Wednesday, 26 September 2012

Pre-operative fasting: let them eat … anything?


It is routine for patients to be fasted before an anaesthetic, usually for six hours, but for morning lists patients are usually fasted from midnight. The fasting is meant to reduce the risk of aspirating (vomit and have gastric contents enter the lungs) during the induction of anaesthesia, which can damage the lungs and be very harmful. But does fasting decrease the risk of this complication occurring? Once again, the benefits of fasting have been overestimated and the harms have been underestimated. And once again, when faced with a difficult decision, doctors have chosen the (seemingly) cautious path, and in doing so have not provided any proven benefit, and have possibly harmed patients. It may not seem like a big deal, but with over 200 million surgical procedures performed in the world each year, it can add up.

The evidence presented is based on a Cochrane review from 2010; a massive undertaking (150 pages for the ‘standard’ version) with many comparisons reported, mainly around different fasting times for fluids and solids, and with outcomes such as aspiration, gastric volume, gastric pH, thirst, hunger, anxiety, pain, nausea and vomiting.

Admittedly the studies reviewed weren’t too adventurous, with the closest fluid intake being 90 minutes pre-operatively, and for food, just over 3 hours. But the results were telling. There were a huge number of comparisons reported, summarised below, but basically there is no direct evidence that different pre-operative fasting times affect the incidence of aspiration of gastric contents. The incidence of that complication is low (but it still occurs, although usually in people who have been adequately fasted, funnily enough) so most researchers chose surrogate outcomes, like gastric volume and gastric pH (with the idea being that the acidity causes more harm to the lungs, and that a high gastric volume increases the risk of vomiting - something never proven).

With all the various comparisons made, there was basically no difference in the risk of aspiration or in gastric volume or pH (with maybe one or two minor exceptions) but patients who were allowed to eat and/or drink within the normal 6 hours time period were less likely to be thirsty, hungry and anxious. The harm they did not measure is the cancellation rate. In my experience, operations are frequently cancelled because the patient had something to eat or drink inside the magic 6 hour period. This causes further distress for the patient, and wastes valuable operating room time.

In trying to summarise the findings, I ended up counting all the comparisons made. There were 124 comparisons reported, with 99 of those comparisons showing no difference in the results between standard fasting and all the different types of non-fasting (different fluids – even coffee and juice - and solids, at different times pre-operatively); 23 comparisons showed the group that were not fully fasted did better (nearly all due to less thirst, hunger and anxiety); and in 2 cases the fasted group did better (for gastric pH).

Not only is there no empirical evidence that fasting decreases complications, but the biological mechanism doesn’t even hold up, with similar gastric volumes and pH in those that were allowed to eat and drink past the 6 hour rule. So we now have another example of no clear benefit, a flawed biological mechanism, and documented harms. Has practice changed?  A little.  Anaesthetists will often tolerate some fluids up to a few hours pre-op, but overall we are still being over-‘cautious’, and in doing so we are not providing any clear benefits, we are making patients thirsty and hungry, and we are cancelling cases unnecessarily.

6 comments:

  1. Hello I read this blog post with great interest...

    In Cardiology (medical, in contrast to surgery), patients are typically kept NPO even before percutaneous procedures such as angiography, stenting, EP studies, and ablation. Some of these case are performed under general anesthesia, but the majority are performed under conscious sedation, without airway protection.

    There has been a rather subdued discussion regarding the utility of NPO status prior to cath-based procedures. Traditionally, as mentioned in your blog post, patients have been asked to remain NPO p MN, or for at least 6 hours. It is an open question as to whether this prevents adverse outcomes (eg nausea, vomiting, aspiration) or if we are exposing patients to harm (hunger, anxiety, hypovolemia, hypoglycemia).

    Historically, nausea, vomiting, and anaphylactic reactions were observed with ionic, hyperosmolar contrast dye. With regard to frequency, nausea >> vomiting >>> aspiration. And of course, "witnessed" vomiting and aspiration was even less common. Most of these observations come from a large Interventional Radiology study, which, may or may not be applicable to Interventional Cardiology.

    In the subsequent decades, there have been further advances in contrast chemistry, to the point where the majority of dye used today is non-ionic and isosmolar. There are far less adverse events with these kinds of dyes.

    Today, the majority of nausea, vomiting, syncope, and aspiration is usually attributed to the vasovagal vents during groin sheath pulling or holding firming pressure in that area. Again, witnessed events are exceedingly rare. In the last several years, radial access for coronary catheterization has become more prevalent and will invariably become the access of choice in the US. The occurrence of vasovagal events with this access site are virtually nil.

    Where do we go from here... a randomized controlled trial would certainly be in order. I certainly believe there is clinical equipoise. Maybe as a start, we could quantify the number of witnessed vomiting and aspiration events in patient's presenting with STEMI... I don't think anyone is sitting at home saying, "I think I'm going to skip lunch in anticipation of STEMI later tonight."

    If any of this research is currently being conducted, I'd really like to hear about it. Fortunately, in the absence of solid data, many institutions are allowing a "light breakfast" (eg. tea and toast) on the morning of procedures.

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    1. Thanks for detailed comment. Argument will probably go on forever. I just want people to consider the evidence and not accept the dogma.

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  2. That's very interesting - I often take things told to me by other subspecialists as a given - especially someting that gets repeated over and over like fasting times. In fact that has been drummed into us from medical school onwards. Interestingly the only major aspiration issue i've seen occured with a well fasted (>9 hours) patient.

    Did the studies use different techniques of induction/tubes vs masks etc when comparing shorter and longer fasting times - as there seems to be a difference in practise if a patient has to be brough to theatre in an emergent setting?

    I guess as you have mentioned that many of our "standard operative" practises are not really that well evidence based - mask wearing doesn't seem to do anything to change infection rates (though orthopaedic procedures were excluded from most studies i've read), and while i'm sure handwashing for prolonged periods may make some difference, numerous papers have used alcoholic handrubs to the same effect.

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    1. Thanks mmv.
      As far as I know, hand washing and masks don't affect infection rates, but like a lot of these things, practice will continue for obvious reasons.
      I read the review right through and there was no mention of any studies on the type of tube / mask used, but they may be part of a different review that I haven't seen.

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    2. Like the practice of wearing a jacket and full hood when preparing for a total joint replacement? Where is the evidence for that? And "space suits" for joints as well. Everyone seems to follow that practice despite the NZ joint registry having its highest incidence of joint infections in cases where surgeons wear "space suits" and have laminar flow.

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    3. Thanks,

      Yep, like a lot of things it relies on the "wobbly tripod" of evidence. See: http://doctorskeptic.blogspot.com.au/2012/08/reasons-to-operate-wobbly-tripod-of.html

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