Monday, February 20, 2017

How long should we fast animals prior to surgery?

Hero: "Clearly you forgot to feed me so I will hang out in your gym gear til you remember."

What’s the hardest thing about taking your pet to the vet? When I talk to clients, they report that - after trying to get the cat into a cat carrier - its fasting.

When dogs or cats undergo a procedure, one of the recommendations is pre-operative fasting. Easier said than done. This is one of the more challenging aspects of treatment of the animals I live with, because they’re not stupid. They know the routine and when breakfast isn’t laid out, they alert me. Repeatedly. Desperately. And unceasingly.

I try all kinds of stunts, for example pretending to sleep in. They know. I adjust my routine to pretend its earlier than it looks. They know.

Humans I know who have undergone procedures under anaesthetic have been almost equally vocal in recounting the inconvenience and challenge of fasting, especially when paired with some sort of laxative preparation (fortunately, animals don’t generally need these).

The reasons we fast animals prior to anaesthetic are to reduce the risk of regurgitation, reduce the risk of aspiration (and aspiration pneumonia), and sometimes because we want an empty gut to work with (for abdominal procedures). It can be hard to tell if they experience post-operative nausea, though some animals clearly do. There are some situations where we don’t have the option to fast – emergency procedures like passing a urinary catheter in a cat with an obstruction, for example.

The American Animal Hospital Association recommends a four hour fast for patients age 6-16 weeks to minimise the risk of post-operative hypoglycaemia. However, for animals over 16 weeks, “overnight” fasting is recommended “for procedures scheduled earlier in the day”. But what counts as overnight? Is dinner the night before allowed? A midnight snack? And what is “earlier in the day”? I’m a morning person so I’d say before 9am, though I have colleagues who’d define morning loosely as something that happens between 10am and 2pm. I suspect the AAHA guidelines are shooting for somewhere in the middle.

One study found no difference in blood glucose, recovery, blood gas or cardiorespiratory parameters in cats anaesthetised with tiletamine-zolazepam fasted for 8, 12 or 18 hours respectively.

In its 2016 Guidelines, the Association of Shelter Veterinarians states that fasting animals over 16 weeks of age for more than 6 hours is unnecessary.

This recommendation appears to be based on a study which found that dogs fed tinned food at half their normal ration three hours before an anaesthetic did not have significantly increased gastric content and might have a reduced incidence of gastro-oesophageal reflux.

It would be helpful to look into the risks of benefits of fasting periods in dogs and cats further so that we can determine optimal fasting times while maximising patient comfort and welfare. 

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