The FAST scan (focused assessment with sonography for trauma) is probably the most frequent application of bedside ultrasound with a moderate sensitivity and very high specificity. It is done as part of our trauma evaluation for blunt or penetrating chest/abdomen/back/pelvic trauma as well as in the evaluation of the unexplained hypotensive patient as part of the RUSH protocol and the patient with a possible ruptured ectopic pregnancy.
The FAST is not, however, to be done fast. It does have limitations. And, it is used as a screening tool (not a diagnostic tool – unless you practice in Italy or other European countries where they do contrast enhanced FAST scans that can detect solid organ injuries – very cool!). It is a screening tool for injuries that cause free intraperitoneal (not retroperitoneal) fluid accumulation. The patient should be supine (trendelenberg is even better), lights should be off (and before you say anything, I know – that never happens), and there should be enough time that has passed for the anechoic (black) free fluid to develop to an amount that can be seen by bedside ultrasound (this time can vary, I’m sure, which is why repeating the FAST scan is very important if a CT scan is not performed or if the patient’s hemodynamic status changes). Each region of the FAST scan should be explored fully and completely. As I’ve said before, don’t half-ass the FAST. Take your time. Be slow and deliberate. Do it right. Do it well. Be complete.
We usually start our FAST by evaluating the right upper quadrant because it’s the most sensitive region to evaluate for free fluid. Why is it the most sensitive area of the FAST? The posterior peritoneum attaches in such a way that free fluid from any injury anywhere will travel to the right upper quadrant in a supine patient (it is the most dependent region in the supramesocolic region). Place your low frequency probe at the right mid-axillary line in the lower rib cage (at the level of the xiphoid process – called the HS line) with the indicator pointed toward the patient’s head (a coronal view):
Assess that your probe is in the correct location by seeing (from top of screen to bottom): the liver on your screen and slowly angling it posteriorly to get the kidney also on your screen which is right above the psoas and then spine (with it’s shadow). If you do not see the liver, you may be too high or too low – slide down or up a rib space to get yourself oriented to the correct region with the depth usually being at 16cm.
Some things to know about performing a COMPLETE right upper quadrant evaluation for the FAST scan:
Be prepared: You will need to evaluate through multiple rib spaces – don’t try to get everything below in one rib space – can’t happen, hardly ever does. So, once you note that you’re in the correct region, you will likely need to slide up a rib space and start the below evaluation:
1. Evaluate above the diaphragm…. to evaluate for intrathoracic free fluid. This may involve having to increase your depth while you slowly fan your probe anterior to posterior. The patient’s breathing will also help you as you will notice the bright white (echogenic) linear diaphragm which hugs the liver will move (right and left) on your screen with every breath. Having the patient take a deep breath and holding it, will allow you better visualization if a rib shadow is in your way. A mirror image of the liver should be seen above the diaphragm (a normal artifact that occurs when ultrasound passes through structures of varying densities and then through air (lung). If no mirror image, and the area above the diaphragm is anechoic (black) – that’s fluid in the thoracic cavity! The spine sign will also help you evaluate for free intra-thoracic fluid, as the spine and it’s shadowing normally stops once it hits the diaphragm when visualizing on the screen from right to left (air is the enemy of ultrasound!). But if there is fluid in the thoracic cavity, you will see the spine continue to traverse past the diaphragm up into the thoracic area (fluid is the lover of ultrasound!).
2. Evaluate below the diaphragm… to evaluate for intraperitoneal fluid. If you increased your depth for #1 above, you will need to adjust it back. Black fluid below the diaphragm between it and the liver is abnormal as seen below:
3. Evaluate between the liver and the entire superior pole of the kidney (classic Morrison’s pouch view). Slowly fan anterior to posterior to visualize the entire superior pole of kidney – you should fan to the extent that the kidney goes completely out of view, comes back into view, then goes back out of view.
4. Evaluate between the left edge of the liver and the entire inferior pole of the kidney. This will also evaluate the right paracolic gutter. I was saving the best for last here. This image is key in not missing free intraperitoneal free fluid as this is where free fluid will be seen first in the right upper quadrant and with more accuracy than any other part of the right upper quadrant view.
Other things to know:
1. Slow and complete fanning is key to identifying free fluid as it allows you to evaluate the entire region of interest, not just one slice. You’re not a CT scanner, you’re an US scanner.
2. Adjust your depth so that your region of interest is large and centered on the screen. If you find that you’re squinting your eyes while you’re evaluating the right upper quadrant, then your depth is too much and thus the region is too small on the screen.
3. New blood is anechoic (black). Ascites is anechoic (impossible to differentiate. Clotted blood is echogenic (shades of gray) – I had a patient who came in once 2 days after being beaten up with abdominal pain – the right upper quadrant was positive showing only gray fluid – be careful not to miss it.
4. Perinephric fat surrounds the kidney in varying amounts depending on the patient. It is echogenic (with shades of gray), but hypoechoic (less bright) than liver and can be mistaken for free fluid. Keep in mind it doesnt move, it is NOT anechoic, and does not appear to be free floating like fluid would be. There’s also a bright line on its edge that doesn’t exist if it were free fluid. Its called the double line sign (one bright line being the outer kidney, the other being the outer edge of the fat). This double line sign helps differentiate it from free fluid as free fluid will not have a double line present.
5. You will detect 300cc of free fluid if you’re great, 500cc if you’re good, and 700 cc if you’re just ok. Be part of the 300!
6. Learn it well before robots take over!
A great lecture/tutorial online by Dr. Mike Stone on Vimeo on the right upper quadrant view of the FAST will put it all together.
For some tips and tricks on the RUQ, take a look at my other post – as sometimes doing all of the above doesnt get you that perfect image.
Why do False Negative FAST scans happen? Well, a prior post describes some of the literature.
The next post will have examples of right upper quadrant images – where we will think about what part of the above described sections is missing from those images and how you would evaluate that – as none of the images will be complete on their own (since it is only one rib space).