science

Preoperative fasting

Preoperative fasting is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Preoperative fasting rather than just read about it. In short: Preoperative fasting is the practice of a surgical patient abstaining from eating or drinking ("nothing by mouth") for some time before having an operation. This is intended to prevent stomach contents from getting into the windpipe and lungs (known as a pulmonary aspiration) while the patient is under general anesthesia.

Key takeaways

Preoperative fasting belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Preoperative fasting to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Preoperative fasting from memory before moving on to harder problems.

Reference excerpt

Preoperative fasting is the practice of a surgical patient abstaining from eating or drinking ("nothing by mouth") for some time before having an operation. This is intended to prevent stomach contents from getting into the windpipe and lungs (known as a pulmonary aspiration) while the patient is under general anesthesia.

Pulmonary aspiration The main hypothesized benefit of preoperative fasting is to prevent pulmonary aspiration of stomach contents while under the effects of general anesthesia. Aspiration of as little as 30–40 mL can be a significant cause of suffering and death during an operation and therefore fasting is performed to reduce the volume of stomach contents as much as possible. Several factors can predispose to aspiration of stomach contents including inadequate anesthesia, pregnancy, obesity, difficult airways, emergency surgery (since fasting time is reduced), full stomach and altered gastrointestinal mobility. Increased fasting times leads to decreased injury if aspiration occurs.

Gastric conditions In addition to fasting, antacids are administered the night before (or in the morning of an afternoon operation) and then once again two hours prior to surgery. This is to increase the pH (make more neutral) of the acid present in the stomach, helping to reduce the damage caused by pulmonary aspiration, should it occur. H2 receptor blockers should be used in high-risk situations and should be administered in the same timing intervals as antacids. Gastroparesis (delayed gastric emptying) may occur and is due to metabolic causes (e.g. poorly controlled diabetes mellitus), decreased gastric motility (e.g. due to head injury) or pyloric obstruction (e.g. pyloric stenosis). Delayed gastric emptying usually only affects the emptying of the stomach of high-cellulose foods such as vegetables. Gastric emptying of clear fluids such as water or black coffee is only affected in highly progressed delayed gastric emptying. Usually, gastroesophageal reflux (GERD) may be associated with delayed gastric emptying of solids, but clear liquids are not affected. Raised intra-abdominal pressure (e.g. in pregnancy or obesity) predisposes to regurgitation. Certain drugs such as opiates can cause marked delays in gastric emptying, as can trauma, which can be determined by certain indicators such as normal bowel sounds and patient hunger.

Minimum fasting times The minimum fasting times prior to surgery have long been debated. The first proposition came from British anesthetists stating that patients should have nothing by mouth from midnight. However, since then, the American Society of Anesthesiologists (ASA), followed by the Association of Anaesthestists of Great Britain and Ireland (AAGBI), recommended new fasting guidelines for the minimum fast prior to surgery. This was based upon evidence by Canadian anesthesiologists who found that drinking clear fluids two hours prior to surgery decreased pulmonary aspiration compared to those nil by mouth since midnight. The following are the recommended guidelines for nil by mouth prior to surgery in healthy patients:

When anaesthesia is required in an emergency, nasogastric aspiration is usually performed to reduce gastric contents and the risk of its pulmonary aspiration.

Unrestricted clear fluids Fasting guidelines often restrict the intake of any oral fluid after two to six hours preoperatively. However, it has been demonstrated in a large retrospective analysis in Torbay Hospital that unrestricted clear oral fluids right up until transfer to theatre could significantly reduce the incidence of postoperative nausea and vomiting without an increased risk in the adverse outcomes for which such conservative guidance exists.

Public information A 2016 systematic review found that the information on the internet often provided inaccurate and out-of-date recommendations on preoperative fasting.

Worked examples

Example 1 — a first encounter with Preoperative fasting

Start with the simplest possible case. Write down what Preoperative fasting claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Preoperative fasting before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Preoperative fasting ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Preoperative fasting

In research: Preoperative fasting appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Preoperative fasting in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Preoperative fasting is common in secondary-school and first-year university syllabi. It links to neighbouring topics Diets, Fasting, Surgery, so understanding it makes those chapters shorter.
In everyday life: Look for Preoperative fasting outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Preoperative fasting in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Preoperative fasting means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Preoperative fasting out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Preoperative fasting in simple terms?

Why does Preoperative fasting matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Preoperative fasting?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Preoperative fasting.