NPO Guidelines and Current Evidence-Based Considerations

by Cameron Goertzen, BMSc, MSc, DDS, MSc (Candidate Dental Anesthesia); Joonyoung Ji, DMD, MSc, DIP. ADBA

Current nil per os (npo) standards promote pre-operative fasting as an approach to reduce the volume and acidity of a patient’s stomach contents to reduce the risks of regurgitation and subsequent pulmonary aspiration. Pre-anesthesia fasting standards apply to any procedure where sedative medications reduce the protective airway reflex that under normal conditions prevent aspiration. The scientific basis for fasting standards are largely reliant on theories of anatomy and physiology and rooted in consensus of expert opinions with limited evidence to support improved clinical outcomes.

Pre-operative fasting standards have been developed by anesthesia societies with almost all following a variant of the “2-4-6-8 rule”. The American Society of Anesthesiologists (ASA) recommends patients to fast from fatty food or meats eight (8) hours prior to surgery, non-human milk or light meal for six (6) hours prior, breast milk for four (4) hours prior, and clear liquids including water, pulp-free juice, and tea or coffee without milk for two (2) hours prior to the anesthetic.1 In Canada, the Canadian Anesthesiologists’ Society’s guidelines are very similar to the ASA guidelines, however, the Canadian Pediatric Anesthesia Society fasting guidelines now encourages children to have clear liquids up to one hour prior to surgery.2,3 The European Society of Anaesthesiology (ESA) pre-operative fasting guideline prohibits solid foods for six hours prior to elective surgery and encourages patients to drink clear fluids up to two hours for adults and one hour for children.4 The concern is that the standards may be too rigid and contribute to patient harm or poor anesthetic outcomes.

Recent research into pre-operative fasting suggests that prolonged fluid fasting can lead to undesirable outcomes such anxiety, dehydration and post-operative nausea and vomiting.5 Likewise, prolonged fasting has resulted in cases of hypoglycemia and hypovolemia, more frequently seen in children.2,5 To improve our understanding of pre-operative fluid management, research has been undertaken to understand if the current fasting standards are actually beneficial for protecting patients from detrimental clinical outcomes.6

Post-operatively, nausea and vomiting are a common complaint of patients who have received sedation/anesthesia, which has been attributed to patient dehydration due to prolonged pre-operative fluid fasting. It has been reported that patients on average fast from liquids for seven hours before surgery despite instruction to maintain fluid intake following current guidelines until two hours prior.7 A recent study published in the European Journal of Anaesthesiology in 2018 by McCracken and Montgomery examined post-operative nausea and vomiting of approximately 10,487 patients, of whom 4,697 had no restrictions on pre-operative intake of clear fluids prior to surgery at Torbay Hospital Day Surgery Unit in the United Kingdom.8 The incidence of nausea within 24 hours post-operatively was reduced from 270/5192 (5.2%) to 179/4724 (3.8%) in patients that could drink up until surgery.8 Likewise, patients with unrestrictive fluid intake were more likely to categorise their surgical experience as ‘very good’. Of the 10,487 patients in the study, there were no adverse events of pulmonary aspiration of gastric contents. Therefore, McCracken and Montgomery suggest that unrestricted consumption of clear fluids before anesthesia may help reduce the rate of postoperative nausea and vomiting.8

Du et al. investigated whether milk or other non-clear fluids require fasting of six hours as recommended in current guidelines.9 In this prospective cross-sectional study, 8- to 14-year-olds fasted overnight and were randomized to drink equal volumes (296 mL) of either apple juice, 2% milk, or Ensure Clear (high-protein clear-fluid). Their research demonstrated that clear liquids were cleared more completely at 90 minutes post-ingestion. However, at 3 to 3.5 hours after ingestion, both clear-fluids and milk were essentially completely cleared. They concluded that differentiation between clear and non-clear liquids in current guidelines is not supported by their results.9 This suggests that current fasting guidelines are too rigid for non-human milk and that four hours, instead of six hours, may be more appropriate.

The key factor in gastric emptying may be the caloric content. Okabe and colleagues examined whether caloric content rather then composition determined gastric emptying.10 Comparing equal volumes of non-human milk and pulp-free orange juice diluted with either gum syrup or water to match the number of calories, this study found that there were no significant differences in liquid gastric emptying time after drinking equal number of calories. They also discovered that ingestion of beverages that do not exceed 220 kcal in calories will be adequately cleared in less than two hours.10 Therefore, liquid gastric emptying may depend primarily upon total caloric content rather type of liquid whether it be non-clear fluids such as milk or clear fluids such as apple juice. Therefore, npo rules for liquids should account for equivalent calories rather then liquid type or volume.

Beach and colleagues examined aspiration and pulmonary adverse events in a prospective database of 139,142 pediatric patients who received procedural sedation/anesthesia across 40 general and children’s hospitals in the United States between September 2007 and November 2011.11 Of the 139,142 pediatric patients, npo status was known for 107,947 patients, and 25,401 patients violated npo as defined as no solid foods for at least eight hours, no non-clear fluids for at least six hours, but having clear fluids within two hours prior to surgery. Of the patients examined, there were zero deaths, 10 aspirations, and 75 major complications, defined as cardiac arrests or any other reason requiring hospital admission. In patients who were npo, aspiration occurred in 8 of 82,546 patients (0.97 events per 10,000), while in patients who were not npo, aspiration occurred in 2 of 25,401 (0.79 events per 10,000) patients.11 To gain perspective, with an incidence of aspiration close to 1 in 10,000, this low risk is equivalent to the lifetime odds of drowning in a bath-tub (1 in 8,078).12 This study concluded that npo status for liquids and solids are not independent predictors of aspiration and that other factors, such has ASA Physical Status and age had higher correlation to major adverse outcomes.11

A study by Andersson et al. examined retrospectively 10,015 pediatric patients at Uppsala University Hospital, Sweden, between 2008 and 2013 who were allowed unlimited clear fluids up until surgery.5 The study’s objective was to determine the incidence of pulmonary aspiration associated with general anesthesia in elective procedures. They found the incidence of pulmonary aspiration to be 3 in 10,000 in pediatric patients with unrestricted fluid intake prior to surgery.5 In the 3 cases of aspiration, the patients did not require post-operative ventilatory support or intensive care and symptoms diminished the day following surgery, without sequelae, after treatment with antibiotics.5

In Ontario, dental anesthesia is a recognized specialty. The Standard of Practice for sedation/anesthesia as created by the Royal College of Dental Surgeons of Ontario (RCDSO) adopted its preoperative fasting standards from the ASA. It represents an opinion or dogma of an expert society to balance optimizing patient status, minimizing patient distress, and maximizing patient safety. However, prolonged fasting may cause dehydration, post-operative nausea and vomiting and contribute to a poor patient experience.6 Clinicians must follow the RCDSO standard, or face consequences from their regulatory body. Until standards evolve, this is a medicolegal requirement. Within that framework, clinicians should ensure that patients receive enough clear fluids until two hours prior to surgery with sedation/anesthesia for the best balance between providing positive outcomes and minimizing risks.

Oral Health welcomes this original article.


  1. ASA. 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 Task Force on Preoperative Fasting and the Use of Pharmacologic Agents to Reduce the Risk of Pulmonary Aspiration. Anesthesiology 126, 376-393, doi:10.1097/aln.0000000000001452 (2017).
  2. Rosen, D., Gamble, J. & Matava, C. Canadian Pediatric Anesthesia Society statement on clear fluid fasting for elective pediatric anesthesia. Canadian journal of anaesthesia = Journal canadien d’anesthesie 66, 991-992, doi:10.1007/s12630-019-01382-z (2019).
  3. Dobson, G. et al. Guidelines to the Practice of Anesthesia–Revised Edition 2018. Canadian journal of anaesthesia = Journal canadien d’anesthesie 65, 76-104, doi:10.1007/s12630-017-0995-9 (2018).
  4. Smith, I. et al. Perioperative fasting in adults and children: guidelines from the European Society of Anaesthesiology. European journal of anaesthesiology 28, 556-569, doi:10.1097/EJA.0b013e3283495ba1 (2011).
  5. Andersson, H., Zaren, B. & Frykholm, P. Low incidence of pulmonary aspiration in children allowed intake of clear fluids until called to the operating suite. Paediatric anaesthesia 25, 770-777, doi:10.1111/pan.12667 (2015).
  6. Lambert, E. & Carey, S. Practice Guideline Recommendations on Perioperative Fasting: A Systematic Review. JPEN. Journal of parenteral and enteral nutrition 40, 1158-1165, doi:10.1177/0148607114567713 (2016).
  7. Raeder, J., Kranke, P. & Smith, I. Free pre-operative clear fluids before day-surgery?: Challenging the dogma. European journal of anaesthesiology 35, 334-336, doi:10.1097/eja.0000000000000805 (2018).
  8. McCracken, G. C. & Montgomery, J. Postoperative nausea and vomiting after unrestricted clear fluids before day surgery: A retrospective analysis. European journal of anaesthesiology 35, 337-342, doi:10.1097/eja.0000000000000760 (2018).
  9. Du, T. et al. Gastric emptying for liquids of different compositions in children. British journal of anaesthesia 119, 948-955, doi:10.1093/bja/aex340 (2017).
  10. Okabe, T., Terashima, H. & Sakamoto, A. Determinants of liquid gastric emptying: comparisons between milk and isocalorically adjusted clear fluids. British journal of anaesthesia 114, 77-82, doi:10.1093/bja/aeu338 (2015).
  11. Beach, M. L., Cohen, D. M., Gallagher, S. M. & Cravero, J. P. Major Adverse Events and Relationship to Nil per Os Status in Pediatric Sedation/Anesthesia Outside the Operating Room: A Report of the Pediatric Sedation Research Consortium. Anesthesiology 124, 80-88, doi:10.1097/aln.0000000000000933 (2016).
  12. Council, N. S. Odds of dying due to injury, United States, 2017 <> (2017).

About the Author

Cameron Goertzen is a current University of Toronto dental anesthesia resident and a University of Toronto DDS graduate. Cameron has a diverse background in research and has published articles in the fields of breast cancer and oral cancer. During his free time, Cameron enjoys playing hockey, cooking/baking, and hiking. Cameron is from Niagara-On-the-Lake and along with his wife, Erin Goertzen, a 1st year pediatric dental resident also at the University of Toronto, hope to practice together following their studies. Jooyoung Ji practices in Ottawa, Ontario.