Ultrasound Assessment of Gastric Fluid Volume in Children Scheduled for Elective Surgery After Clear Fluid Fasting for 1 Versus 2 Hours
A Randomized Controlled Trial
June 2023 by Dr David Stoeter
This small randomized single-blinded controlled trial is the second such trial comparing gastric fluid volumes after a 1-hour versus a 2-hour clear fluid fast in children undergoing general anaesthesia for elective surgery. It is the first utilising ultrasound. The authors (from a major university hospital in Cairo, Egypt) postulated that a 1-hour fast would result in significantly higher gastric volumes. They did indeed demonstrate that the volumes roughly doubled, and suggest that a 1-hour fast may result in an unsafe stomach. The questions the reader is specifically interested in, however, are: Is there a gastric fluid volume at which the risk of aspiration is significantly increased? Does a 1-hour fluid fast lead to a significantly greater proportion of patients having gastric volumes above this threshold?
The former question has as yet not been clearly answered. It would require a study of 10,000-20,000 patients in order to reliably demonstrate an increase in aspiration rates above baseline levels. What we do have is attempts to define a normal distribution of gastric volumes in small (ranging 34 to 538 patients) observational studies of both adults and children with and without risk factors for regurgitation or aspiration events. These normal distributions seem to demonstrate that around 95% of children have volumes less than 1.5ml/kg and that there are always small groups of outliers with larger volumes occupying the remaining 5% who have volumes greater than this regardless of clear fluid fasts of 2, 5 or 6-hours. This study demonstrates that whether a 1 or 2-hour fast is enforced, a child’s gastric volume never lies above this 1.5ml/kg threshold. Other risk thresholds of 1.25ml/kg and 0.8ml/kg (as mentioned by the authors) seem to be based on an arbitrary grading system that has not been related to aspiration risk nor to a normal distribution of the population of interest.
Strengths of the study included a robust overall design, a fairly well powered sample size, standardisation of quantity and caloric content to a maximum of 3ml/kg and 0.42kcal/ml respectively in keeping with standard practices in many countries and international recommendations. They did not comment on exactly how successful they were at enforcing the precise 1 and 2-hour fasts. It would also have been helpful to have some more parameters reported such as mean, standard deviation and range in order to establish the nature of the normal distribution in each sample. Their reference to several difference risk thresholds (0.8ml/kg, 1.25ml/kg and 1.5ml/kg) is likely confusing for practical application.
In summary, this study shows that whilst gastric volumes are higher after a 1-hour versus a 2-hour fast, this does not seem to place patients outside of the normal distribution of what is understood to be a fasted stomach and would, contrary to the authors conclusions, support a 1-hour minimum fast given that we know that most patients who apply this rule in practice fast longer than 1 hour. It would be helpful in the future to better define the normal distribution of gastric volumes in fasted children for the sake of studies like this one since some of the statistical reporting in existing studies is not always crystal clear on this point. A multicentre study is likely necessary to generate the numbers needed to relate this to aspiration risk.