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Obesity and Anaesthesia

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Obesity and Anaesthesia
Photo by Towfiqu barbhuiya / Unsplash

Take home messages

  • Obesity has a whole host of impacts on how you conduct your anaesthetic
  • The more of the risks you mitigate against, the smoother the procedure
  • See rule #6

A rant

"It's a five minute job, just a quick GA," quipped the surgical SHO.

The patient in question had a 2cm x 2cm abdominal wall abscess and had flat out refused to have it incised and drained under local anaesthetic, so the plan was for a 'quick GA'.

She was 132kg, smoked 15 a day, had 'moderate' asthma - and otherwise had no diagnosed medical conditions.

After positioning her on a bariatric trolley, ramped with Oxford HELP pillow, THRIVE and five full minutes of preoxygenation, she promptly desaturated to 78% within thirty seconds of muscle relaxation, requiring fairly sporting two-person facemask ventilation to re-oxygenate her. Mercifully the intubation was relatively easy on this occasion.

The surgical SHO was correct - it was a five minute job.

Followed by five minutes of bronchospasm, five minutes of resedation for dysynchrony with the ventilator, and five minutes of jaw thrusting after the tube was out to hold her airway open.

Rule number 6 - a simple operation does not equal a simple anaesthetic.

Obesity is a growing problem

Literally. And it's getting worse.

It will come as no suprise that obesity increases the risk of adverse events in anaesthesia and surgery, for a variety of anatomical, physiological and metabolic reasons.

  • A BMI of 40 or more increases risk all by itself
  • A BMI of 30 or more increases risk when another comorbidity is present
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Day surgery is usually inappropriate if BMI is greater than 50 kg/m2

What are the categories of obesity?

  • Class 1 - BMI 30-35
  • Class 2 - BMI 35-40 - ASA 2
  • Class 3 - BMI >40 - ASA 3

The terms 'morbid' and 'supermorbid' have been replaced by the above classes.

What problems does it cause?

This is an exceedingly popular exam question, because it's an enormous topic and very relevant to modern-day anaesthesia, with a greater proportion of the population becoming obese year on year.

The UK has the sixth highest level of obesity, with a whopping 64% of the population either overweight or obese, and as you might expect this is a greater issue in lower-income areas.

The easiest way to address this question is probably by breaking it down into the following topics:

  • Airway
  • Breathing
  • Circulation
  • Pharmacology
  • Logistics

Airway problems

  • Large neck
  • Reduced neck movement
  • Greater pharyngeal soft tissue mass
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You don't need to do an RSI just because of obesity, unless there are other indications.

Breathing problems

Obesity is an independent risk factor for developing postoperative pulmonary complications. 

  • Obstructive sleep apnoea (and all of its complications)
  • Obesity hypoventilation syndrome
  • Reduced FRC
  • Rapid atelectasis due to weight on thorax
  • Larger closing capacity*

*This is the volume of the thoracic cage below which the small airways start to close. If the functional residual capacity is smaller than the closing capacity, then airway closure will occur in an anaesthetised patient, producing rapid desaturation and increased ventilatory pressures.

Circulation problems

  • Pulmonary hypertension
  • Underlying undiagnosed heart disease
  • VTE risk
💡
Post operative DVT risk is 10x higher in obese patients

Logistics issues

  • Bariatric beds and tables
  • More people needed to move patient
  • Positioning implications - see below
  • Hover mattress
  • Hoists
  • Pressure points - particularly gluteal ischaemia if lying supine for a long time
  • Inducing in theatre reduces the extra unnecessary risks of disconnecting and moving a high risk patient
💡
Note that a bed or operating table's safe weight limit might be different in the head-down or head up positions so check first!

Tube it or lose it

The fourth national audit project found that obese patients were twice as likely to have airway problems during an operation, particularly when a supraglottic airway was used.

If you would kindly refer to the third rule of anaesthesia - if you've thought about a tube, it's time to tube - it will come as no surprise that intubation is the safest airway plan in the obese patient, especially if:

  • The abdomen is going to be insufflated for laparoscopy
  • The patient is going to be in extreme positions (head down/lateral/prone)
  • The patient has a lot of thoracic adipose tissue, needing high ventilatory pressures

So if in doubt, get a tube out.

Of course you can use a supraglottic airway if you're comfortable and experienced doing so and the patient and procedure are suitable, but we'll leave that one up to you.

💡
If the BMI is over 40, it's harder to justify a supraglottic airway

Which factors, specific to obesity, make intubation more difficult?

  • Neck circumference >42cm
  • BMI >50kg/m2
  • Central adiposity
  • Large breasts or thoracic mass
  • Reduced safe apnoeic time

If you're concerned about the possibility of needing to do front of neck access, you can ultrasound the neck before induction to identify the cricoid location, depth and presence of vasculature above it.

Do I need an arterial line or special monitoring?

Not necessarily, but sometimes an invasive arterial line might work better than an NIBP cuff, especially if they have large, short limbs.

Reasons to ask for the ultrasound

  • Difficult IV access
  • Visualisation of neck anatomy
  • Regional anaesthesia
  • Because it's fun

Pharmacological considerations

Obese patients have more fat, but also have more non-adipose tissue than patients with a normal BMI.

  • Lean body weight takes this into account, and is therefore larger than ideal body weight

Lean body weight is probably better generally for most drugs, except for:

  • Vasopressors including adrenaline and noradrenaline should be scaled to ideal body weight
  • Suxamethonium should be scaled to total body weight
💡
Note that pharmacokinetic models, such as TCI models, aren't validate in obese patients, so BIS/EEG monitoring is advised.

What are the benefits of TIVA for an obese patient?

  • Quicker emergence
  • Less PONV
  • Lower risk of awareness during airway manipulation
  • Less laryngospasm

After the operation

Early mobilisation is the golden rule of postoperative rehabilitation, and this can be more difficult for overweight and obese patients, so ensuring they have adequate pain relief and the right facilities and support to mobilise as early as possible is crucial.

It's not just about BMI

Weirdly enough, a lot of data suggests that a 'bit' of obesity (i.e. class 1 and 2) actually seems to reduce your perioperative morbidity and mortality when compared with those of normal weight, and actually being underweight is associated with the highest perioperative risk.

The most likely reason is that this is because obesity is measured by BMI, and BMI is a fairly terrible way of measuring someone's body fat distribution, especially if a patient has higher muscle mass, which as we all know - is vastly denser than fat.

This guy's BMI is at least 'overweight'

Bear in mind, BMI as a measuring tool was developed 200 years ago. It only accounts for mass, and does not make any distinction as to what type of tissue is contributing, or how metabolically broken it is.

For example - sarcopenic obesity is being overweight despite reduced muscle mass, which as you might expect - is very bad news.

Surely there's a better way to measure obesity?

Location, location, location

It's one thing to consider the amount of adipose tissue a patient is carrying, but it's equally important to think about the distribution of that fat throughout the body.

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Fat distribution predicts morbidity and mortality much better than BMI

WHO definition of central obesity

Waist circumference

  • >102 cm for men
  • >88 cm for women
  • >90 cm for Asian men
  • >80 cm for Asian women

For whom is obesity even more of an issue?

There are some groups for whom obesity increases perioperative risk to an even greater extent than others:

  • Asian
  • African-Caribbean
  • Elderly

Positioning

Many obese patients with describe being unable to lie flat on their back because they become breathless, largely because the abdominal contents press upwards on the diaphragm, reducing tidal volumes and increasing basal atelectasis.

So it will come as no surprise, therefore, that lying an obese patient flat on their back for induction of anaesthesia is distinctly suboptimal, as it drastically reduces their functional residual capacity and the safe apnoeic time for an already difficult intubation.

If in doubt - ramp the patient!

The Oxford HELP pillow is designed to optimise the patient's position for airway management and oxygenation. Learn to use it, and don't be scared to ask for it if you have any concerns about intubation.

💡
During laparoscopy for upper abdominal surgery the patient should be in the 20° reverse Trendelenburg position with 45° hip flexion to reduce airway pressures

Why does ramping the patient prior to induction help?

  • More comfortable with less shortness of breath
  • Easier laryngoscopy
  • Improved FRC
  • Easier facemask ventilation

A free CRQ

Tap the image for a free CRQ on difficult intubation from our wonderful colleagues at frca-revision.com

Useful Tweets and Resources

References and Further Reading

Primary FRCA Toolkit

While this subject is largely the remit of the Final FRCA examination, up to 20% of the exam can cover Primary material, so don't get caught out!

Members receive 60% discount off the FRCA Primary Toolkit. If you have previously purchased a toolkit at full price, please email anaestheasier@gmail.com for a retrospective discount.

Discount is applied as 6 months free membership - please don't hesitate to email Anaestheasier@gmail.com if you have any questions!

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