Looking to reduce and prevent dangerous complications from anesthesia and sedation during surgery, hospitals and surgical centers are ramping up emergency training for medical staff and keeping a closer eye on patients on the operating table.

Anesthesia is the part of surgery many patients may understand the least, yet it can result in a wide range of complications, from cardiac arrest to damaged vocal chords. A growing body of research is showing that existing medical conditions, such as obesity, diabetes, sleep apnea, asthma, heart disease and some hidden genetic conditions, can put patients at higher risk for complications.

As more surgical procedures are taking place in outpatient centers with faster-acting drugs such as propofol and quicker patient releases, some medical experts worry the facilities aren't adequately equipped to handle anesthesiology emergencies. Patients must be closely monitored for breathing problems and overdose, especially the elderly.

The death in September of Joan Rivers, 82 years old, at a New York outpatient endoscopy center was due to a lack of oxygen during a throat procedure while sedated with propofol, according to the office of the city’s medical examiner, which described it as “a predictable complication of medical therapy” but didn't elaborate in a statement earlier this month. It is not clear exactly what caused Ms. Rivers to be deprived of oxygen.

Although some form of anesthesia is used in the more than 100 million surgical procedures each year, ranging from nerve blocks that numb pain to general anesthesia drugs that immobilize patients in a deep sleep, how anesthesia works on the brain remains something of a mystery.

“Evolution didn’t intend for patients to be rendered unconscious and impervious to pain so someone could slice them open and close them up again,” says David Gaba, a professor of anesthesia at Stanford University School of Medicine and founding director of the simulation center at the Veterans Affairs Palo Alto Health Care System in California. “We don’t completely understand the mechanisms of general anesthesia.”

What can go wrong with anesthesia can be surprising and fast-acting. An inherited disorder known as malignant hyperthermia can be triggered by certain inhaled anesthesia drugs, causing a fast, steep rise in temperature and muscle contractions that may be fatal.

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