The instruction to fast before an operation, often summarized by the Latin phrase “nil per os” (NPO), is the most important safety instruction a patient receives prior to surgery. This rule means “nothing by mouth,” and it ensures a patient’s stomach is empty when they are placed under anesthesia. Failing to fast introduces a significant risk that could lead to serious, life-threatening complications. This measure plays a direct role in protecting the patient during the procedure.
Understanding Pulmonary Aspiration
The primary danger of having food or liquid in the stomach during surgery is pulmonary aspiration. This occurs when stomach contents—including liquids, partially digested food, or acidic gastric juices—enter the lungs instead of the digestive tract. The risk increases dramatically when a patient is unconscious.
Aspiration can trigger two types of severe lung injury. Highly acidic stomach fluid causes immediate chemical injury to lung tissues, known as aspiration pneumonitis, which can rapidly lead to respiratory failure and acute inflammation. If solid food particles are inhaled, they can mechanically obstruct the airways, leading to breathing difficulties and collapse of the lung segments. Pulmonary aspiration frequently results in severe morbidity and can be fatal.
The Physiological Role of Anesthesia
Food in the stomach becomes hazardous due to the profound physiological effects of general anesthesia. Anesthetic agents induce unconsciousness, which eliminates the body’s natural protective reflexes. Essential reflexes, such as the gag reflex and the ability to cough or swallow forcefully, are suppressed when the patient is fully anesthetized.
Anesthesia medications also relax various smooth muscles throughout the body. This relaxation includes the muscular rings, or sphincters, that normally act as one-way valves in the digestive system, most notably the lower esophageal sphincter. When this sphincter relaxes, the pressure from stomach contents can easily overcome the weakened barrier.
With the lower esophageal sphincter relaxed and protective airway reflexes eliminated, stomach contents can passively regurgitate up the esophagus and into the throat. The material can then spill into the trachea and be inhaled directly into the lungs. This passive regurgitation poses the greatest threat during the induction of anesthesia and when the breathing tube is removed.
Current Fasting Guidelines
To minimize the risk of pulmonary aspiration, specific time frames for fasting have been established by professional bodies, such as the American Society of Anesthesiologists (ASA). These guidelines are based on the rate at which different types of food and drink empty from the stomach.
The shortest recommended fasting period is for clear liquids, which are permitted up to two hours before the procedure. Clear liquids include water, fruit juices without pulp, clear tea, and black coffee.
A light meal, such as toast or crackers without high fat content, requires a fasting period of at least six hours before anesthesia administration. Non-human milk and infant formula also require a minimum of six hours of fasting, as they have a similar rate of gastric emptying to solid foods.
Meals that are heavy, fried, or contain fatty foods take the longest to digest, necessitating a minimum fasting window of eight hours or more. These timeframes are general minimums for healthy patients undergoing elective surgery and may be extended by the care team if the patient has underlying conditions that delay gastric emptying.