In the July issue of ACEP news: there was an article which highlighted a multi-center study’s results of over 6,000 kids that discusses the FAST scan in the pediatric population.(study has yet to be published, as I cannot find it anywhere) (FAST = focused assessment with sonography for trauma). It was discussed at SAEM as well. The findings are not surprising: FAST scan is done with low frequency in kids and when it is, it has a low sensitivity and high specificity (if negative, it does not rule out injury). But, one of the exciting parts of it was that low and moderate-risk kids got fewer CT scans when a FAST scan was performed. One of the main authors is Dr. James Holmes, from UC Davis, who has studied ultrasound in trauma extensively, most recently highlighted in JAMA assessing adult patients and the predictors of injury, concluding the FAST scan being the most accurate.
This topic is near and dear to my heart as it was my residency research project where we gathered data for the 3 years that I was at UC Irvine, with preliminary results upon my graduation, and then more data gathering for the 2 years after my graduation culminating into the publication.
To sum it all up with regard to FAST scans: depends on operator (be a rockstar sonographer), ambient light (the darker the better), patient position (trendelenberg is best, but supine will do), time to FAST (the longer the better) and…
1. Its tough to study FAST scan in kids. Why? Well, they usually do well and have no injury. The studies done over the years have a very low power because of that alone. Lastly, the sensitivity and specificity is tough to evaluate as each study defines a true positive/negative differently. Some compare the FAST to any amount of free fluid on CT (even if its not due to injury and only physiologic); some compare the FAST results to whether the patient needed admission, or operative intervention; others do it somewhere in between the two.
2. Most studies have the following results: Sensitivity 60-85%. Specificity 90-99%. A positive FAST – you’re done – its positive (go to CT if stable; to OR if not – but definitely not going home). A negative result, you still may miss injury/fluid so correlate clinically, which means a good physical exam with your suspicion of injury. These patients can go home, be observed with serial exams/FAST, or CT depending on those factors.
The FAST scan does not diagnose injury. It screens for intraperitoneal free fluid. period.
Keep in mind what the FAST scan will miss:
1. Injuries that do not cause free fluid
2. Injuries causing retroperitoneal free fluid
3. Injuries that cause <300 cc intraperitoneal free fluid (the lower the fluid amount the more likely to miss)
4. Injuries causing free fluid where the FAST scan is done too early in free fluid accumulation, and, therefore, will not detect it
Keep in mind FAST scan limitations:
1. Pelvic injuries and free fluid – even though it’s the most dependent part, free fluid here can be missed – bowel gas scatter, empty bladder, large area for fluid to accumulate, retroperitoneal fluid accumulation
2. Solid organ injury – will only be suspicious if significant free fluid develops (>300 cc)
3. Hollow viscus injury – may not develop free fluid or the amount of free fluid in time to identify on FAST
4. Pancreatic injury – may not develop free fluid
5. Diaphragm injury/perforation
The final point: repeat your FAST scans, especially if you don’t CT, but your patient should be kept supine to increase your accuracy