A Point-of-care tool for aspiration risk assessment
A Point-of-care tool for aspiration risk assessment
1) Why the antrum?
a. The distal regions of the stomach (antrum and pylorus) are most consistently seen in ultrasound. The body usually has a larger air content that may interfere with the exam and the fundus is of difficult US access.
b. The antral contents are reflective of overall stomach contents.
c. The antral size is the least variable and the one that correlates best with gastric volume.
2) What can I do if I cannot visualize the antrum?
a. Use the landmarks (identify liver and aorta/IVC, then focus in the region caudad to the liver and anterior to the vessels.
b. Change the patient’s position (supine to right lateral decubitus (RLD)
c. Ask the patient to take a slow deep breath: this may move the transverse colon more caudally bringing the antrum into view.
d. Try rotating and tilting the probe.
e. In 2-3% of patients the antrum may not be identifiable.
3) How can the stomach be distinguished from the colon or the small bowel?
a. The stomach has a characteristic thick multi-layered wall. The most prominent layer is the hypoechoic muscularis propriae that is almost always apparent. The colon and small bowel have a thin wall.
b. Scanning from right to left can highlight the different stomach areas (pylorus- antrum –body)
4) Aorta versus inferior vena cava (IVC): does it matter which vessel is on the screen?
Both the IVC and the Aorta lie posterior to the pylorus and the antrum and both can be identified in the course of the exam. For estimating gastric volume, measuring the antrum ( in the RLD) at the level of the aorta is recommended for greater precision.
5) What are the limitations of gastric ultrasound?
a. The stomach may not be easily found in 2-3% of subjects despite proper scanning technique
b. The findings may not be reliable in subjects with previous gastric surgery (e.g. partial gastrectomy, gastric by-pass) or large hiatus hernias. Volume estimations in particular will likely be inaccurate in these subjects.
6) How long does the exam take?
Usually less than 5 minutes.
7) Can you estimate any volume accurately?
The model presented here has been validated from 0 to 500 mL.
8) Does 1.5mls/kg refer to Ideal Body Weight or Total Body Weight?
This refers to total body weight.
9) Is the model reliable?
Volume estimates are highly reliable provided the scanning and measurement technique described are followed systematically.
10) Can you explain the effect of age on predicted volume?
For a given gastric fluid volume, older patients tend to have a higher antral CSA than their younger counterparts. The reason for this observation is unknown; however, it could possibly be explained by a more compliant gastric wall in older patients.
11) In a patient with symptomatic gastroesophageal reflux disease (GERD) does an empty stomach suggest that endotracheal intubation is not necessary?
Not necessarily. GERD is a complex disease that affects many parts of the gastrointestinal tract. For example, duodeno-gastric reflux is also common in these patients. So an empty stomach at a given point in time does not guarantee the stomach will remain empty for the duration of the surgery. Therefore, in patients with symptomatic or severe reflux, a conservative approach to management including endotracheal intubation may be warranted.
12) If I am unable to visualize the stomach, can I assume it is empty?
Although it is easier to visualize the stomach when it’s full, an exam should be considered inconclusive if it is unable to find the organ. This can occur in 2-3% of subjects. One possibility is that it could be “hidden” posterior to the colon. The most prudent approach in this situation is to guide management according to patient history.
13) Do you think widespread use of gastric ultrasound will replace the American Society of Anesthesiologists’ (ASA) fasting guidelines?
Fasting guidelines are based on best available evidence and are an important component of current standard of care. However, they are only applicable to relatively healthy patients presenting for elective surgery. Fasting intervals may be unreliable in subjects with significant comorbidities (diabetes, severe renal or liver dysfunction, GERD) or who are acutely ill. These are likely the situations where gastric ultrasound is most useful to individualize risk assessment and guide management.
14) Should every patient have a preoperative gastric ultrasound exam?
This is not recommended. In situations where the pretest probability of a full stomach is either very low (healthy elective fasted patient) or very high (e.g. actively vomiting, known proximal bowel obstruction, ingestion of solids within 2-3 hours), gastric ultrasound will add little to the clinical assessment, and may even “confuse the picture”. Rather, like all other diagnostic tests, gastric ultrasound is most useful in situations of clinical equipoise (i.e. pretest probability of 50%), when gastric content is truly uncertain.
15) Are full stomachs common despite appropriate fasting intervals?
In healthy fasted elective surgical patients, the incidence of a full stomach is extraordinarily low (about 2%). This is likely more common in patients with significant chronic comorbidities. In urgent or emergency situations, fasting intervals are unreliable.
16) Is it necessary to begin the scan in the supine position given that the majority of the information is obtained from the RLD position?
Scanning in the supine position is helpful for two reasons: 1) If solid or thick fluid is observed in the supine position, the stomach can be considered full and the exam complete 2) Scanning in both supine and the RLD position allows for a semi-quantitative evaluation of volume ( grade 0-2).