Anaesthetic care with the obese patient

With just under half of all cats and dogs and a third of rabbits being considered overweight by vets in the UK, we take a look at the extra care we take when anaesthetising our patients

Posted: 14 September 2017

Anaesthetic care with the obese patient

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With just under half of all cats and dogs and a third of rabbits being considered overweight by vets in the UK, it’s no wonder that we are seeing more and more problems associated with overfeeding. Obesity is an energy imbalance that results in an animal being more than 20% above its ideal weight. With this weight gain comes significant risks, a problem more and more of us have to take into account when operating and anaesthetising in practice.

Why does obesity increase the risk?

Obese patients can be safely anaesthetised as long as any problems associated with cardiovascular and respiratory function are recognised and appropriate anaesthetic and analgesic drugs and doses are used.

Some of the more serious conditions to be aware of as a result of excessive body fat include;

  • Increased respiratory effort – fat in the thorax restricts lung capacity
  • Cardiac impairment – fat deposits around the heart, congestive heart failure and cardiomegaly are commonly found
  • Circulatory disorders – hypertension, aortic compression, poor perfusions of limbs
  • Respiratory diseases - pulmonary hypertension and embolisms
  • Hormonal complications - diabetes mellitus, Cushing’s disease and hypothyroidism
  • Skeletal problems - osteoarthritis of weight-bearing joints

Pre-anaesthetic testing

When compared to patients with a normal BMI, the pre-anaesthetic examination and health check in the obese animal needs even more care and attention. A thorough physical examination should be performed checking in detail both the respiratory and cardiovascular systems. Whenever possible, additional tests should be offered, including blood pressure measurement, ECG’s and thoracic x-rays to check for normal or abnormal function.

Blood tests should always be offered to check renal function, hydration status, electrolyte balance, full haematology and urinalysis. It may also be indicated to check hormone levels if diabetes, Cushing’s or hyperthyroidism is suspected.

Prior to any anaesthetic it may also be necessary to provide intravenous fluids, antibiotics, heart medications, insulin or hormone therapy. For some animals the risk of an anaesthetic may outweigh the risk of waiting until some weight loss has been achieved, in which case a weight loss program may be the best way forward.

Which anaesthetic?

Choosing the right anaesthetic depends on many factors. The results of the clinical examination and ancillary tests are important as are the length of anaesthetic required and the degree of pain that’s expected. The best drugs are those that provide a rapid induction and recovery and have good pain relief. They also need to have minimal effects on the cardiovascular and respiratory function. Alpha-2 agonist drugs are best avoided because of the significant cardiovascular-depressing effects of these drugs.

Calculating the dose

In general obese animals should have drug doses calculated according to their lean body weight and not their obese weight. This helps to avoid an overdose. As a guide animals with body condition scores greater than 6/9 should have the induction dose reduced by 20% or more and then the anaesthetic given ‘to effect’.

Care with cardiac disease

Protocols using opioid drugs (eg. butorphanol and buprenorphine) can provide adequate sedation and analgesia prior to induction. When used appropriately they can also help to reduce the induction dose and levels of inhalant agent required to maintain anaesthesia, so minimising the impact on the cardiovascular system.

Obese patients should be continually monitored with ECG’s, blood pressure testing, pulse oximetry and end-tidal carbon dioxide measurement during anaesthesia.

Care with respiratory disease

Drugs that depress the respiratory system (eg. opioids, barbiturates, and propofol) should be used with caution in animals with respiratory disease. Close attention should be paid to respiratory function throughout. Use effective respiratory monitoring - check respiration rates and monitor capnograph.

Control of the airway with an appropriately sized endotracheal tube is essential. Many overweight patients have difficulty with ventilation. Fat deposits in the tissues surrounding the airways can obstruct airflow, particularly in the pharyngeal region.

Supplemental oxygen and should be continued during recovery until the animal has regained full consciousness and complete airway control.

Care with liver disease

Patients with raised liver enzymes or known hepatic dysfunction may be best suited for induction with propofol because of the extra-hepatic mechanisms of metabolism.

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