Anaesthesia for the obese patient

Question 1

Obesity

Epidemiology

Answer 1

The prevalence of obesity has tripled over the past 40 yrs and there
are now more than 650 million obese adults worldwide.

Increases are greater in developing compared to developed countries

UK and continental Europe
The UK has the sixth highest prevalence of obesity according
to the Organisation for Economic Cooperation and
Development (OECD). Some 64% of UK adults are overweight
or obese, with lower socioeconomic groups over-represented.
Only five European countries have a prevalence of obesity
amongst adults of <20%. In these countries, more than half of
all adults are overweight.

Ireland 2nd highest in europe

Question 2

Obesity

Epidemiology

Answer 2

The prevalence of obesity has tripled over the past 40 yrs and there
are now more than 650 million obese adults worldwide.

Increases are greater in developing compared to developed countries

UK and continental Europe
The UK has the sixth highest prevalence of obesity according
to the Organisation for Economic Cooperation and
Development (OECD). Some 64% of UK adults are overweight
or obese, with lower socioeconomic groups over-represented.
Only five European countries have a prevalence of obesity
amongst adults of <20%. In these countries, more than half of
all adults are overweight.

Ireland 2nd highest in europe

Question 3

Categorisation of obesity

Answer 3

Categorisation of obesity
Obsolete classifications include ‘morbid obesity’, ‘super obesity’,
‘super-morbidly obese’ and ‘super super morbidly obese’.

BMI
BMI is a commonly used measurement of obesity, its advantage being that it is easy to calculate.

BMI does not describe the composition and distribution of body tissue (muscle/adipose) or metabolic state.

These are important factors in terms of
pathophysiology,
perioperative risk and management.

BMI can be useful to alert teams and allow planning and preparation.

Table 1 details the latest classification of obesity according to BMI, and the ASA grade corresponding to each
class.

Question 4

Table 1 WHO classification of obesity and ASA grades

Answer 4

Category BMI (kg/m2) ASA grade

Underweight <18.5 1

Normal weight 18.5e24.9 1

Overweight 25 1

Pre-obese 25-29.9 1

Obese 30 1
Class 1 30-34.9 1

Class 2 35-39.9 2

Class 3 >40 3

Question 5

BMI and the ‘obesity paradox’

Answer 5

Class 3 obesity is associated with increased postoperative
morbidity and mortality.

However, overweight, Class 1 and 2
obese patients often have a lower incidence of complications
and mortality than patients of normal weight.

This is found across a range of surgical procedures including hip fractures
(up to 35% less chance of death within 1 yr).2

Patients with low
BMI (<18.5 kg m2 ) tend to have the highest mortality rates of
the different BMI groups.

This ‘obesity paradox’ whereby obesity may convey health benefits
is not fully understood.

It is proposed that rather than direct health benefits from
excess adipose tissue that the ‘obesity paradox’ is a consequence of unrecognised differences between BMI groups,
for example cigarette smokers and patients
with disease-induced
weight loss, but the ‘obesity paradox’ is still seen in studies
where smoking has been accounted for

Question 6

Age

Answer 6

Increasing age is associated with reduced functional reserve

and can be associated with an increased postoperative mortality and morbidity.
In obesity this association is not fully supported.

Age >65 yrs was found to be an independent risk factor for postoperative complications in Class 3 obese patients undergoing
posterior lumbar fusion.4

The development of severe life threatening complications
after bariatric surgery was not
linked to age in another study.

Overall mortality was

Question 7

Distribution of fat

Answer 7

The terms ‘apples’ and ‘pears’ are commonly used to describe
predominantly central (abdominal/visceral) and peripheral
adipose tissue, respectively.

Central obesity is defined by
WHO as a waist circumference
>102 cm and >88 cm for men
and women, respectively,

and in the Asian population >90 cm
and >80 cm.

Adipose tissue distribution can be accurately defined by CT
and MRI.

Simply measuring waist circumference or visual
assessment of the fat distribution of patients will identify
those with greater perioperative risk and associated
comorbidities.

Fat distribution predicts risk more accurately than BMI;
central obesity has associated greater risks of metabolic syndrome (MetS), cardiovascular disease, difficulty in airway and
ventilation management and overall perioperative risk

Question 8

Presence of comorbidities

Answer 8

Many of the comorbidities associated with obesity are known
to increase perioperative risk.

Sleep disordered breathing is
associated with difficult airway management, unplanned
tracheal reintubation and postoperative cardiopulmonary
complications.

Risks can be reduced by good preoperative
optimisation and compliance.

The degree to which comorbidities increase perioperative risk for obese patients is not clear, and studies investigating this may be affected by the
‘obesity paradox’.

Question 9

MetS

Answer 9

Metabolic syndrome comprises a cluster of conditions and is
increasingly recognised as a major contributor to perioperative morbidity and mortality.

Compared with the obese patient who does not have MetS there is an increased risk of developing cardiovascular disease and an even greater risk of
developing Type 2 diabetes mellitus.

Numerous international
diagnostic criteria exist,
each requiring the presence of three
of the factors listed in Table 2.

Obesity and MetS cause significant increases in
postoperative mortality in all classes of obesity,
with greater risks of postoperative cardiac complications (2-3 risk),
pulmonary complications (1.5-2.5 risk), acute kidney injury, stroke
and development of sepsis compared with patients of normal
weight

Question 10

Table 2

Health conditions associated with obesity, suggestive features and the perioperative actions required. ABG, arterial blood gas;
BiPAP, bilevel positive airway pressure; CPET, cardiopulmonary exercise testing; DM, diabetes mellitus; ECHO, echocardiogram; HDL,
high-density lipoproteins; HDU, high dependency unit; LFTs, liver function tests; METS, metabolic equivalents; NAFLD, non-alcoholic
fatty liver disease; NASH, non-alcoholic steatohepatitis; OHS, obesity hypoventilation syndrome; OSA, obstructive sleep apnoea; SBP,
systolic BP; SDB, sleep disordered breathing

Question 11

Resp

Answer 11

Conditions Suggestive features Actions required before surgery

Respiratory

Sleep-disordered breathing (OSA, OHS)

Shortness of breath

SpO2<95% breathing air

STOP-BANG >=5

OHS-BMI>30,
hypercapnia when awake,
raised HCO3,
hypoxia,
exclusion other causes of hypoventilation

ABG initially

Spirometry

CPET if abnormalities found in above tests

Airway planning
4x increased risk of difficult intubation and
difficult mask ventilation

Refer for further investigation and treatment

Commence CPAP before surgery and continue after surgery

BiPAP sometimes necessary for improvement in symptoms
(especially OHS)

Plan for postoperative HDU/ICU admission if symptoms not
improved by time of surgery

Question 11

Resp

Answer 11

Conditions Suggestive features Actions required before surgery

Respiratory

Sleep-disordered breathing (OSA, OHS)

Shortness of breath

SpO2<95% breathing air

STOP-BANG >=5

OHS-BMI>30,
hypercapnia when awake,
raised HCO3,
hypoxia,
exclusion other causes of hypoventilation

ABG initially

Spirometry

CPET if abnormalities found in above tests

Airway planning
4x increased risk of difficult intubation and
difficult mask ventilation

Refer for further investigation and treatment

Commence CPAP before surgery and continue after surgery

BiPAP sometimes necessary for improvement in symptoms
(especially OHS)

Plan for postoperative HDU/ICU admission if symptoms not
improved by time of surgery

Question 12

Asthma

Answer 12

Dyspnoea
Wheezing

Asthmatic symptoms common
but reversibility with b2- agonists not always found
cause is partly chronic proinflammatory state from
excess adipose tissue,
and fat within/
around chest/abdomen causing small airway collapse

Weight loss:
symptoms from both ‘classical’ asthma and fatr elated wheeze will improve

Question 13

Cardiovascular

Answer 13

Cardiovascular

Hypertension
Left ventricular hypertrophy
Left ventricular failure
Conduction abnormalities
Cardiomyopathy

Clinical signs of heart failure
History of cardiac syncope.
Increased SBP
Reduced exercise capacity

Preoperative ECG
ECHO if structural or functional disease suspected
Referral to cardiologist
Medical management and optimisation before surgery

Right heart failure Pulmonary hypertension
resulting from sleep-disordered
breathing
Polycythaemia

Question 14

Reduced functional capacity

Answer 14

Difficult to assess in obese
patients

Ability to achieve 4 METS
indicates fitness and low risk patient

Assess ability to walk on a flat level surface
Assess climbing stairs

CPET:
not routine, but can be used where assessment suggests high
risk patient
equipment may not tolerate patients’ weights
obesity can confuse interpretation of results

Question 15

Metabolic

Answer 15

Metabolic
Diabetes mellitus

Deranged serum glucose,
HbA1C,
or associated complications

Optimise glucose control with referral to endocrinologist
Avoid delaying surgery based on HbA1C concentrations only

Question 16

Liver involvement

Answer 16

Liver involvement
(NASH/NAFLD)
Evidence of cirrhosis, deranged
LFTs
Liver-shrinking diet (<1000 calories per day) can reverse the
disease processes somewhat
Metabolic syndrome Central obesity, hypertension,
impaired glucose handling or DM,
increased triglycerides, decreased
HDL cholesterol
Actively seek components of metabolic syndrome (three or
more required for diagnosis)

Question 17

Obesity surgery mortality risk score

Answer 17

OS-MRS
Risk factors Points scored
BMI>50 kg m2 1

Male 1

Hypertension 1

Age >45 yr 1

Risk factors for PE (previous VTE,
preoperative vena cava filter, OSA/OHS,
right heart failure, pulmonary
hypertension)
Maximum 1

Risk prediction can be facilitated with the obesity surgerymortality risk score (Table 3). The score can be used to plan the
need for postoperative care; a score of 4e5 indicates a highrisk patient and should prompt consideration of postoperative admission to critical care

Question 18

Organisation and equipment

Preparation and dignity

Answer 18

Preoperative assessment should assess the ability of a patient
to mobilise on the day of surgery and after surgery to enable
planning of equipment, personnel and identification of any
potential hazards or unsuitability

Preparation and dignity

Obese patients are mindful of societal views about their size;
concerns about their self-image figure highly.

Dignity is important in the obese individual.

Ensure theatre gowns of an appropriate size are available,
appropriately-sized underwear,
and rooms are accessible with toilets and showers that can
accommodate the patient’s size and weight.

The risk of deep vein thrombosis is increased in the obese,
with up to 10 times
greater incidence after surgery.11

Deep vein thrombosis risk should be assessed and prophylaxis measures following national guidance used for all obese patients.

Question 19

Moving and positioning

Answer 19

Moving and positioning

Appropriate equipment availability to transfer the patient is
required.

This may include transport from home into the
hospital, in which case appropriate vehicles are necessary.

Chairs, trolleys, beds and operating tables must be safe for use
up to or above the patient’s weight, and through the full
anticipated range of positions and movements required (e.g.
operating tables may be safe to use for a given weight in the
neutral position, but not able to be positioned head down or
head up at maximum weight).

It should be confirmed before surgery that the patient’s accommodation is acceptable: it is mandatory to maintain their dignity and safety.

Adjuncts can be utilised to accommodate patients where it is confirmed the
equipment can hold the weight, such as extra-wide extensions for operating tables and arm gutters.

Wherever possible the patient should maintain their own
mobility (e.g. to walk to theatre and position themselves on
the operating table). Gel pads should be used to protect pressure points. Attention to these areas during anaesthesia is
vital; numerous reports define crush injury from patient
positioning (classically from gluteal ischaemia with prolonged
pressure at the time of surgery) and rhabdomyolysis. At the
end of surgery, transfer from the operating table to ward bed
can be facilitated with slide sheets or hover mattresses. The
patient can remain on these devices to aid postoperative care

Question 20

Monitoring and ventilation

Answer 20

Invasive arterial monitoring is not routine for obese patients.

Non-invasive BP cuffs should be of the correct size.

The shape of the arm may make accurate measurement impossible on
the upper arm, in which case the forearm can be used.

Anaesthetic ventilators should be capable of delivering high
driving pressures and able to deliver PEEP.

The potential for incomplete neuromuscular block antagonism demands
routine neuromuscular monitoring.

Question 21

Ultrasound

Postoperative

Answer 21

Ultrasound
Ultrasound visualisation may be useful to facilitate successful
regional blocks and in cases of difficult venous access.

Postoperative
Walking aids and structures that can be attached to hospital
beds to improve patient independence and rehabilitation
should be available.

Question 22

Induction of anaesthesia

Position

Answer 22

Induction of anaesthesia

Induction of anaesthesia should be undertaken in the operating theatre. This mitigates unnecessary risk created by
transferring an anaesthetised obese patient into the operating
room, and makes the management of any adverse events
more practical with sufficient assistance available.

Patient position

Anaesthesia should be induced in the head up
ramped position with elevation of the upper body
until the ear tragus and sternal notch are aligned horizontally.

This can be achieved by folding sheets and blankets under the head and neck or using specialised pillows.

This position offers advantages:

(i) increased comfort for the patient;
(ii) functional residual capacity is maintained;
(iii) reduced dyspnoea;
(iv) bag-mask ventilation is facilitated;
(v) laryngoscopy is improved.

Question 23

Airway

Answer 23

Airway

Standard airway devices and adjuncts should be available
including supraglottic airway devices (SAD),
video laryngoscopes and, when indicated, fibreoptic endoscopes.

There are
no special or preferred choices of equipment for airway
management in obese patients.

Tracheal intubation is the recommended technique for
airway management. Whatever aid to intubation is used, it
should be one with which the anaesthetist is most familiar
with and which offers the best chance of success. For experienced practitioners, after full assessment and consideration
of the proposed surgical procedure, the use of an SAD may be
appropriate. This should be with a second-generation device,
and caution is strongly advised when BMI is >40.

Obesity is associated with a higher risk of developing
airway problems under anaesthesia. The UK NAP4 study
revealed twice the rate of adverse events, particularly with the
use of SAD, and a greater failure rate of rescue techniques.13
Routine airway assessment defining indicators of potential
difficult laryngoscopy or difficult ventilation should always be
performed, particularly identifying:

Question 24

Predictors of difficult airway

Answer 24

(i) Mallampati score 3 (predicts difficult facemask ventilation and intubation)14;
(ii) neck circumference >42 cm (predicts difficult
intubation)15;
(iii) BMI >50 kg m-2 (independent predictor of both difficult
intubation and facemask ventilation)16;
(iv) the presence of a beard;
(v) symptoms of gastro-oesophageal reflux disease.

The

Question 25

Def difficult aiwary

Answer 25

varying definitions of a ‘difficult airway’ lead to difficulty in identifying good predictive features. Neck circumference appears to be one of the best predictors in the morbidly
obese.17

Obese patients have increased oxygen consumption and
CO2 production because of increased total body tissue mass
and increased work of breathing. This can lead to hypercapnia
and hypoxaemia when ventilation is impaired. Excess adipose
tissue reduces chest wall compliance, which reduces functional residual capacity (FRC) to closing capacity, leading to
atelectasis. Further reductions in FRC during general anaesthesia, when lying supine and during pneumoperitoneum, will exacerbate this

The onset of hypoxaemia can be delayed by preoxygenation of the lungs, induction of anaesthesia in the semi-upright
position with application of CPAP or high-flow nasal oxygen,
and avoidance of prolonged apnoea. Currently there is no
evidence to demonstrate one technique is superior to another.
Routine awake fibreoptic airway management is not indicated unless a specific reason is identified at preassessment.
A backup plan for airway management should be agreed
should the primary plan fail. This must be communicated to
the whole theatre team, and should follow the Difficult Airway
Society guidelines.
Front of neck airway access may be more difficult with a
risk of complications. This should be considered when planning airway management; in high-risk cases it may be
appropriate to identify the depth of the cricothyroid membrane, vascular tissue, and mark relevant landmarks to
improve the chance of success of emergency front of neck
access.

Question 26

Routine RSI

Answer 26

Obesity is associated with an increased incidence of known
risk factors for aspiration, such as hiatus hernia and diabetes
mellitus with autonomic neuropathy causing delayed gastric
emptying. However, evidence reveals that obesity alone does
not increase the risk of reflux and pulmonary aspiration. In
the absence of any risk factors for aspiration (e.g. a patient
who is unfasted, or who has intra-abdominal pathology)
performing an RSI is not routinely required.

Failed intubation after induction of anaesthesia requires
antagonism of neuromuscular block. When sugammadex is
available there is much greater control of the antagonism of
rocuronium-rather than suxamethonium-induced neuromuscular block, with longer or shorter block duration as
required. A recent article in this journal offers analysis of the
factors that influence how an RSI is conducted.18

Question 27

Awareness

Answer 27

Awareness
The NAP5 study revealed an increased incidence of awareness
in obese patients shortly after induction of anaesthesia.19 This
is attributable to the rapid redistribution of i.v. anaesthetic
agents. In order to reduce occurrence, anaesthetists should
ensure adequate dosing of i.v. anaesthetic agent, prompt delivery of maintenance anaesthetic agent and further i.v. bolus(es) of anaesthetic agent before airway manipulation or protracted airway manoeuvres.

Question 28

Principles of drug dosing

Answer 28

Principles of drug dosing

The principles of dosing anaesthetic drugs for obese patients
have recently been described in this journal.20

Lean body weight reflects excess non-adipose tissue of obese patients.

This can be considerably greater than ideal body weight,
generally peaking at 100 kg and 70 kg for male and female
patients, respectively.

Lean body weight is usually the appropriate scalar to calculate drug doses,
with a few exceptions.

Question 29

Exceptions to lean body weight dosing

Answer 29

Emergency medications such as noradrenaline (norepinephrine) and adrenaline (epinephrine) are dosed according to ideal body weight,

suxamethonium according to total body weight

and the minimum dose of atropine according to lean body weight,
as lower doses can cause paradoxical bradycardia