Obesity Physiology

Question 1

Define obesity

Answer 1

BMI > 30kg/m^2

NICE categories are as follows:
18.5-24.9 = normal
25 - 29.9 = overweight
30 - 34.9 = obesity class 1
35 - 39.9 = obesity class 2
>40 = obesity class 3

Over 60% of UK adults are overweight or obese, particularly in lower socioeconomic groups

2021 PErioperAtive CHildhood obesitY (PEACHY) study demonstrated worryingly high rates of obesity in children requiring surgery under GA.

BMI = Mass/height²

Question 2

When does obesity impact perioperative risk

Answer 2

Once BMI reaches 40kg/m², or if there are significant comorbidities.

Obesity hypoventilation syndrome and OSA are of concern, and patients are more sensitive to respiratory depressant effects of opioids.

BMI >50 likely precludes day surgery

Previous bariatric surgery can delay gastric emptying

Central obesity (apple shape) has greater perioperative risk than thigh and waist obesity (pear shape)

Question 3

What are the anaesthetic implications of obesity?

Answer 3

Cardiovascular
Stroke and PE risk
HTN & IHD
Arrhythmias
Heart Failure
Difficult IV access

Respiratory
Difficult airway (reduced mouth/neck range of motion), large neck & soft tissues, raised intragastric pressure & reflux risk
Obesity Hypoventilation Syndrome
OSA & Opioid sensitivity
Pulmonary HTN
Reduced thoracic compliance
Worse V/Q mismatching
Smaller FRC and larger closing capacity, causing ‘wheeze’

Neurological
Headache, poor sleep, depression

GI
Gallstones, fatty liver
Deranged liver enzymes
Hiatus hernia

Metabolic
Hypercholesterol, DM, Increased BMR

GU
Menstrual issues, erectile dysfunction, fertility problems, PCOS

Question 4

How does obesity change your anaesthetic plan?

Answer 4

Induction
Difficult cannulation may require US
Monitoring (such as BP cuffs) may need changing
Additional staff/equipment for moving & handling
Consider regional if possible
RSI isn’t essential unless specifically indicated
Consider performing in theatre to reduce risk during transfer from anaesthetic room
Optimal patient positioning (Oxford HELP & ramped position - align tragus to sternal notch)
Preoxygenate (consider THRIVE)
Consider videolaryngoscopy
Call for help early
Anticipate early desaturation & functionally difficult airway
Higher risk of awareness, particularly with volatile agents and prolonged airway manipulation

Intraop
Recruitment manoeuvres
Caution with airway pressures, especially in reverse trendelenburg
Pressure areas
Side extensions on operating table
Table weight limit - especially if traversing

Extubation
Greater risk of desaturation (loss of FRC & increased closing capacity) - ideally sit patient up
Extubate only once fully awake, avoid post-op opioids
Be ready to reintubate
Consider extubation onto BiPAP/THRIVE

Question 5

What is metabolic syndrome?

Answer 5

A group of conditions:
Obesity
Impaired glucose tolerance
Hypercholesterolaemia Hypertension

Significantly increases mortality for all classes of obesity:

Cardiac/pulmonary complications
AKI
Stroke
Sepsis

Question 6

How does obesity affect drug dosing and pharmacokinetics?

NEEDS FIXING

Answer 6

Dosing depends on fat-solubility & volume of distribution of the drug.

Total, ideal, and lean body weight are used.

Lean body weight: (Boer/James/Hume Formulae)
Propofol, Thiopentone, Opioids, Non-depolarising NMBDs, local anaesthetics

Ideal/Adjusted body weight: Propfol, neostigmine, sugammadex, antibiotics

Total body weight: Suxamethonium, LMWHs

Absorption
Gastric stasis, IM more difficult to administer

Distribution
Fat soluble drugs have a larger VD, and there is increased body water, plasma volume, and CO

Metabolism
Slightly increased hepatic enzyme activity, increased esterase levels in blood & tissues

Elimination
Increased CO increases renal clearance