FRCA Notes

Perioperative Fasting

The curriculum asks for "indications for preoperative fasting and appropriate regimens", as well as "Local feeding/starvation policies and the reasons behind them" in the obstetric setting.

The March 2026 CRQ on perioperative fasting was well, answered, although marks were lost on carbohydrate loading, the physiological effects of prolonged fasting, and "the speed of gastric emptying for clear fluids."

There is a separate page on fasting in paediatric patients.

Resources


Rationale for fasting

  • To reduce the risk of pulmonary aspiration of gastric contents; see the aspiration under anaesthesia page for definitions, risk factors and management
  • The consequences of aspiration depend heavily on what is aspirated:
    • Solids cause airway obstruction and asphyxia
    • Clear fluids of near-neutral pH are rarely of clinical significance
  • This distinction underpins modern guidance: a long fast for solids, a short fast for clear fluids

Fasted is not the same as empty

  • Approximately 2.5L of saliva and gastric juice pass through the stomach each day regardless of oral intake
  • A residual gastric volume of up to 1.5ml/kg is now regarded as normal in fasted adults
    • Around 6% of appropriately fasted elective patients still have a significant fluid volume, and ~1.7% have solid content, on gastric ultrasound
  • Drinking clear fluids up to 2hrs pre-operatively does not increase residual gastric volume or lower pH; if anything, volume is slightly reduced
  • Residual gastric volume is therefore a poor surrogate for aspiration risk

Aspiration requires a coincidence of factors

  • A sufficient volume of gastric content
  • Delayed or inadequate gastric emptying
  • Impaired lower oesophageal sphincter function, or a sphincter overwhelmed by raised gastric pressure (e.g. straining under light anaesthesia)
  • Regurgitation of that content
  • Sufficient regurgitated material reaching the bronchi
  • Material harmful enough to damage the lungs

  • Most aspiration events stem from failure to recognise risk factors and adapt the anaesthetic technique, rather than from the length of the fast
  • Modern incidence of perioperative aspiration is approximately 1 in 7,000 anaesthetics

Clear fluids

  • Emptying is exponential: the rate of emptying is proportional to the volume remaining in the stomach
    • Drinking more therefore speeds up emptying
    • Liquids pass along the lesser curvature (the 'Magenstrasse') directly to the duodenum

  • Rate of emptying is governed mainly by calorie content, as duodenal receptors limit delivery to roughly 200kcal/hr
    • Milk and orange juice empty at the same rate if matched for calories
    • Fat empties more slowly than protein, which empties more slowly than carbohydrate
    • Osmolality and carbonation have little effect

  • Gastric half-time for water is approximately 10 - 15 minutes and ~95% of an ingested clear fluid has left the stomach within 1hr
  • Tea with a splash of milk (up to ~20% of volume) has a half time of ~23 minutes; essentially the same as black tea
  • Milk in large quantities curdles in the stomach and behaves like a solid so drinks made predominantly from milk are treated as solids
  • Inter-individual variability is high (~30%), and emptying is episodic rather than smooth

Solids

  • Emptying is approximately linear (zero order), after a lag phase of ~1hr while food is broken down, although particles >2mm are retained by the pylorus
  • Moderate meals empty at roughly 1-1.5g/min
  • Time to empty increases greatly with meal size and fat content
    • After a very large meal (~1.7kg), ~30% of solids may remain at 6hrs
    • This is the rationale for the ASA advising 8hrs or more after fried or fatty food or meat

Factors affecting gastric emptying

  • Many conditions which delay emptying affect solids far more than liquids

Factor Solids Clear fluids
Opioids Delayed (morphine ~halves emptying rate) Delayed
Labour Delayed Delayed
Diabetic gastroparesis Delayed Normal or even accelerated
GLP-1 receptor agonists e.g. semaglutide Delayed (retained solids on ultrasound) Less affected
Trauma e.g. hip or wrist fracture Delayed (pain, stress, opioids) Not delayed
Vagotomy Delayed Rapid
Obesity Not delayed Not delayed
Time of day; luteal phase Slower in the evening (~50%) and luteal phase (~36%) Unaffected



Intake ESAIC (2011) ASA (2017; updated 2023) CPOC Sip Til Send (2025)
Solid food 6hrs 6hrs after a light meal; 8hrs or more after fried or fatty food or meat 6hrs
Milk and milky drinks 6hrs; drinks made predominantly from milk count as solids 6hrs (non-human milk) 6hrs
Clear fluids Encouraged until 2hrs; tea or coffee with up to 1/5 milk regarded as clear 2hrs; type of fluid more important than volume Unlimited until 2hrs; then sips up to 170ml/hr until sent for
Carbohydrate drinks Safe until 2hrs, including in diabetics Until 2hrs Until 2hrs, if given locally
Chewing gum, sweets, smoking Should not lead to cancellation or delay; nicotine should still be discouraged Chewing gum should not delay surgery Avoid for 6hrs; if taken, anaesthetist weighs up risks

Clear fluids

  • Traditionally defined as a fluid through which newsprint can be read
  • Water, fruit juice without pulp, squash, black tea or coffee
  • Carbonated drinks are permitted by the ASA but excluded from the CPOC protocol, although carbonation does not affect gastric emptying
  • Alcohol is excluded

Other points

  • Patients with obesity, GORD or diabetes, and pregnant women not in labour, can follow the same fasting guidance although these factors may still alter the overall anaesthetic technique
  • Regular medications should be continued with a small volume of water (up to ~150ml)
  • Routine use of antacids or prokinetic premedication is not recommended for elective surgery in non-obstetric patients as there is lack of demonstrable clinical benefit
  • Where acid suppression is used, H2 antagonists are more effective than PPIs at reducing gastric volume and raising pH
  • Patients should be actively encouraged to drink clear fluids until the permitted time; the guidance is a minimum, not a target

  • In practice adults fast for a median of 9 - 12hrs for fluids, rather than the recommended 2hrs
  • The effects are greatest in the elderly, frail and multimorbid i.e. the patients with the least physiological reserve

Cardiovascular

  • Dehydration and relative hypovolaemia
  • Greater haemodynamic instability on induction of anaesthesia or neuraxial block
  • Impaired cardiac output
  • Pre-operative oral rehydration has been shown to reduce transient ischaemic ECG changes after spinal anaesthesia in older patients

Neurological

  • Duration of fluid fasting is a modifiable risk factor for postoperative delirium
  • Anxiety, fatigue and malaise
  • Caffeine withdrawal headache in regular coffee drinkers

Renal

  • Dehydration increases the risk of postoperative AKI, particularly in renal surgery
  • Pre-operative dehydration in hip fracture patients is associated with a fourfold increase in postoperative complications

Gastrointestinal

  • Thirst and hunger, which are consistently rated among the most unpleasant parts of the surgical experience
  • Increased PONV
  • Does not reliably produce an empty stomach

Nutritional and metabolic

  • Switch to a catabolic, 'fasted' state before the surgical stress response has even begun
  • Insulin resistance and perioperative hyperglycaemia which is associated with increased infection, reoperation and mortality
  • Ketosis
  • Raised inflammatory markers e.g. IL-6

Endocrine

  • Amplified stress response; ↑ cortisol, catecholamines and glucagon
  • Omission of medications if patients are wrongly told 'nil by mouth' includes everything

MSK

  • Protein breakdown and loss of muscle mass
  • Reduced postoperative muscle strength, impairing mobilisation

  • Despite the 2hr limit of on fluids, median fluid fasting remains up to 12hrs
  • A fixed 2hr cut-off requires predicting the start time of anaesthesia, which is rarely possible beyond the first case on a list

Contributing factors

  • System:
    • Unpredictable lists, re-ordering and on-the-day cancellations (12 - 16% of operations)
    • Poor communication between theatre and ward, worsened by staff shortages
  • Staff:
    • Ingrained 'nil by mouth from midnight' culture
    • Fear of aspiration and of cancellation if the 2hr window is breached so staff fast patients 'to be on the safe side'
    • Medico-legal concern among anaesthetists
  • Patient:
    • Unclear instructions meaning patients often arrive having drunk nothing since the night before

  • Intravenous fluids are not the answer as patients on IV fluids awaiting surgery fast for nearly twice as long for oral fluids (12.5 vs 6.5hrs) with more thirst

  • Developed in NHS Tayside (2021) and now recommended by the Centre for Perioperative Care (CPOC) for adoption by all UK hospitals
  • Patients sip small volumes of clear fluid after the 2hr point until they are sent for by theatre
  • Applies to elective, obstetric and paediatric patients, and those having sedation, unless a senior clinician opts an individual patient out

CPOC recommended protocol (adults)


Time before surgery Permitted intake
Within 6hrs No food, chewing gum, sweets, fizzy drinks or soup
6hrs until 2hrs Unlimited clear fluids: water, black tea or coffee, squash, juice without bits
2hrs until sent for Sips of clear fluid; up to 170ml per hour (one standard NHS cup)

  • Local variations exist e.g. 150ml/hr, or tea and coffee with up to 15ml milk
Advantages Disadvantages
Removes the need to predict theatre start times No adequately powered evidence of safety for aspiration
Dramatic reduction in actual fluid fasting times Relies on patients limiting their own intake
Less thirst, nausea, headache and anxiety; improved satisfaction Requires education of ward, theatre and administrative staff
Better hydration; fewer IV drips needed Variation between local protocols may confuse staff and patients
Cheap and simple; no reported increase in adverse events Anaesthetist concern regarding medico-legal exposure

Exclusions

  • Significant symptomatic GORD or hiatus hernia
  • Class 3 obesity (BMI >40)
  • Documented gastroparesis e.g. diabetic autonomic neuropathy
  • Bowel obstruction, ileus or a known obstructing GI lesion
  • Stroke affecting swallowing, or patients on thickened fluids
  • Significant dementia or learning difficulty preventing self-limitation of intake
  • Procedures on the stomach e.g. gastroscopy or TOE


  • A component of enhanced recovery (ERAS) pathways
  • Typically a clear, iso-osmolar carbohydrate drink given the evening before surgery (~800ml) and ~400ml (12.5% carbohydrate; ~50g, 200kcal) up to 2hrs pre-operatively

Rationale

  • Stimulates endogenous insulin release, switching the patient from a fasted to a fed metabolic state before surgery
  • Attenuates the catabolic response to fasting and surgery

Benefits

  • Reduced postoperative insulin resistance and hyperglycaemia
  • Preservation of muscle mass and strength
  • Less hunger, thirst and anxiety than a flavoured water placebo
  • Reduced PONV in some studies
  • No increase in residual gastric volume compared with placebo or fasting; 600ml of a dilute glucose drink has cleared the stomach within 30 minutes
  • Evidence for reduced length of stay or mortality is inconsistent

Potential issues

  • Safety evidence comes from products designed for perioperative use so not all carbohydrate drinks can be assumed to behave the same way
  • Diabetes is not a contraindication although blood glucose should be monitored
  • Patients with bowel obstruction or known gastroparesis
  • Cost and availability; patients may need to be given drinks to take home

Emergency surgery

  • Fasting does not convert the patient into a 'fasted elective' state; RSI decisions should be based on risk factors, not the fasting time alone
  • The benefit of delaying for gastric emptying must be weighed against the risks of delay
  • Emergency lists are heterogeneous so blanket fasting policies may be unhelpful
  • Gastric ultrasound may be helpful

Procedural sedation

  • Some guidelines recommend unrestricted clear fluids before sedation for patients at no or low risk of aspiration
  • The CPOC Sip-Til-Send protocol also applies to sedation

Breaches of fasting

  • Sips of fluid do not justify postponement; upper GI secretions alone can reach ~2ml/min
  • Chewing gum, a boiled sweet or a cigarette shortly before induction is not necessarily a reason to cancel, although a retained bolus of gum is an airway hazard and should be removed
  • Up to 13% of patients admit to ignoring fasting instructions ± lying as they fear surgery will be cancelled; so clear and realistic instructions improve honesty and safety

Background

  • Fear of aspiration has shaped obstetric practice and withholding oral intake in labour was the norm until the 2000s
  • Aspiration is now rare:
    • GA is used for only 5-10% of Caesarean deliveries in high-income settings
    • Incidence of ~5 per 1,000,000 deliveries, mostly during Caesarean section under GA, with no maternal deaths

Gastric emptying in pregnancy and labour

  • Pregnancy per se does not significantly delay gastric emptying, including in obese women at term, and gastric acid secretion is unchanged
  • However, the lower oesophageal sphincter should be regarded as incompetent at term, resolving within ~48hrs of delivery
  • Emptying is normal in early labour but becomes delayed as labour advances

  • Opioids cause marked gastric stasis:
    • Parenteral opioids, and bolus doses of epidural or intrathecal opioid, significantly delay emptying
    • Low-dose epidural infusions with fentanyl do not appear to delay emptying until the total fentanyl dose exceeds ~100mcg
    • Effective epidural analgesia may actually hasten emptying, by relieving pain
  • Overall, gastric emptying across pregnancy and labour is unpredictable, so uniform assumptions about risk should be avoided

Oral intake in labour

  • Labour is metabolically demanding and fasting contributes to fatigue, dehydration and maternal ketosis (which may affect fetal acid-base balance)
  • Thirst is a major source of distress; >75% of women want to drink in labour

  • One study (BMJ, 2009) found no difference in spontaneous vaginal delivery, labour duration, operative delivery or neonatal outcome between a light diet and water only
  • Light diet is beneficial at preventing ketosis but increases gastric volume
  • Isotonic, glucose-containing 'sports drinks' prevent dehydration and ketosis without increasing gastric volume so are a reasonable middle-ground

Guidance on oral intake in labour


Body Recommendation
NICE (NG235) May eat a light diet in established labour if no opioids given and no risk factors making GA likely
ESAIC (2011) Clear fluids as desired; solids discouraged in active labour, but low-residue foods (e.g. toast, biscuits) reasonable for low-risk women. Units performing much emergency obstetric surgery under GA should probably not allow eating
WHO; RANZCOG Low-risk women may eat and drink in labour
ASA Avoid solids in active labour; encourage clear fluids, especially electrolyte drinks; tighter restriction if high risk. Access to neuraxial analgesia should not depend on fasting status

  • The trend is towards a flexible, individualised approach rather than rigid protocols, considering:
    • Likelihood of requiring GA or urgent operative delivery
    • Maternal comorbidities e.g. obesity, pre-eclampsia, diabetes
    • Labour progress
    • Presence of effective neuraxial analgesia
    • The woman's own preferences, through shared decision-making

  • Women at higher risk of operative delivery (e.g. pre-eclampsia, twins) or who have received parenteral opioids should not eat
  • Hydration is maintained with limited clear fluids or IV fluids, and regular oral H2 antagonists should be considered

Elective Caesarean section

  • Sip-Til-Send has been used safely, and is associated with less thirst, nausea, light-headedness and anxiety
  • Gastric ultrasound studies have shown no clinically relevant increase in gastric volume
  • A spinal anaesthetic is not risk-free in this regard: loss of consciousness from a high block or vasovagal episode carries its own unprotected-airway risk
  • Unlike non-obstetric patients, all women undergoing LSCS should receive antacid prophylaxis
Scenario Regimen
Elective Oral H2 antagonist (or PPI) the night before and 60 - 90 minutes before surgery ± consider metoclopramide 10mg
Emergency under regional IV H2 antagonist at the decision for surgery
Emergency under GA 30ml 0.3M sodium citrate within ~20 minutes of induction