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Fasting guidelines


Some things in emergency medicine go together, like peanut butter and chocolate – take monkey bars and supracondylar fractures, for example. Jack fell off the monkey bars after school and has a deformed arm. With the help of some inhaled methoxyflurane and intranasal fentanyl, he has been placed in an above-elbow backslab and is awaiting theatre.

The orthopaedic team is on the way down to consent him and wants to know when he last ate. Jack had emptied his lunch box by the middle of the morning but was still starving.

So hungry was Jack that his mother had gone to the local drive-through and picked up some fries. Time slows down to a crawl, the scream of “Nooooooooooooooooooo” on your lips as he crams a fistful of the salty delights into his face…

So, what is the big deal with fasting for theatre?

Way back when (1883, to be precise), this man, Joseph Lister, said…”

“While it is desirable that there should be no solid matter in the stomach when chloroform is administered, it will be found very salutary to give a cup of tea or a beef-tea about two hours previously…”

Regurgitation of stomach contents whilst under anaesthesia is a very real risk, as Curtis Mendelson pointed out in 1946. It may be associated with pneumonitis and aspiration pneumonia and potentially cause significant morbidity and mortality, though the incidence is exceedingly small, in the realm of 0.7-0.1%.  

But we have not evolved to go for prolonged periods with an empty stomach, and going more than six hours without eating may be bad for us if we need an operation.

Besides making us (and out-patients) hangry, it may lead to dehydration and hypoglycaemia, especially in infants and neonates.  Prolonged fasting does not guarantee an empty stomach (Bouvet 2017, Van de Putte 2017).

It is always worth returning to the primary literature to see where these recommendations regarding aspiration risk come from. The data from Roberts and Shirley is based on data obtained from experiments on a single neonate substitute, a rhesus monkey, who had aliquots of acidic stomach contents dripped into its bronchioles. It was extrapolated that patients were at increased risk if they had a residual gastric volume greater than 0.4mls/kg and a pH of less than 2.5. However, it should be pointed out that, according to Schreiner (1998), the residual gastric fluid volume ranges from 0.3ml/kg up to 4.5ml/kg in normal, healthy individuals at the time of induction. Yet, we do not see a massive number of cases of aspiration. There must be more than just gastric volume at play; we use it as a (relatively inaccurate) surrogate marker for aspiration risk.

What are the current fasting guidelines?

As with many guidelines, they are based on the group’s consensus opinion coupled with a small evidence base. A fascinating historical account, retold by Mesbah and Thomas (2017), tells of William Beaumont, an American military surgeon, who studied a fur trapper with a fistula from the stomach to the outside world. He observed that a hearty meat and potatoes meal might take up to 5 hours to be extruded, but clear fluids would be expelled almost immediately.

The well-known 2-4-6 rule was based on a survey of 110 specialists – practitioners from the Association of Paediatric Anaesthetists. While everyone seemed to agree that patients should fast for solids for 6 hours and clear liquids for 2, there was variability regarding milk.  

Not all milk is created equal. A range of fasting times has been suggested depending on whether breast milk, formula or cow’s milk was drunk (Ed. Sorry, hipsters, but I cannot find any information on soy or almond milk). A recent small-volume, cross-sectional study published in the BJA suggests that by about 3-3.5 hours, all fluids (apple juice, 2% milk and Ensure) should have been cleared. But given that the gastric emptying half-life for breast milk is about 25 minutes and formula is 51 minutes, it makes biological sense that formula-fed infants should fast for longer.

ANZCA recommends the following:-

(Ed. note: Thanks to B.J So for pointing out the new changes)

So what about that nice cup of tea that Lister suggests? Is it considered a clear fluid? The European Society of Anesthesiology Guidelines thinks it is – well, all but one group member did – though very little evidence (outside of experimental models) shows it is safe. Certainly, large volumes of milk are liable to curdle and increase the risk of aspiration. Perhaps it depends on how you make your tea, as they define a clear fluid as one that you can read newsprint through.  There is also emerging literature to suggest that carbohydrate-rich fluids up to 2 hours before anaesthetic may enhance recovery after surgery.

The guidelines from the Royal Children’s Hospital in Melbourne sensibly suggest that elective patients can drink clear fluids up to an hour before theatre. This is most likely based on data from a large Swedish study looking at a 6-4-0 regime in 10,015 children undergoing a surgical procedure (HT Andrew Weatherall). Andersson et al. found 3 cases (3 per 10,000) in their dataset. None of these three cases suffered from serious harm.

And what about if the patient has been chewing gum? Only a handful of studies have examined the impact of gum on gastric pH and emptying. Two showed an increase in the gastric volume of around 10mls – certainly not enough to cause a clinically significant increase in the risk of aspiration. The European guidelines suggest that routine surgery should not be cancelled because the patient has been chewing gum, whereas the RCN guidelines take the opposite viewpoint. But this recommendation has nothing to do with gastric emptying or pH but is related to the risk of swallowing the gum.

Pre-operative fasting makes sense for elective procedures, but this boy needs to have an operation tonight. Again, the data on fasting in injured children is lacking. I was taught that gastric emptying slows at the time of the trauma, so the time from the last meal to the time of the trauma was more indicative of stomach contents. Pain, opiates, and stress can all impact gastric motility. Given that we continue to produce secretions – 1ml/kg/hr of saliva and 0.6ml/kg/hr of gastric juices – the longer we leave it, the more full the stomach gets. So, I reached out to some experts…

Rather than fixating on how long it has been since the patient last ate, it might be better to look with an ultrasound. Andrew Weatherall, a paediatric anaesthetist and all-around good guy, wrote about it on Songs or Stories. Go to for a full run-down on how to perform this procedure. This is what you might see…

So, what does this mean to me?

Question the surgeon when they ask a child to fast from midnight for the morning operating list. What do they mean by ‘fast’?

Whilst I am sure most wards are fully aware of fasting guidelines, it is not the case for emergency departments, where patients may linger for some time before going to the ward or theatre. Chat with your friendly anaesthetic service and ask them what their guidelines are.

Addendum May 2018

The combined European societies have released a consensus statement suggesting that, based on lack of evidence of harm, clear fluids can be taken up to ONE hour before an elective procedure.

(Thanks to Peter Van de Putte and the team at for their kind permission to use their US clips)

Selected References

Abola RE, Gan TJ. Preoperative Fasting Guidelines: Why Are We Not Following Them?: The Time to Act Is NOW.

Andersson H, Zarén B, Frykholm P. Low incidence of pulmonary aspiration in children allowed intake of clear fluids until called to the operating suite. Pediatric Anesthesia. 2015 Aug 1;25(8):770-7.

Apfelbaum JL, Caplan RA, Connis RT, Epstein BS, Nickinovich DG, Warner MA. Practice Guidelines for Preoperative Fasting and the Use of Pharmacologic Agents to Reduce the Risk of Pulmonary Aspiration: Application to Healthy Patients Undergoing Elective Procedures An Updated Report by the American Society of Anesthesiologists Committee on Standards and Practice Parameters Anesthesiology 2011; 114:495–511

Bouvet L, Desgranges FP, Aubergy C, Boselli E, Dupont G, Allaouchiche B, Chassard D. Prevalence and factors predictive of full stomach in elective and emergency surgical patients: a prospective cohort study. BJA: British Journal of Anaesthesia. 2017 Feb 16;118(3):372-9.

Bouvet L, Mazoit JX, Chassard D, Allaouchiche B, Boselli E, Benhamou D. Clinical assessment of the ultrasonographic measurement of antral area for estimating preoperative gastric content and volume. Anesthesiology: The Journal of the American Society of Anesthesiologists. 2011 May 1;114(5):1086-92.

Brady MC, Kinn S, Ness V, O’Rourke K, Randhawa N, Stuart P. Preoperative fasting for preventing perioperative complications in children. The Cochrane Library. 2009 Jan 1.

Cook-Sather SD, Litman RS. Modern fasting guidelines in children. Best Practice & Research Clinical Anaesthesiology. 2006 Sep 30;20(3):471-81.

Desgranges FP, Gagey Riegel AC, Aubergy C, Queiroz Siqueira M, Chassard D, Bouvet L. Ultrasound assessment of gastric contents in children undergoing elective ear, nose and throat surgery: a prospective cohort study. Anaesthesia. 2017 Nov 1;72(11):1351-6.

Dowling Jr JL. ” Nulla per os [NPO] after midnight” reassessed. Rhode Island Medicine. 1995 Dec;78(12):339-41.

Du T, Hill L, Ding L, Towbin AJ, DeJonckheere M, Bennett P, Hagerman N, Varughese AM, Pratap JN. Gastric emptying for liquids of different compositions in children. BJA: British Journal of Anaesthesia. 2017 Oct 9;119(5):948-55.

Emerson BM, Wrigley SR, Newton M. Pre‐operative fasting for paediatric anaesthesia A survey of current practice. Anaesthesia. 1998 Apr 1;53(4):326-30.

Maltby JR, Sutherland AD, Sale JP, Shaffer EA. Preoperative oral fluids: is a five-hour fast justified prior to elective surgery? Anesth Analg 1986;

Maltby JR. Fasting from midnight–the history behind the dogma. Best practice & research clinical anaesthesiology. 2006 Sep 30;20(3):363-78.

Mendelson CL. The aspiration of stomach contents into the lungs during obstetric anesthesia. American Journal of Obstetric and Gynecology 1946; 52: 191205.

Mesbah A, Thomas M. Preoperative fasting in children. BJA Education. 2017 Jun 30.

Nordcren B. The rate of secretion and electrolyte content of normal gastric juice. Acta Physiologica Scandinavica. 1963;58(Suppl. 202).

Roberts RB, Shirley MA. Reducing the risk of acid aspiration during cesarean section. Anesthesia & Analgesia. 1974 Nov 1;53(6):859-68.

Royal College of Nursing. Clinical Practice Guidelines: Perioperative fasting in Adults and Children. London, Royal College of Nursing. November 2005

Schmitz A, Schmidt AR, Buehler PK, Schraner T, Frühauf M, Weiss M, Klaghofer R, Kellenberger CJ. Gastric ultrasound as a preoperative bedside test for residual gastric contents volume in children. Pediatric Anesthesia. 2016 Dec 1;26(12):1157-64.

Schreiner MS. Gastric fluid volume: is it really a risk factor for pulmonary aspiration?. Anesthesia & Analgesia. 1998 Oct 1;87(4):754-6.

Smith I, Kranke P, Murat I, Smith A, O’Sullivan G, Søreide E, et al. Perioperative fasting in adults and children: guidelines from the European Society of Anaesthesiology Eur J Anaesthesiol 2011;28(8):556–569

Van de Putte P, Vernieuwe L, Jerjir A, Verschueren L, Tacken M, Perlas A. When fasted is not empty: a retrospective cohort study of gastric content in fasted surgical patients. BJA: British Journal of Anaesthesia. 2017 Feb 16;118(3):363-71.

Williams C, Johnson PA, Guzzetta CE, Guzzetta PC, Cohen IT, Sill AM, Vezina G, Cain S, Harris C, Murray J. Pediatric fasting times before surgical and radiologic procedures: benchmarking institutional practices against national standards. Journal of pediatric nursing. 2014 Jun 30;29(3):258-67.



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3 thoughts on “Fasting guidelines”

  1. Hey Andrew, FYI your “ANZCA” link points to an old version (2016) of the PS07 statement. The latest PS07 document (2017) updated a few things including the fasting guidelines in children.

    Key differences:

    For kids >6 months:
    • 6 hours – limited solid food, formula milk, breast milk
    • 1 hour – clear fluids up to 3ml/kg/hr (new)

    (as an aside, the 2016 document didn’t actually allow breastmilk at 4 hours for this age group either)

    For infants <6 months:
    • 4 hours – formula milk
    • 3 hours – breast milk
    • 1 hour – clear fluids up to 3ml/kg/hr (new)

    ANZCA PS document directory:
    Latest PS07 document:

  2. Thanks for pointing out the lack of evidence behind GA fasting guidelines. The APA survey probably reflects the result of many years of anaesthetists practicing using the ASA guidelines for adults, also based on a member survey.
    It should also be noted that fasting prior to ED procedural sedation has a somewhat better evidence base, with many studies finding that pre-procedural fasting does not reduce the risk of vomiting or aspiration. One of the better summaries of this evidence is found in the ACEP guideline: