By the 1960s the term nil by mouth (or its Latin variant NPO, nil per os) after midnight had become the widely accepted guideline for all surgical patients. If you recently had an elective procedure, you might know that it has not changed much since—fasting before surgery, meaning no food and no water, is still advice routinely given to preoperative patients. Yet the evidence—and medical practice, and even the recommendations—have evolved since Mendelson. Medical practice has yet to catch up.
For one thing, anesthesiologists no longer use ether, a substance known to make patients nauseated. They also employ endotracheal tubes, which protect the airways from the aspiration of stomach contents. Knowledge about digestion has increased to the point where the rate of calories leaving the stomach is predictable: A spate of studies on gastric emptying found that patients who consume clear fluids two hours prior to an operation do not have higher gastric volumes than those who fast for longer. In 1999, the tide of mounting evidence pushed the American Society of Anesthesiologists to amend its preoperative fasting guidelines: Patients are now instructed to have a light meal six hours before a procedure and clear fluids—drinks that you can see through, such as pulp-free juices, black coffee, or tea without milk and cream—until two hours prior to the operation. Guidelines in other countries were similarly amended.