Avant factsheet
Managing surgical risks
in obese patients
Obese patients can pose a unique set of challenges for
surgeons and anaesthetists who need to be mindful of the
increased risks during surgery for these patients that require
specific care and precautions to take.
Sleep-related breathing disorders range from obstructive sleep apnoea
(OSA) syndromes through to central apnoeic and hypoventilation
syndromes. Obese patients are prone to OSA and obesity
hypoventilation syndrome (OHS).
Most obese patients presenting for elective surgery are relatively healthy
and their perioperative risk is not increased compared to patients of
normal weight. In fact, overweight and moderately obese healthy
patients undergoing non-bariatric surgery have paradoxically lower
crude and adjusted mortality rates compared with patients of normal
weight. This has been named ‘the obesity paradox’. However, this
benefit is lost if obese patients suffer from the metabolic syndrome or
other comorbidities. Hypotheses to explain the obesity paradox include:
increased energy stores; reduced systemic inflammatory response with
adipose tissue acting as a sink for inflammatory proteins and cytokines;
or perhaps increased perioperative clinical vigilance displayed by those
caring for obese patients. This benefit displays a J-shaped relationship,
with the highest perioperative complication rates seen in underweight
and morbidly obese patients.
Severe OSA is estimated to occur in 10–20% of patients with BMI > 35 kg/
m2 and is often undiagnosed. Overall, a diagnosis of OSA is associated
with twice the incidence of postoperative oxygen desaturations,
respiratory failure, postoperative cardiac events and ICU admission. The
presence of multiple and prolonged oxygen desaturations increases the
sensitivity to opioid-induced respiratory depression.
Many obese patients require specific perioperative care compared
with non-obese patients. Specific attention should be made to assess
for sleep-disordered breathing and those at higher risk for venous
thromboembolism (VTE) disease.
All patients with significant obesity should have a formal anaesthesia
review well before surgery, allowing time for appropriate investigations
and planning.
Pathophysiology of obesity and fat distribution
Not all fat within the body is identical. Unlike peripherally deposited fat,
intra-abdominal fat is more metabolically active. Patients with centrally
distributed fat (‘android’ or ‘apple-shaped’) are at greater perioperative
risk than those with peripherally distributed fat (‘gynecoid’ or ‘pearshaped’), and are far more likely to exhibit the metabolic syndrome of
hypertension, insulin resistance and hypercholesterolemia.
Preoperative preparation
All obese patients should have their Body Mass Index (BMI) calculated.
Recording high BMIs on the operating theatre list assists in forewarning
operating theatre staff that additional preparation, equipment and time
may be required in caring for these patients.
Appointing a lead-anaesthetist in each Anaesthetics Department in
overseeing the operating theatre suite’s approach to managing obese
patients is recommended by some anaesthetic societies.
Unexpected mortality in patients with severe OHS is higher than in
individuals with OSA alone. OHS exists when an obese individual (BMI
>30 kg/m2) with sleep disordered breathing develops awake alveolar
hypoventilation (PaCO2 >45 mmHg), which cannot be attributed
to other conditions such as lung disease, neuromuscular disease,
hypothyroidism, or pleural pathology. An awake oxygen saturation
(SpO2) <93 % in a patient with morbid obesity should alert the clinician
to the possibility of OHS, and a high serum bicarbonate level is a clue
that the patient is chronically hypercapnic. The combination of chronic
hypoxaemia and hypercapnia makes patients with OHS particularly
susceptible to the effects of anaesthetic agents and opioids. Such agents
may precipitate acute-on-chronic hypoventilation and even respiratory
arrest in the early postoperative period.
Intraoperative management
Anaesthetising the patient in the operating room may lessen the
challenges involved in transporting an asleep obese patient, and
decrease the risk of accidental awareness due to disconnection
during transfer.
Equipment and patient handling issues:
▶▶ Use appropriately-sized arterial pressure cuff (too small will overestimate BP).
▶▶ The anthropometric changes associated with obesity can make
venous access and performance of peripheral nerve blockade
technically difficult. The use of ultrasound can help significantly.
▶▶ Know the weight limits of all lifting and support equipment.
▶▶ Use appropriately-sized beds and chairs.
▶▶ Ensure staff-training to enable provision of safe handling of obese
patients and to reduce staff injury.
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Ventilation implications
Management of obese patients undergoing sedation
Obesity results in reduced functional residual capacity, atelectasis
and increased shunting in dependent lung regions. Obesity is also
associated with decreased lung and chest wall compliance. Resting
metabolic rate, work of breathing and minute oxygen demand is all
increased. Therefore, such patients demonstrate rapid and early oxygen
desaturation with general anaesthesia.
▶▶ Pre-procedure assessment should be similar for patients undergoing
general anaesthetic.
▶▶ Airway obstruction may occur with sedation only.
▶▶ The ‘solo-practitioner’ model is a high-risk undertaking for such
patients. The proceduralist should not be responsible for sedation
and airway management as well.
Specific ventilation implications in managing obese patients include:
▶▶ These patients will desaturate rapidly once apnoeic.
▶▶ P
re-oxygenation prior to anaesthesia is vital, ideally with the patient
in the semi-erect position.
References
1.
Mutter TC, Chateau D, Moffatt M et al. A matched cohort
study of post-operative outcomes in obstructive sleep apnea.
Anaesthesiology 2014; 121:707-18.
2.
Nightingale CE, Margarson MP, Shearer E et al. Perioperative
management of the obese surgical patient 2015. Association of
Anaesthetists of Great Britain and Ireland, Society for Obesity and
Bariatric Anaesthesia. Anaesthesia 2015;70:859-876.
3.
Castro-Añón O, Pérez de Llano LA, De la Fuente Sánchez S et al.
Obesity-hypoventilation syndrome: increased risk of death over
sleep apnea syndrome. PLoS One 2015;10: e0117808.
▶▶ Higher ventilator inflation pressures are usually required, owing to
decreased chest wall and respiratory compliance.
▶▶ Application of extrinsic positive end-expiratory pressure (PEEP) is
important to reduce small airways collapse.
Post-operative management
Obesity alone does not mandate high dependency unit (HDU)
postoperative care. The need for postoperative HDU admission should
be determined by: the patient’s pre-existing comorbidities; the type of
surgical procedure performed; the likelihood of untreated OSA and the
anticipated need for parenteral narcotics; and local hospital factors, for
example, skill-mix of ward staff.
The routine use of mechanical and early chemical deep vein thrombosis
prophylaxis should be considered, as obese patients are at higher risk of
VTE disease. Ideally, these patients should be mobilised early.
Post-operative considerations when management of obese patients
with sleep-disordered breathing
▶▶ Airway obstruction is a major concern in the postoperative period –
consider admission to HDU.
▶▶ Before discharge from the post-anesthesia care unit, ensure there are
no signs of apnoea or hypopnoea. Ongoing hypoventilation would
usually mandate admission to HDU.
▶▶ Monitor pulse oximetry postoperatively on the ward until Sp02
returns to baseline without supplemental oxygen, and parenteral
opioids are no longer required.
▶▶ Use long-acting opioids and sedatives with caution.
▶▶ Use multimodal analgesia, including local anaesthetics.
▶▶ Use the patient’s CPAP machine early in the postoperative period.
▶▶ Postoperative acoustic respiratory rate monitoring on the ward, in
addition to continuous Sp02 monitoring, adds an extra margin of
safety patients with sleep-related breathing disorders.
▶▶ OSA occurs more commonly in REM sleep, which predominates on
the second night post-surgery.
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avant.org.au
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