Why do doctors have patients refrain from drinking water after midnight on the day before a surgery?

Answer 1

When they "put you to sleep" with deep sedation or anesthesia, the rings of muscle which separate the stomach from the esophagus (swallowing tube) relax, and stomach contents can run "uphill". (Some people with chronic heartburn have this problem wide awake.) If that happens, or you just plain vomit, the can stomach contents can be inhaled into your lungs, where the stomach acid, juices, and food particles will cause damage and severe respiratory distess, or drown you. At the least, you could develop pneumonia from having breathed in all that germy and corrosive mess, and people have died from the aftermatch of that event. Nowadays, they have a tube going into your main air passage with a balloon on it which usually keeps the stomach contents from getting into the air passages, but no equipment is fail-safe. Sometimes the balloons leak or pop. Probably best to follow the rules, although people do cheat and get lucky. This is not current practice. Patients are made NPO (nothing per oral) after midnight for solid food and milk products now. Clear fluids may be taken by mouth after midnight for most patients (and especially in pediatric patients without IV fluids) to protect against dehydration the next morning. This has been known for about 20 years now. However, there are some patients that must be kept NPO for everything when they have medical problems that slow or stop gastric emptying. There are standard texts that tell when post midnight they should be NPO for water too. In a normal healthy person that is to undergo elective surgery, the gastric emptying half-life for clear fluids is about 20 minutes. At the time of surgery, we are looking for a gastric volume of about 25ml or less and a gastric pH of less than 2.5. This is for protection against pneumonia if the person is to regurgitate and aspirate gastric fluids. I believe that most patients who are at home and are going for surgery in the morning are told to be NPO for everything by mouth, as they may not understand and might eat some solid foods, milk products (which can become solid when exposed to gastric acid), or liquids that include some solid or particulate matter (such as Orange Juice with orange pulp.)