Preoperative fasting

Fasting

Fasting is primarily the act of willingly abstaining from some or all food, drink, or both, for a period of time. An absolute fast is normally defined as abstinence from all food and liquid for a defined period, usually a single day , or several days. Other fasts may be only partially restrictive,...

is the practice of a patient abstaining from oral food and fluid intake

Nil per os

Nil per os is a medical instruction meaning to withhold oral food and fluids from a patient for various reasons. It is a Latin phrase which translates as "nothing through the mouth". In the United Kingdom it is translated as nil by mouth .Typical reasons for NPO instructions are the prevention...

 for a time before an operation is performed. This is intended to prevent pulmonary aspiration

Pulmonary aspiration

Pulmonary aspiration is the entry of material from the oropharynx or gastrointestinal tract into the larynx and lower respiratory tract...

 of stomach

Stomach

The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

 contents during general anesthesia.

Pulmonary aspiration

The main reason for preoperative fasting is to prevent pulmonary aspiration

Pulmonary aspiration

Pulmonary aspiration is the entry of material from the oropharynx or gastrointestinal tract into the larynx and lower respiratory tract...

 of stomach contents while under the effects of general anesthesia. Aspiration of as little as 30-40ml 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

Pregnancy

Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...

, obesity

Obesity

Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health, leading to reduced life expectancy and/or increased health problems...

, 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, antacid

Antacid

An antacid is a substance which neutralizes stomach acidity.-Mechanism of action:Antacids perform a neutralization reaction, increasing the pH to reduce acidity in the stomach. When gastric hydrochloric acid reaches the nerves in the gastrointestinal mucosa, they signal pain to the central nervous...

s 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

PH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 of the acid

Acid

An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

 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

Gastroparesis

Gastroparesis, also called delayed gastric emptying, is a medical condition consisting of a paresis of the stomach, resulting in food remaining in the stomach for a longer period of time than normal. Normally, the stomach contracts to move food down into the small intestine for digestion. The...

 (delayed gastric emptying) may occur and is due to metabolic

Metabolism

Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 causes (e.g. poorly controlled diabetes mellitus

Diabetes mellitus

Diabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced...

), decreased gastric motility (e.g. due to head injury) or pyloric obstruction (e.g. pyloric stenosis

Pyloric stenosis

Pyloric stenosis is a condition that causes severe vomiting in the first few months of life. There is narrowing of the opening from the stomach to the intestines, due to enlargement of the muscle surrounding this opening , which spasms when the stomach empties...

). Delayed gastric emptying usually only affects the emptying of the stomach of high-cellulose

Cellulose

Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

 foods such as vegetables. Gastric emptying of clear fluids such as water or black coffee

Coffee

Coffee is a brewed beverage with a dark,init brooo acidic flavor prepared from the roasted seeds of the coffee plant, colloquially called coffee beans. The beans are found in coffee cherries, which grow on trees cultivated in over 70 countries, primarily in equatorial Latin America, Southeast Asia,...

 is only affected in highly progressed delayed gastric emptying.

Occasionally, gastroesophageal reflux may be associated with delayed gastric emptying of solids, but clear liquids are not affected. Raised intra-abdominal

Abdomen

In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

 pressure (e.g. in pregnancy or obesity) predisposes to regurgitation

Regurgitation (digestion)

Regurgitation is the expulsion of material from the mouth, pharynx, or esophagus, usually characterized by the presence of undigested food or blood.Regurgitation is used by a number of species to feed their young...

. 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 be nil by mouth

Nil per os

Nil per os is a medical instruction meaning to withhold oral food and fluids from a patient for various reasons. It is a Latin phrase which translates as "nothing through the mouth". In the United Kingdom it is translated as nil by mouth .Typical reasons for NPO instructions are the prevention...

 from midnight. However, since then, the American Society of Anesthesiologists

American Society of Anesthesiologists

The American Society of Anesthesiologists is an association of physicians, primarily anesthesiologists, that share a common goal of raising the standard of the medical specialty of anesthesiology and the improvement of patient care by fostering and encouraging education through research and...

 (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

Nil per os

Nil per os is a medical instruction meaning to withhold oral food and fluids from a patient for various reasons. It is a Latin phrase which translates as "nothing through the mouth". In the United Kingdom it is translated as nil by mouth .Typical reasons for NPO instructions are the prevention...

 since midnight. The following are the recommended guidelines for nil by mouth prior to surgery:

Age	Solids	Clear liquids
<6 months	4 hours	2 hours
6-36 months	6 hours	3 hours
>36 months (including adults)	6 hours	2 hours

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