detection of free fluid in the abdomen

Abdominal ultrasound in the eFAST exam is primarily focused on the detection of intra-abdominal free fluid as a marker of visceral organ injury. Further, ultrasound can be used to help guide paracentesis in the case of differentiation of hemoperitoneum from simple ascitic fluid. Herein, we describe critical elements of this examination for learners newly approaching this examination. For further reading please refer to references 1 and 2 and view the flipped classroom video in the training section of this web-page.

Although the learning outcomes for the intra-abdominal free fluid (IAFF) element of the eFAST exam are relatively straight-forward, there are a number of important limitations that must be considered. Notably, US is not necessarily a substitute for clinical judgement, nor does it replace more advanced imaging modalities.

Probe selection often depends on probe accessibility, user-preference and comfort. The most common probe for examination of free fluid is the curvilinear probe, as its broad-footprint, low-frequency and high penetration depth are favourable for abdominal examination.

The following images outlines multiple factors that may limit findings of the eFAST exam. While this is not all-inclusive, the categorical considerations are simply described under technological, acquisition/interpretation-related, and patient-specific factors. Further, learners must be aware that selected patients may require additional imaging regardless of their sonographic findings, which may limit the applicability of their focused examination.

On top of these limitations, several other factors can limit the ability to sonographically detect IAFF (3).

The 3-point technique is the most common approach to detection of IAFF. The first quadrant often examined is opportunistic, but generally starts at the RUQ (as it is the first or second place IAFF will accumulate). Importantly, the sacral promontory divides the upper and lower torso and its respective dependent locations. Learners often find the pelvic views most challenging to acquire and interpret.

In the upper quadrants, the examination focuses on 4 distinct areas, specifically interrogating for the presence of an anechoic space. In the RUQ, the sub-diaphragmatic (1), hepatorenal (2), infra-hepatic (3) and inferior pole of kidney (4) should be examined. The left substitutes the spleen for the liver with similar anatomical points. Only when all spaces are clearly negative is the quadrant declared, "negative".

These comparison image demonstrates a normal reference image compared to a positive study (with an anechoic strip in the hepato-renal space). The hepato-renal space is the most anatomically dependent space in the RUQ. Notably one study has found the liver tip to be more sensitive (4), highlighting that this exam should really be performed with vigilance.

The image to the right shows normal transverse (axial) supra-pubic images. Bladder decompression is often a mitigating factor, preventing full sonographic visualization of the pelvic space. In males, free fluid accumulates in the recto-vesicular space. In females, the recto-uterine pouch (of Douglas) is the most inferior peritoneal reflection where fluid accumulates. Fluid can also accumulate in the vesico-uterine space.

Image right demonstrates frankly positive transverse pelvic images. Importantly, we will not cover longitudinal pelvic images in this sub-section for the sake of brevity.

Procedural guidance

Typically, paracentesis is performed in the left lower quadrant, where the abdominal wall is most thin, in the safest area of approach. While this technique is relatively simple, we would encourage those interested in performing this technique to seek training by a skilled professional to ensure this technique is performed carefully.

References

1. Patel NY, Riherd JM. Focused assessment with sonography for trauma: methods, accuracy, and indications. Surg Clin North Am. 2011;91(1):195-207.

2. Boniface KS, Calabrese KY. Intensive care ultrasound: IV. Abdominal ultrasound in critical care. Ann Am Thorac Soc. 2013;10(6):713-724.

3. Meyers MA. Dynamic Radiology of the Abdomen: Normal and Pathologic Anatomy, 4th, Springer-Verlag, New York 1994. p.55.

4. Lobo V, Hunter-Behrend M, Cullnan E, et al. Caudal Edge of the Liver in the Right Upper Quadrant (RUQ) View Is the Most Sensitive Area for Free Fluid on the FAST Exam. West J Emerg Med. 2017;18(2):270-280.