Anyone who has had a general anaesthetic will be well aware of the need to fast beforehand. ‘Nil by mouth’ (NBM) or ‘NPO’ (nil per os, os being the Latin for mouth) instructions are part of everyday life on pre-operative wards.
This withholding of food and liquids before a general anaesthetic is necessary is because of the risk of the full stomach emptying all or parts of its contents into the patient’s lungs. Once general anaesthesia is induced, the protective reflexes that would normally cause us to cough and splutter in the event of the stomach emptying in the wrong direction are lost along with most other reflexes.
Solid material poses the biggest threat since it can mechanically obstruct the airway. Indeed, this was recognised as early as 1946 by Curtis Mendelson when he described an aspiration syndrome and several fatalities in pregnant patients under general anaesthesia.
However, what is often forgotten about this ground-breaking work is that whilst solid aspiration was indeed a significant issue, aspiration of liquid contents did not result in any long term issues or fatalities even in an era of very different levels of postoperative care that we enjoy today. This led to historical fasting guidelines which have remained fairly universally followed ever since, in that solids should be avoided for at least six hours before general anaesthesia and liquids for two hours.
In practice, however, these rules translate into much longer fasting times due to the unpredictability of the operation day and patients’ understandable anxiety not to fall foul of any rules that may lead to delay or cancellation. Most studies in paediatrics adhering to a two hour clear fluid fasting policy result in an average fasting time of more than six hours.
This is an unnecessarily long time on an already stressful day for parents and patients alike. This is particularly true for young patients who do not understand why they cannot have a drink. Parents, who themselves may well be anxious about the impending procedure, have the added stress of keeping their child fasted beyond what they are used to and having to deny their repeated pleas for a drink.
Recently these rules have been challenged, demonstrating that a more liberal policy of allowing drink up to the time of being called to the operating theatre does not result in any greater incidence of aspiration than more conservative rules in more than 10,000 patients.
Several UK paediatric centres are now recognising the suffering and adverse physiological effects (such as greater drops in blood pressure at the start of anaesthesia) that prolonged fasting can cause. Many have moved to a one hour clear fluid fasting policy that allows for a drink on arrival at the admission unit in most cases. There is, naturally enough, anxiety amongst many anaesthetists that a more liberal regime may result in a higher rate of aspirations that may in turn lead to respiratory complications but the available literature does not support this view.
Of course aspiration rates continue to be monitored and dealt with promptly and effectively if and when they occur, as they always have been. Anaesthetists are risk-averse by nature and are understandably conservative about challenging dogma but we all recognise that, especially in young children, we need to be more patient-focussed in this regard.
Featured image credit: Rubber duck by Photographer2015. CC0 public domain via Pixabay.