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Why Am I Allowed to Drink Clear Fluids Before Surgery, but Not Eat?

Surgeons often instruct patients to refrain from consuming food or drink before surgeries. However, even when patients aren't allowed to eat, they can often drink "clear fluids". Why aren't clear fluids counted as consumption, and what purpose does fasting before surgery even serve?

While preparing for my recent knee surgery I was given a lot of instructions. Where to park, how to dress, when to arrive, what to bring and even what (not) to eat and drink. Like many surgical patients, I was given a time after which I was no longer allowed to eat and a different time after which I was not allowed to drink clear fluids.

Being a Diet Coke lover, I immediately wondered what it is about brown, opaque Diet Coke that makes it not OK to consume close to surgery. What is it about clear, colourless Sprite that makes it fine? As a more direct comparison, why would white grape juice be OK but not purple grape juice? I wondered if somehow the dyes in the drinks could interfere with surgical imaging or complicate emergency procedures I could need like intubation. Maybe the colourings could affect my mouth, stomach or urine in a way that made surgery more difficult somehow.

If you’ve already spotted the folly in my logic, you’re several hours faster than me. What I eventually realized is that I, and likely many patients put on a clear liquid diet, was fundamentally misinterpreting the word “clear.” While I heard “clear” as in colourless, what the doctor meant was clear as in transparent or free of small particles.

Once you realize the reasoning behind clear-fluid diets for surgical patients, it becomes obvious that non-cloudy is what doctors mean. But I, like most patients, wasn’t given the reasoning, only the instruction. So, allow me to explain to you what wasn’t explained to me: why food matter in the stomach during surgery can lead to pulmonary aspiration, and why clear fluids help prevent this.

In 1946 Curtis Mendelson published this paper examining the details surrounding 66 women who, while giving birth, had some of their stomach contents travel into their lungs. Mendelson concluded that this aspiration was occurring while patients were under general anesthesia. While sedated their throat reflexes were inhibited, which allowed food matter from the stomach to travel up the esophagus and enter the trachea and lungs. In some cases, this resulted in the patient asphyxiating or choking to death, whereas in others it went undetected until after labour had been completed when it caused infections, pneumonia, and other pulmonary complications.

Thus, to help avoid pulmonary aspiration he recommended several things, including that women in labour not be allowed to eat, as they might need to be unexpectedly placed under general anesthesia. These recommendations have been adjusted and refined in the last 74 years, but Mendelson’s main direction, that those preparing to be sedated should fast, has stuck.

The reason clear, particulate-free liquids are allowed when eating isn't has to do with how quickly foods and drinks of different compositions leave the stomach. While food, especially fat- or protein-rich food, can take up to 8 hours to leave your stomach, studies have shown that clear fluids like coffee, water or pulp-free orange juice are clear of your stomach within two hours or faster. Therefore, clear fluids can be safely consumed up until two hours before surgery. This is true for almost all patients, including those who are pregnant, young, obese or anxious.

Thus, patients should be allowed to freely consume clear fluids like apple juice, coffee, Gatorade, tea (with no or little milk), or chew gum, up to two hours before surgery. This practice has some obvious benefits. For one, patients are less thirsty. I was given the traditional “nil par os” after midnight order (NPO, Latin for “nothing by mouth”) but because my operation took place in the early morning, I only went about three hours without water. Nonetheless, I still felt quite parched. When I woke up after my surgery the very first thing I asked was if I could have a drink.

However, if a patient is scheduled for afternoon surgery and given an NPO order, they could be going 12 hours or more without drinking. That’s not fun for anyone, but for the elderly or those with medical conditions, it could be quite harmful. For children or infants, going several hours without fluids can increase their discomfort and make them grumpy or upset, making the lives of parents, and the jobs of nurses, surgeons, and anesthesiologists more difficult.

Beyond patient comfort, allowing clear fluids, and specifically giving patients a carbohydrate-rich beverage preoperatively, can help them avoid developing insulin resistance and possibly lead to shorter hospital stays. There’s also evidence that unrestricted access to clear fluids preoperatively can decrease the occurrence of nausea after surgery. Plus, as this article points out, given that saliva production and gastrointestinal secretions lead to about 2 milliliters of liquid entering your stomach per minute anyways, small sips of water will hardly make a large difference in the gastric volume of a patient.

So, there’s a good reason that you can’t eat before surgery, namely, pulmonary aspiration and pneumonia. But if there’s no good reason to avoid drinking until two hours before surgery, why do so many hospitals still prevent patients from drinking for more hours than necessary?

Well, doctors likely believe that simpler instructions such as “stop eating and drinking at midnight” are easier to follow for patients than “stop eating and drinking non-clear fluids at midnight and stop drinking clear fluids at 6 a.m.” They may have never questioned the standard NPO order or have seen the policy statements by organizations like the Canadian Anesthesiologists’ Society.

But perhaps most importantly, because doctors are being needlessly cautious. Even though an abundance of evidence tells us that clear fluids are gone from the stomach within two hours, they’d rather leave a bigger window just in case. However, the negative consequences of leaving this window have to be considered too. When they are, and when the research is examined critically by experts, the results are the two-hour recommendations that all hospitals should really be following.

Next time I have surgery, if there is a next time, I will definitely be questioning any NPO orders I get, and I’d encourage any medical professionals reading to inquire into the fasting protocols at their hospitals. Maybe there’s a good reason that a doctor wants a patient to fast for longer than recommended, but it should not be standard practice.

To read about how, in the end, I didn't need to be fasting at all, click here!


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