence of atrial fibrillation (Cadby et al., 2015). The mechanism may be an increased sympathetic tone associated with recurrent episodes of apnoea and arousal.
The relative risk of deep venous thrombosis and pulmonary embolism for obese patients is twice that of non-obese patients. The disparity is greatest for patients less than 40 years (Stein, Beemath and Olson, 2005). Despite this knowledge, pulmonary embolism (PE) remains a leading cause of morbidity and mortality in bariatric centres, with an estimated 2.4 per cent incidence of symptomatic venous thromboembolism and 0.3 per cent incidence of fatal PE (Gould et al., 2012).
Pro-coagulant changes in obesity include enhanced platelet activity, increased circulating pro-coagulants, impaired fibrinolysis and vascular endothelium activation. Obesity related hormones, insulin resistance, chronic inflammation and inactivity are contributing factors (Freeman, 2010).
The co-morbidities known to increase the risk of fatal VTE in bariatric surgery are a past history of VTE, BMI >60 kg/m2, central obesity, venous stasis disease and a diagnosis of OSA/OHS (Sapala et al., 2003). Interventions recommended to reduce VTE risk are intermittent pneumatic lower-limb compression devices combined with anticoagulant prophylaxis.
Typical dosing regimens for unfractionated heparin and low molecular weight heparin (LMWH) may under-dose morbidly obese patients. Previous international guidelines recommended thrice-daily UH or twice-daily LMWH dosing (Geerts et al., 2008); however, a recent update recommended seeking pharmacy advice for bariatric dosing (Gould et al., 2012).
Insertion of a vena cava filter preoperatively may be considered in very high-risk patients. This includes obese patients with a history of prior PE, prior iliofemoral DVT, evidence of venous stasis disease, known hypercoagulable state or increased right-sided heart pressures (PAP >40mmHg) (Mechanick et al., 2008).
Genetic predisposition and excess body weight are the major causes of type 2 diabetes (T2DM). An estimated 90 per cent of patients with T2DM are obese (Pedersen, 2013). The mechanisms are complex, but adipose tissue is known to increase resistance to insulin-mediated glucose uptake.
As yet, there is insufficient evidence to recommend screening for T2DM in all obese surgical patients. In practice, however, all obese patients older than 45 years and those with at one risk factor for T2DM – including physical inactivity, history of cardiovascular disease, hypertension or dyslipidaemia – justify HbA1c screening under international guidelines (American Diabetes Association, 2015). The preoperative assessment and management of patients with diabetes is covered elsewhere in this textbook.
Kidney Function Assessment
High BMI is a strong independent risk factor for developing chronic kidney disease (CKD) (Hsu et al., 2006). Possible mechanisms for obesity-related CKD include glomerular hyper-filtration, chronic inflammation and kidney compression and infiltration by visceral fat (Hall et al., 2015).
Perioperatively, there are the additional risks of acute kidney injury (AKI) caused by hypovolaemia, hypotension and nephrotoxins. Obese patients undergoing bariatric surgery have up to six times the risk of AKI compared with other general surgical patients (Weingarten et al., 2013). The risk is greatest at the extremes of BMI and in patients with metabolic syndrome (see Table 18.3) (Glance et al., 2010).
Assess baseline renal function for all obese patients undergoing surgery where fluid shifts, blood loss or exposure to nephrotoxins might be expected. The preoperative diagnosis and management of kidney disease is covered elsewhere in this text.
Anatomical and physiological changes in obese patients make airway management more difficult. In the UK’s Fourth National Audit Project (NAP4), obese patients accounted for 42 per cent of major airway complications. Difficulties with tracheal intubation, aspiration, extubation problems and airway trauma were the most common complications. Of the obese patients who suffered a major airway complication, 25 per cent died or suffered brain damage (Cook et al., 2011). For details of airway assessment please refer also to Chapter 7 of this volume.
Predicting Difficult Intubation
It is debatable whether obesity alone increases the likelihood of difficult tracheal intubation. Clinical trials comparing direct laryngoscopy and intubation in obese and normal-weight subjects are conflicting (Brodsky et al., 2002; Juvin et al., 2003). A large cohort study of 91,332 surgical patients concluded a BMI >35 kg/m2 was only a weak predictor of difficult and failed intubation (adjusted odds-ratio 1.34; 95% CI 1.19–1.51, P <0.0001) (Lundstrøm et al., 2009).
The best predictors of difficult airway management in obese patients are listed in Table 18.6. Absolute weight, BMI, gender, mouth opening and mandibular recession are not independent predictors of difficult intubation in obese patients, but taken together may assist with airway planning (Juvin et al., 2003). Please see also Chapter 7, page 87.
|Variable||OR||95% CI||P-value||Author||Outcome measure|
|Mallampati Score III or IV||3.93*||2.65–5.84||<0.0001||(Juvin et al., 2003)||DI|
|(Brodsky et al., 2002)|
|2.54||1.18–3.85||0.009||(Leoni et al., 2014)||DMV|
|Short neck (subjective)||2.65||1.47–4.79||0.034||(Cattano et al., 2014)||DMV|
|Reduced mobility of cervical spine||2.29||1.51–3.48||<0.0001||(De Jong et al., 2015)||DI|
|Neck circumference ≥43cm||2.23^||1.35–3.71||0.002||(Cattano et al., 2014)||DMV & DI|
|(Leoni et al., 2014)|
|(Brodsky et al., 2002)|
|Age ≥ 49||2.03||1.24–3.32||0.005||(Cattano et al., 2014)||DMV|
|Limited jaw protrusion||1.98||1.03–4.28||0.046||(Leoni et al., 2014)||DMV|
|Obstructive sleep apnoea||1.96||1.19–3.22||0.009||(De Jong et al., 2015)||DI|
Legend: DI = Difficult intubation; DMV = Difficult mask ventilation; OR = odds-ratio; CI = Confidence interval, cm=centimetre,
Predicting Difficult Mask Ventilation
The incidence of difficult mask ventilation (DMV) in surgical populations is 1–2 per cent (Han et al., 2004; Kheterpal et al., 2006). In obese populations, the incidence of DMV is reported as high as 14 per cent (Cattano et al., 2014).
A BMI > 30 kg/m2 is the strongest independent predictor of DMV (Kheterpal et al., 2006). It seems that once the threshold BMI of 30 kg/m2 is reached, further obesity does not substantially increase the risk (Leoni et al., 2014).
The independent predictors of DMV in obese populations are listed in Table 18.6. Other predictors of DMV from the general literature include male gender, lack of teeth, history of snoring or OSA, neck radiation changes and the presence of a beard (Kheterpal et al., 2006; Langeron et al., 2000).
A common question in pre-assessment: Is this obese patient suitable for day-case surgery? The first issue is whether the healthcare facility is adequately equipped and staffed for the obese patient. This includes appropriately rated bariatric trolleys, operating tables and patient-lifting devices. The second issue is whether the patient can be safely discharged home post-operatively given the patient’s co-morbidities, the invasiveness of the surgery and analgesic needs.
A systematic review of obese patients undergoing ambulatory surgery failed to find any difference in perioperative complications or readmission rates for patients with a BMI <50 kg/m2 (Joshi et al., 2013). Overall, however, the rates of ambulatory surgery for very obese patients are low, so the true rate of complications and readmission cannot be certain. A conservative approach to day surgery for patients with a BMI >40 kg/m2 is recommended.