The instruction to fast before surgery, often referred to by the Latin phrase nil per os (NPO), meaning “nothing by mouth,” is a non-negotiable safety measure designed to protect the patient from a severe complication that can occur during a procedure. This restriction on eating or drinking for a specific period is not arbitrary but is based on the physiological effects of anesthesia on the body’s natural defense systems. By ensuring the stomach is empty, medical teams significantly reduce the risk of contents entering the lungs, which is the primary danger associated with having food or drink before an operation. The strict adherence to these guidelines allows the surgical team to administer anesthesia with the highest possible degree of safety.
The Critical Risk of Aspiration
The main danger of having food or liquid in the stomach during surgery is pulmonary aspiration, which occurs when stomach contents are regurgitated and then inhaled into the lungs. This is a life-threatening event because the body’s protective reflexes are suppressed by anesthesia, allowing foreign material to enter the respiratory tract. The inhaled material travels down the trachea, the tube leading to the lungs, instead of the esophagus, which leads to the stomach.
Aspiration can lead to two major complications: obstruction and chemical injury. Large particles of undigested food can physically obstruct the airway, leading to immediate choking and a lack of oxygen. More commonly, the highly acidic stomach fluids and smaller food particles cause a severe chemical burn to the delicate lung tissue, a condition called aspiration pneumonitis.
Stomach acid has a very low pH, and its presence in the lungs triggers a rapid, intense inflammatory reaction that damages the air sacs and small airways. This damage can severely impair the lungs’ ability to exchange oxygen and carbon dioxide, potentially leading to respiratory failure. Estimates suggest that aspiration can occur in about 1 in every 2,000 to 3,000 cases involving anesthesia, and it is associated with a high rate of severe pulmonary complications and mortality. Therefore, pre-operative fasting is the most effective preventative measure against this serious risk.
How Anesthesia Affects the Digestive System
General anesthesia creates the risk of aspiration by temporarily neutralizing the body’s protective mechanisms. The drugs administered work by depressing the central nervous system, which causes muscle relaxation throughout the body, including the involuntary muscles of the digestive tract. This relaxation includes a critical structure known as the lower esophageal sphincter (LES), a ring of muscle that acts as a valve between the esophagus and the stomach.
Anesthesia agents, such as volatile anesthetics and opioids, reduce the tone and effectiveness of the LES, making it easier for stomach contents to passively flow back up into the esophagus and throat. This passive reflux is normally prevented by the LES and by the body’s natural reflexes. Furthermore, the anesthesia suppresses the protective reflexes, such as coughing, gagging, and swallowing, that would normally clear the throat and prevent aspiration.
The process of inducing anesthesia and securing the airway is when the risk is particularly high, as the muscles of the throat and mouth are relaxing. The inability to clear the airway combined with a relaxed LES means that if the stomach is full, the contents can easily enter the trachea. Additionally, stress, pain, and the use of certain medications before surgery can slow down the natural process of gastric emptying, meaning food remains in the stomach for a longer duration than usual.
Practical Fasting Guidelines and Exceptions
Pre-operative fasting guidelines are based on the rate at which different substances leave the stomach. The general rule for most elective procedures is an eight-hour fast for a heavy meal, especially those high in fat, protein, or fried foods, which take the longest to digest. A lighter meal, such as toast or crackers, requires a minimum fasting period of six hours before the procedure.
Liquids are cleared from the stomach much faster, leading to shorter fasting times. Clear liquids can typically be consumed up to two hours before the scheduled time of surgery. These include:
- Water.
- Plain black coffee or tea.
- Pulp-free fruit juices.
It is important to note that non-human milk, such as cow’s milk or formula, is treated like a solid food due to its fat and protein content, requiring a six-hour fast, while breast milk for infants requires a four-hour fast.
There are also specific exceptions that patients should discuss with their surgical team. Patients are usually allowed to take their necessary prescription medications, such as those for blood pressure or heart conditions, with a small sip of water during the fasting period.
Chewing gum and hard candies are generally discouraged because they stimulate the production of gastric juices, which increases the volume and acidity of stomach contents, raising the risk of aspiration. These guidelines are minimum recommendations for healthy patients undergoing elective surgery, and patients with conditions that delay gastric emptying may need a longer period of fasting.
Anesthesia agents, volatile anesthetics and opioids, reduce the tone and effectiveness of the LES, making it easier for stomach contents to passively flow back up into the esophagus and throat. This passive reflux is normally prevented by the LES. Furthermore, the anesthesia suppresses the protective reflexes, such as coughing, gagging, and swallowing, that would normally clear the throat and prevent aspiration.
The process of inducing anesthesia and securing the airway is when the risk is particularly high, as the muscles of the throat and mouth are relaxing. The inability to clear the airway combined with a relaxed LES means that if the stomach is full, the contents can easily enter the trachea. Additionally, stress, pain, and the use of certain medications before surgery can slow down the natural process of gastric emptying, meaning food remains in the stomach for a longer duration than usual.
Practical Fasting Guidelines and Exceptions
Pre-operative fasting guidelines are based on the rate at which different substances leave the stomach. The general rule for most elective procedures is an eight-hour fast for a heavy meal, especially those high in fat, protein, or fried foods, which take the longest to digest. A lighter meal, such as toast or crackers, requires a minimum fasting period of six hours before the procedure.
Liquids are cleared from the stomach much faster, leading to shorter fasting times. Clear liquids, which include water, plain black coffee or tea, and pulp-free fruit juices, can typically be consumed up to two hours before the scheduled time of surgery. It is important to note that non-human milk, such as cow’s milk or formula, is treated like a solid food due to its fat and protein content, requiring a six-hour fast, while breast milk for infants requires a four-hour fast.
There are also specific exceptions that patients should discuss with their surgical team. Patients are usually allowed to take their necessary prescription medications, such as those for blood pressure or heart conditions, with a small sip of water during the fasting period. Chewing gum and hard candies are generally discouraged because they stimulate the production of gastric juices, which increases the volume and acidity of stomach contents, raising the risk of aspiration. These guidelines are minimum recommendations for healthy patients undergoing elective surgery, and patients with conditions that delay gastric emptying may need a longer period of fasting.