FRCA Notes

The prevalence of obesity has tripled over the past 40yrs, with increases are greater in ‘developing’ compared to ‘developed’ countries
The UK has the sixth highest prevalence for obesity; 64% of adults are overweight or obese
Only five European countries have a prevalence of obesity of <20%, although the prevalence of overweight adults in these countries is still >50%
In the USA, 36% are obese and 89% overweight, with similar rates in Canada, Australia and New Zealand

BMI is commonly used as a measure of obesity
It is easy to calculate and can aid in planning and preparation
It does not describe important pathophysiological factors influencing perioperative risk such as body composition, the distribution of tissue (muscle vs. adipose) or the metabolic state

Category	BMI (kg/m²)
Underweight	<18.5
Normal weight	18.5 - 24.9
Overweight	≥25
(Pre-obese	25 - 29.9)
Obese	≥30
Class 1 obesity	30 - 34.9
Class 2 obesity	34.9 - 39.9
Class 3 obesity	≥40

Patients who are overweight or obese can be ASA 1, although class 2 obesity (ASA 2) and class 3 obesity (ASA 3) generate a higher risk

Body composition

Normally total body weight is comprised of 80% lean body mass (also known as fat-free mass) and 20% fat weight
In the overweight individual, lean body weight increases slowly relative to the increase in fat weight, so the ratio of total body weight to lean body weight increases
There is also an increase in total body water

Different weights used for drug dosing

Total body weight (TBW)

Ideal body weight (IBW)

I.e. what the patient should weigh if they had a normal ratio of lean mass to fat
Calculated using the Broca formula: height in centimetres minus 100 (man) or 105 (woman)

Lean body weight (LBW)

The weight of everything except body fat
Exceeds ideal body weight
Various calculations exist including the Janmahasatian formula
Plateaus at 100kg in men and 70kg in women

As IBW and LBW are calculated using height and sex, they may be the same for obese and non-obese individuals of the same height and gender
Giving obese patients highly lipid soluble drugs using IBW- or LBW-based dosing may result in lower plasma concentrations than anticipated
Equally, giving poorly lipid soluble drugs accordingy to IBW or LBW may result in lower plasma concentrations as they do not account for the excess fat mass

Adjusted body weight (ABW)

Acknowledges that obese individuals have increased lean body mass and an increased volume of distribution for drugs
Uses ideal body weight + a correction factor multiplied by the excess between ideal and total body weight
I.e. ABW = IBW + C(TBW - IBW)
The use of a drug-specific correction factor reflects differing drug solubility
Sometimes the ABW40 is used, where the correction factor is set at 40% (a.k.a Servin's formula), for calculating ABW for the purposes of propofol infusions

Respiratory

Increased risk of difficult facemask ventilation and intubation

Altered respiratory mechanics

Increased O₂ consumption & CO₂ production
Lower chest wall compliance → increased work of breathing
Reduced FRC, especially when supine
Closing capacity may encroach on FRC, making patients prone to hypoxia due to shunt
Linear increase in A-a gradient with BMI

Increased incidence/risk of:

Sleep disordered breathing (OSA, OHS, non-obstructive hypoxia)
Asthma

Chronic inflammatory state caused by excess adipose tissue
Compression of small airways due to increasing fat deposition within and around the chest & abdomen
Reversibility not always seen with β₂ agonists
Weight loss improves symptoms

Pulmonary hypertension

Cardiovascular

Increased overall blood volume but lower blood volume-per-body weight i.e. 50ml/kg rather than 70ml/kg
Increased cardiac output
Increased risk of peri-operative bleeding/blood-loss

Increased risk of cardiovascular comorbidities

Hypertension (10x)

Ischaemic heart disease

Heart failure

Arrhythmias

Peripheral vascular disease

Venous thromboembolism

Gastrointestinal & metabolic

Increased incidence of:

Insulin resistance ± T2DM
Hiatus hernia
GORD
NASH/Fatty liver/cirrhosis
Hypercholesterolaemia
Osteoarthritis

Metabolic syndrome (3 or more from):

Central obesity
Hypertension
Impaired glucose handling or outright diabetes
Raised triglycerides
Reduced HDL cholesterol

Raised intra-abdominal pressure increases risk of abdominal compartment syndrome

Physiological variable	Pharmacokinetic consequences
↑ Total body water	↑ V_D
↑ Fat mass	↑ V_D
↑ Plasma protein levels	↑ Protein binding
↑ Lean body weight	↑ Clearance (and because CO is ↑)

Absorption

Largely unchanged due to preserved gastrointestinal function

Distribution

Increased volume of distribution due to increases in both total body water and fat mass
Increased protein binding due to higher levels of plasma proteins; may reduce free drug fraction

Metabolism

Cardiac output is higher, as is hepatic and renal blood flow, so drug delivery to organs of metabolism is increased
This increases clearance and metabolism of flow-limited drugs with a high extraction ratio e.g. propofol, ketamine and morphine

Conversely metabolic pathways may be hindered by coalescing comorbidities such as fatty liver disease, chronic kidney disease, smoking or the effects of therapeutic drugs
Obesity in-and-of itself does not cause consistent effects on hepatic metabolism

Elimination

Increased cardiac output increases GFR and therefore renal clearance
Clearance correlates with lean body weight and therefore is increased as LBW increases in obesity

Propofol

Use LBW for induction boluses as this correlates well with cardiac output (onset) and plasma clearance (offset)

For TCI, neither Marsh nor Schnider models are not validated at extremes of weight
Indeed, both become inaccurate at a BMI of >37 (females) or >42 (males)
Overall require careful monitoring and titration to ensure adequate anaesthesia without overdosing

Marsh

Scales doses linearly with total body weight and may cause over-dosing if TBW used in obese patients
Does scale the central compartment according to weight
The algorithm does not correct body weight so one should input a calculated LBW
Preferable to use Servin's formula (ABW40) for propofol infusion using Marsh model

Benefits from a higher dose to reflect distribution of lipophilic propofol into excess fatty tissue, but may cause exaggerated cardiovascular effects

Schnider

May be more suitable as it uses age, height, weight and sex to derive compartment sizes and calculates an individualised k_e0
Does, however, use a fixed central compartment size
The internal algorithm calculates LBW from TBW using the James formula, so should input the TBW

Thiopentone

Use IBW for induction dose due to increased V_D and longer elimination half-life

Fentanyl

Rapid onset
Rapid offset in obesity due to high V_D, rapid redistribution (highly lipid soluble) and increased clearance
However, can accumulate significantly when infused due to high lipid solubility and V_D, resulting in prolonged elimination half-life and clinical effect
Similar effect seen in alfentanil although the prolonged effect is more related to low clearance
For both fentanyl and alfentanil, LBW dosing is recommended

Remifentanil

Dosing by TBW can produce cardiovascular depression so dosing should use LBW
Minto model

Not validated in obesity
Uses weight, height and age to calculate a LBW
Cautious titration is required

Morpine & friends

Obese patients are at risk of opioid-related respiratory depression due to existing OSA and accumulation of morphine + active metabolites
Dose by LBW with cautious titration and monitoring
Codeine and tramadol can pose similar risks due to idiosyncratic metabolism (codeine) and active metabolites (tramadol)

NMBA

Suxamethonium: dosing by TBW is appropriate as it accounts for increase pseudocholinesterase activity

Non-depolarising agents

As they are small polar molecules with a small VD, rate of clearance is largely related to metabolism
As metabolism correlates with LBW, this should be used for dosing
Use of TBW leads to prolonged action

Reversal agents

Little data exists for sugammadex and evidence supports the use of IBW, ABW40 or TBW, although SOBA guidance suggests using ABW
Use ABW for neostigmine

Local Anaesthetics

Dosing by LBW or IBW is recommended
However, increased plasma protein levels may reduce free fraction of LA and therefore increase dose requirements
Central neuraxial techniques may be unpredictable because of the effects of obesity on the epidural space

Volatile agents

Decreased pulmonary uptake due to reduced FRC is offset by the increase in cardiac output
The peripheral compartment is poorly vascularised somewhat mitigating its increased size
More lipid-soluble agents may accumulate for prolonged procedures, but for short procedures there is little difference
Pharmacokinetics of sevoflurane and desflurane unaltered

The health risk from obesity is not uniform, with increased risk of obesity-associated health conditions in Black African-Caribbean, Asian and elderly populations

BMI

Obesity does not necessarily increased the risk of perioperative morbidity and mortality

Class 3 obesity is associated with increased post-operative morbidity and mortality
Conversely, class 1 & 2 patients often have a lower incidence of complications and mortality than normal weight patients, a finding across a range of surgeries
Underweight patients carry the highest risk of mortality among the different categories of BMI

This may be due to unrecognised differences between BMI groups rather than a beneficial effect from adipose tissue per se

Age

Increasing age is associated with reduced functional reserve, and carries an increased risk of post-operative morbidity and mortality
In obesity, this association is not as concrete

In Class 3 obese patients undergoing PLIF, age >65yrs was independently associated with increased risk of post-operative complications
Overall mortality was increased 3x in patients undergoing bariatric surgery aged >55yrs compared to those <55yrs

Distribution of fat

Central (abdominal/visceral) obesity is defined as:

Waist circumference >102cm (men) or >88cm (women)
(Or >90cm and >80cm in Asian men and women)

Adipose tissue distribution can be defined by CT and MRI
However simple measurement of waist circumference can identify those with greater perioperative risk
The distribution of fat predicts mortality more accurately than BMI

Central obesity has greater associated risks of:

Difficult airway/ventilation management
Cardiovascular disease
Metabolic syndrome
Overall peri-operative risk

Presence of comorbidities

The degree to which comorbidities increase peri-operative risk for obese patients is not clear
OSA is associated with difficult airway management, unplanned re-intubation and post-operative cardiopulmonary complication

Metabolic syndrome comprises a cluster of conditions and contributes majorly to perioperative morbidity and mortality, with increased risk of:

Developing cardiovascular disease
Developing T2DM
Post-operative cardiac complications (2-3x)
Post-operative pulmonary complications (1.5-2.5x)
Stroke
Sepsis

Functional capacity

The ability to achieve >4 METs indicates a lower risk
However, obese patients may not exert themselves to this degree and this does not alone define an unfit patient

Sarcopenic obesity is characterised by reduced muscle mass and functionality in the presence of obesity

There are trends towards adverse outcomes, particularly with age
Varying diagnostic criteria result in the exact risk being unclear
The presence of sarcopenic obesity is suggested by:

Poor grip strength
History of immobility
Slow gait

History and examination

Document weight and BMI
Elucidate functional status

Identify important factors such as:

Whether fat distribution is central or peripheral
The presence of metabolic syndrome
SpO₂ <95% on room air
Presence of other comorbidities that require investigation and optimisation

Investigations

These will be guided by the individual's comorbidities and the surgery planned

Respiratory

Resting SpO₂ on air
History: dyspnoea, wheeze
Screening e.g. STOP-BANG
FBC to check for polycythaemia
ABG to check for hypercapnoea and/or raised bicarbonate
Spirometry

Cardiovascular

Resting HR, NIBP
History: exertional chest pain, palpitations, overt features of heart failure, syncope
12-lead ECG
Exercise capacity ± CPET
TTE
Cardiology input

Metabolic

Serum glucose
HbA_1c
LFTs
Actively seek components of metabolic syndrome
Diabetes/endocrine input

Risk assessment

Risk prediction can be facilitated with the Obesity Surgery Mortality Risk Score (OS-MRS); it is only validated in bariatric surgery but may be used in non-bariatric surgery
The score can be used to plan the need for postoperative care; a score of 4-5 indicates a high-risk patient and should prompt consideration of post-operative HDU admission

Risk Factor	Score
BMI >50kg/m²	1
Male	1
Hypertension	1
Age >45yrs	1
Any risk factor for PE: OSA/OHS Previous VTE Right heart failure Pulmonary hypertension IVC filter inserted pre-operatively	Max 1

Considerations for day surgery

Obesity in-and-of itself is not a contraindication to day surgery
Other than a requirement for long-acting post-operative opioids, there are no significant anaesthetic or surgical factors which will otherwise contraindicate the obese patient having day surgery
Several patient factors may make day surgery inappropriate, such as:

Poor functional capacity
Unstable respiratory or cardiac disease
Metabolic syndrome
OS-MRS of 4 or 5
Previous VTE

Optimisation & planning

Consider NIV for patients with OSA/OHS
Medical management of cardiovascular and metabolic comorbidities
Weight loss where possible
Senior anaesthetist and senior surgeon on the list
Allow extra list time for both anaesthetic and surgical components
Plan for HDU/ICU post-operatively if appropriate; diabetes, respiratory disease and open abdominal surgery are associated with higher risk of unanticipated ICU admission

General equipment

Suitable gowns and theatre-wear of appropriate size

Appropriate transfer equipment should be available:

Patient should maintain their own mobility as much as possible
Equipment such as chairs, trolleys and beds should be safe to use at patient’s weight
May require use of hover mattresses or similar devices
Adequate staff members present to facilitate safe transfer

Oxford HELP pillow or ramping device
Large IPC's or TEDS (10x increased risk DVT)
Sufficient staff to move the patient
Once on the operating table, require:

Extra-wide extensions or arm gutters
Gel pads to protect pressure points as prolonged pressure can cause ischaemia and rhabdomyolysis (classically gluteal ischaemia)

Anaesthetic equipment

Large NIBP cuff; may need to use forearm for measurement
Do not routinely require intra-arterial BP monitoring
Predict difficult intubation therefore VL and associated difficult airway kit
Long spinal/epidural and regional needles if RA planned
Ultrasound machine in case of difficult vascular access
Ventilator capable of delivering suitable driving pressure and PEEP
Neuromuscular monitoring as higher potential for incomplete reversal of NMBA

Induction & airway

Obesity is associated with a higher risk of developing airway problems under anaesthesia, with NAP4 demonstrating:

2x rate of adverse events (esp. with SAD)
Higher failure of rescue techniques

Routine airway assessment should be performed, with particular emphasis on identifying:

Mallampati III (predicts difficult facemask ventilation and intubation)
Neck circumference >42cm (one of the best predictors of difficult intubation)
BMI >50 kg/m² (independent predictor of both difficult intubation and facemask ventilation)
The presence of a beard
Symptoms of gastro-oesophageal reflux disease

Induction

In theatre to mitigate unnecessary transfer risk and allow greater space and access to assistance
Induce in ramped position as maintains FRC, reduces dyspnoea and facilitates BVM/laryngoscopy

Airway choices

Likely to require intubation and therefore need appropriate equipment to facilitate this, including video laryngoscopy and adjuncts
A SAD may be appropriate; a 2ndgeneration device should be used and should definitely be used with caution if BMI >40kg/m²
An airway plan should be vocalised and DAS guidelines followed
Routine fibreoptic intubation is not recommended
FONA may be more difficult if required and have higher rate of failure/complications; may be appropriate in high risk cases to identify the depth of the cricothyroid membrane, vascular tissue, and mark relevant landmarks to improve the chance of success

Obesity is associated with an increased incidence of known risk factors for aspiration

However, obesity alone does not increase risk of reflux and pulmonary aspiration
In the absence of other risk factors, routine performance of RSI is not required

There is a higher risk of hypercapnoea and hypoxia, as increased body tissue mass and work of breathing leads to greater oxygen consumption and CO₂ production
FRC is reduced to closing capacity and causes atelectasis and hypoxia, due to:

Excess adipose tissue, which reduces chest wall compliance
Lying supine
General anaesthesia
Effect of pneumoperitoneum

Maintenance of anaesthesia

NAP5 revealed an increased incidence of awareness in obese patients shortly after induction of anaesthesia, due to rapid redistribution of anaesthetic agents
In order to reduce occurrence, anaesthetists should ensure:

Adequate dosing of IV agent
Prompt delivery of maintenance anaesthetic agent
Further bolus(es) of anaesthetic agent before airway manipulation or protracted airway manoeuvres
Using processed EEG-based depth of anaesthesia monitoring to reduce risk of awareness

TIVA with propofol offers a number of potential advantages over volatile anaesthesia for the obese patient:

Reduced incidence of laryngospasm
Maintained anaesthesia during protracted airway manipulation
Rapid offset with ‘clear-headed’ emergence
Reliable clearance of hypnotic agents
Reduced PONV

Using volatile agents with a rapid offset of action (low blood:gas partition coefficient) e.g. desflurane/sevoflurane should limit adipose absorption and reduce risk of re-sedation at emergence

Ventilation

Obesity is an independent risk factor for developing postoperative pulmonary complications
Current recommendations for ventilation include:

Lung protective volumes (6-8ml/kg)
Plateau pressure <30cmH₂O
PEEP titrated to respiratory and cardiovascular state, typically 8 - 10cmH₂O

Judicious use of recruitment manoeuvres where there is suspicious of atelectasis/collapse
Minimising the effect of positioning on ventilation is important, e.g. Trendelenburg position in lower abdominal surgery with laparoscopy

Reversal of NMBA

The hazards of airway/respiratory problems during emergency are greater in the obese population
To mitigate for this ensure:

Ventilate with F_iO₂ 1.0 until P_EO₂ >0.9
Sit upright or at least ramped
Full reversal of NMBA with appropriate doses of sugammadex or neostigmine
Only extubate once fully awake

Effect of previous bariatric surgery

Patients may have undergone previous bariatric surgery
This may influence conduct of anaesthesia

Adjustable gastric banding

High risk of aspiration even with prolonged fasting times
Antacid premedication + RSI technique
Avoid routine NGT insertion as risk of band displacement or perforation
Do not deflate band without discussing with a bariatric surgeon

Sleeve gastrectomy

Routinely intubate even if no other risk factors for aspiration
High risk of gastric reflux even in the absence of symptoms

Malabsorptive surgery

Small bowel shortening can affect oral bioavailability of drugs, in particular post-operative analgesia

Recover in upright position
Consider HDU care for those with existing hypoventilatory issues, need for NIV or other need for closer monitoring

Multi-modal opioid-sparing analgesic strategy

Aim to avoid opioids where possible to limit respiratory depression
If using, use short-acting opioids e.g. fentanyl
If IV opioids required continuous saturations monitoring should be in situ

Multi-modal anti-emesis as standard

Robust venous thromboprophylaxis

Early mobilisation
Chemical prophylaxis based on total body weight dosing

Obesity