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.