General anesthesia is a state of controlled unconsciousness achieved by administering powerful medications that temporarily alter brain function, allowing surgical procedures to be performed without pain or awareness. Due to the profound effect these drugs have on the body, a mandatory safety rule requires patients to abstain from eating or drinking before the procedure. This rule, known as Nil Per Os (NPO), meaning “nothing by mouth,” is enforced to protect the patient from a specific, serious complication that can occur during the anesthetic process.
The Immediate Danger: Pulmonary Aspiration
The primary risk associated with having food or liquid in the stomach during general anesthesia is pulmonary aspiration. This occurs when stomach contents, including acidic digestive juices, are regurgitated and inhaled into the lungs. Though rare, aspiration can have devastating consequences.
Mechanical Obstruction and Chemical Injury
The danger of aspiration involves both mechanical obstruction and chemical injury. Undigested food particles that enter the airway can physically block the breathing tubes, leading to lung collapse and severe respiratory distress.
More concerning is the chemical injury caused by the stomach acid. Aspirating even a small volume of this acidic fluid causes a severe inflammatory reaction in the lung tissue called chemical pneumonitis. This condition can rapidly destroy the lining of the lungs and lead to aspiration pneumonia, acute respiratory distress syndrome (ARDS), and a high mortality rate.
How General Anesthesia Affects Protective Reflexes
General anesthesia creates the environment for aspiration by temporarily deactivating the body’s natural defense systems. In a conscious person, the pharyngeal and laryngeal reflexes (gag and swallowing reflexes) trigger a forceful cough or swallowing action to prevent contents from entering the windpipe. These reflexes are the primary protection against aspiration.
Anesthetic medications suppress the central nervous system, causing protective reflexes to become inactive. This loss of muscle tone means that if stomach contents regurgitate, there is no automatic mechanism to clear the airway.
General anesthesia also causes the lower esophageal sphincter (LES) to relax. The LES is a muscular valve that acts as a barrier to keep stomach acid and food from flowing backward. When relaxed by anesthetic drugs, stomach contents can easily travel up into the pharynx and be inhaled. The combination of a relaxed LES and absent protective reflexes necessitates an empty stomach before receiving general anesthesia.
Specific Fasting Guidelines
To ensure the stomach is empty and the risk of aspiration is minimized, specific fasting protocols are followed for elective procedures. Guidelines differentiate substances because foods and liquids empty from the stomach at varying rates.
The standard fasting time for clear liquids (water, black coffee, or fruit juice without pulp) is two hours before the procedure. The time frame is extended for more substantial intake, as these items take longer to digest.
For more substantial intake, the required fasting times are:
- Breast milk requires a four-hour fasting period.
- Infant formula, nonhuman milk, or a light meal (such as toast and tea) generally require six hours.
- Meals that include fried or fatty foods, as well as meat, necessitate a longer fasting period of eight hours or more.
Patients who need to take necessary medications, such as those for blood pressure or seizures, are typically allowed to do so on the morning of surgery with a small sip of water. These are general standards, and specific medical conditions like diabetes or hiatal hernia can slow gastric emptying, requiring individualized instructions. Patients must always follow the precise instructions provided by their care team, as deviation from the NPO rule often results in the procedure being delayed or canceled.