Pearls & Pitfalls for Trauma Ultrasonography
1. Blood appears anechoic (black) when acute and free flowing, but hypoechoic (gray) when subacute or clotted. When blood is gray it is much easier to miss, especially if you are not looking for it. Blood in the pericardial space that is related to trauma is often hypoechoic (gray).
2. The sensitivity of ultrasound in trauma improves with Trendelenburg positioning, repeat imaging and if you know how and where to look (experience). Always image the inferior tip of the liver, as this area may be positive when Morrison’s pouch is not. Also, turn down the gain on the pelvic view if the bladder is full, otherwise the enhancement behind the bladder may “white out” a small pocket of fluid. The sensitivity is ~85% if done serially, but as low as 24% in some studies. Usually >200ml of fluid can be detected
3. The specificity of the FAST exam is about 95%. False positives may include fluid mimics such as the prostate, psoas, a perinephric fat pad, severe hydronephrosis, fluid filled bowel or stomach, or a large blood vessel. They may also include other causes of free fluid such as ascites, urine, physiologic fluid, and inflammatory fluid from infection, pancreatitis or ischemic bowel.
4. If you cannot see the heart at all on the parasternal view, consider a small anterior left sided pneumothorax until proven otherwise. The chest film usually will not be sensitive enough to pick this up. If the patient is going to the OR consider either going to chest CT first or placing an empiric chest tube if the patient remains unstable and must be rushed to surgery.
5. Ultrasound can also be used to evaluate directly for pneumothorax or hemothorax. For hemothorax use the curvilinear probe to image above the diaphragm for fluid. Ultrasound is more sensitive than x-ray as it can detect as little as 20ml of fluid. For pneumothorax, use the linear small-parts probe in the sagittal plain. If safe, elevate the head of the bed 60 degrees and scan 2-3 interspaces at the most anterior part of chest. Look for the absence of the normal lung sling & comet tail artifacts that move with respiration. Sensitivity has been reported at ~95%, which is better than chest x-ray. False positives include mainstem intubation, bleb, infiltrate, contusion, ARDS, atelectasis, adhesions, and pulmonary fibrosis
6. If you work in a trauma center and don’t have an ED dedicated bedside ultrasound machine yet, you should. Get together with the trauma surgeons and request the hospital buy one and put on a course to train or re-train your doctors.