2. Obesity

Question 1

How do you classify obesity

Answer 1

<18.5 Underweight

18.5–24.9 Ideal weight

25–29.9 Overweight

30–39.9 Obese

40–49.9 Morbidly obese

50–59.9 Super obese

60–69.9 Super super-obese

> 70 Hyper-obese

BMI = weight (kg)/height (m2)

Question 2

What are the physiological
changes associated with obesity
that complicate anaesthesia?

Answer 2

> Airway

• Higher incidence of difficult face mask ventilation and difficult intubation

Question 3

Respiratory

9

Answer 3

> Respiratory

• Increased O2 consumption and
CO2 production, which may be
associated with chronic hypercarbia.

• Decreased chest wall compliance
causes increased work of breathing.

• Decreased functional residual capacity (FRC), especially when supine, means that closing capacity may encroach on FRC; once BMI
exceeds 40, FRC falls to 1 L or less.

• Increased incidence of
obstructive sleep apnoea (OSA).

• Obesity hypoventilation syndrome occurs

i.e. low O2 saturations not
caused by obstruction).

• Increased incidence of asthma.
• Increased incidence of pulmonary hypertension.

• These factors result in a high
risk of peri-operative hypoxia.

Question 4

> Cardiovascular
x 2

> Metabolic
1

Answer 4

> Cardiovascular

• Increased blood volume and cardiac output.

• Increased incidence 
 ischaemic heart disease, 
arrhythmias,
hypertension, 
hyperlipidaemia, 
heart failure 
and cor pulmonale.

> Metabolic
• Increased incidence of type 2 diabetes mellitus.

Question 5

> Gastrointestinal

3

Haematology
1

Answer 5

> Gastrointestinal

• Increased incidence of hiatus hernia,
gastro-oesophageal reflux disease,
fatty liver
and cirrhosis

• Decreased rate of stomach emptying

• Raised intra-abdominal pressure,
which can cause compartment syndrome

> Haematology
• Higher incidence of venous thromboembolism

Question 6

What are the pharmacodynamic and pharmacokinetic changes that occur?

PK

x 3

Calc for IBW

Answer 6

> Pharmacokinetics

• Altered volume of drug distribution (VD)
and elimination

• The use of ‘ideal’ or ‘actual’ body weight during drug dose calculation must be considered

> Ideal body weight in kg

• Height (cm) – 100 for adult males
• Height (cm) – 105 for adult females

Question 7

Volatiles

Answer 7

> Volatiles

• Sevoflurane and desflurane –
pharmacokinetics not significantly altered.

• Halothane –
increased reductive hepatic metabolism,
increasing the risk of halothane hepatitis.

Question 8

> Induction agents

Answer 8

• Thiopentone –

increased VD and
longer elimination half-life.

Suggested dose is 7.5 mg/kg
based on ideal body weight.

• Propofol –
induction dose should be based
on ideal body weight.

When used for maintenance (TIVA),
calculate dose using actual body weight,

as propofol does not accumulate in obese patients.

Question 9

> Opioids

> Muscle relaxants

Answer 9

• Fentanyl –
pharmacokinetics not altered,
use actual body weight.

• Morphine –
accumulates in body fat,
use ideal body weight.

> Muscle relaxants

• Suxamethonium –
use actual body weight.

• Rocuronium –
use ideal body weight.

Question 10

What are the anaesthetic considerations for an obese

patient presenting for surgery?

Answer 10

> Pre-operative assessment

1
• When planning elective surgery,
pre-operative assessment should
involve a multi-disciplinary team.

2
• Ensure sufficient time is allocated
for the patient on the operating list.

Both the anaesthetic and surgery are likely to be technically more difficult and take longer.

3
• Assess and establish co-morbidities to help define ‘risk’.

This will be dependent on the individual patient and the proposed surgery.

The severity of symptoms, e.g. angina, may be masked by a sedentary lifestyle.

Request further investigation as appropriate,
e.g. lung function tests, ECHO.

4
• Anaesthetic history,
including thorough airway assessment,
grade of previous intubations and patient’s ability to lie flat, if necessary.

5
• Consider regional techniques where possible.

6
• Optimise the patient for surgery if possible,
e.g. weight loss programme and
optimisation of medical conditions.

7
• Liaise with critical care to arrange
appropriate post-operative care,
e.g. HDU/ICU and non-invasive ventilation.

8
• Ensure a senior anaesthetist is available for the case.

9
• Inform theatres in advance to ensure bariatric equipment is available (e.g. electric bed, operating table, hover mattress and large blood pressure cuffs).

Question 11

> Induction

Answer 11

> Induction

• Pre-medicate with antacid prophylaxis
or a proton pump inhibitor.

• If possible, get the patient to climb
onto the operating table and position themselves.
If not, use aids such as a hover mattress.

• Anticipate difficult intravenous access and use ultrasound if necessary.
• Have a low threshold for invasive blood pressure monitoring, as cuffs can be inaccurate.
• Position the patient 30° head up.
• Consider awake fibre-optic intubation.
• Use a short-handle laryngoscope with a long blade and ensure the difficult airway trolley is available

.
• Ensure strict pre-oxygenation (end-tidal oxygen >90%).

Question 12

> Maintenance

Answer 12

• Use short-acting anaesthetic agents,
e.g. desflurane, sevoflurane,
propofol, remifentanil.

• Ventilate using PEEP.
• Ensure pressure areas are padded and monitored.

• Ensure adequate fluid input
given larger surface area of patient.

• Monitor and maintain temperature.

Question 13

> Post-operative management

Answer 13

> Post-operative management

.
• Ensure there are sufficient people
present to move the patient safely.

• Extubate awake and sitting up.
Consider extubating onto CPAP.

• Prescribe supplementary O2 for
at least 24 hours post-operatively.

• Admit to a suitable level of care,
based on your assessment of risk.

• Thromboprophylaxis.
• Multimodal analgesia.
• Early physiotherapy and mobilisation.