Intra-abdominal free fluid
Ultrasound-based Diagnosis & management
This section will review the use of ultrasound for detection and drainage of free fluid in the peritoneal cavity. We will review a step-wise process to explore 1. ultrasound-based diagnosis of a free fluid in the abdomen 2. common pathologies that may resemble intra-abdominal free fluid and 3. ultrasound-based procedural guidance. Although we have selected a number of drainage devices that are available in the Edmonton zone, the essentials of this procedure remain the same, regardless of the device.
For the sake of simplicity, this page will not review the differential diagnosis (e.g. hemoperitoneum vs cirrhotic ascites), biochemical analysis, decision to whether a patient requires diagnostic or therapeutic drainage, decision to hold anticoagulation/address coagulation factors or adjunctive imaging.
Key references for this approach are at the end of this page.
a Step-wise approach
Step 1: COnfirm presence of intra-abdominal free fluid
Recognize free fluid
Free fluid in the abdomen can be detected in a variety of spaces from the left upper quadrant to the right upper quadrant to the pelvis. In this clip, we can see an anechoic (black) strip interrupting the hepato-renal space.
Detection for the purpose of diagnostic &/or therapeutic drainage
While free fluid can be detected in a variety of spaces (e.g. FAST exam), sampling is *most commonly performed in left or right lower quadrants, away from the large abdominal viscera and where the abdominal wall is likely to be less thick. Generally, the depth is set at between 12 and 15 cm.
Step 2: Rule out false positives or confounders
Ensure clear landmarks
The margins of the abdomen should be distinct. Confirmation of sub-diaphragmatic structures can easily be confirmed by recognizing such structures as the kidney. In this clip, we do not have enough information to clearly define the boundaries of the abdomen and risk falsely concluding the presence of free fluid in the abdomen.
False positive intra-abdominal free fluid
In some cases, dilated bowel next to the abdominal wall may resemble a pocket of free fluid. Be careful to look for features of small/large intestines including transiting particles, circular folds (plicae circulares, small intestine) or haustra (large intestine).
Step 3: Consider ultrasound-based (gross) quantification and characterization of fluid
Quantification of abdominal free fluid
While actual quantification of free fluid in the abdomen is challenging, many experts perform a rough "eyeball" measurement for the purposes of predicting safety in sampling (if indicated). In this clip, we can see a small amount of anechoic space interrupting the bowel from the abdominal wall. This site is likely to be a challenge for safe passage.
Typically free fluid in the abdomen is anechoic (black). Whereas ascites from cirrhosis is often more serous, free fluid from bleeding or purulence may demonstrate exudative properties including mixed echogenicity and strands. In this clip we can see the presence of multiple loculations, suggesting a complex intra-abdominal collection.
Step 4: SIte selection for safe needle passage
Site-selection for drainage
You can see in this clip two calipers have been placed to measure the thickness of the abdominal wall and the distance between the abdominal wall and adjacent bowel. While there is no clear "safe" distance, many suggest at least 2-3 cm is likely the safest distance between the abdominal wall and adjacent viscera. Once you have selected a "safe" site , we suggest you mark the spot with a marker or blunt plastic cannula.
Ensure no underlying vessels
Conventional blind landmarking suggest the safest site is at least 6 cm lateral to the midline to avoid the inferior epigastric artery. Many experts recommend a blind landmark as 3cm superior and 3 cm medial to the anterior superior iliac spine. A linear high-frequency probe with power Doppler can be used at the selected site to rule out large vessels as shown in the 2D + power/ color Doppler.
Step 4: Perform paracentesis
The most commonly selected site is left lower quadrant with the patient in slight left decubitus. This maximizes the fluid pocket and permits safe passage of a needle.
Static vs dynamic guidance of paracentesis
Dynamic guidance refers to directly visualizing a needle as it traverses a tissue ("real-time guidance"). This technique is commonly used in vascular access. Static guidance refers to identification and localization of a structure for safe puncture (without real-time guidance). For the purposes of paracentesis, we advocate for a pre-planned static-guidance based technique to ensure full attention on needle trajectory and depth. Prior to puncture, we recommend visualizing the space in two orthogonal planes to ensure a safe distance from visceral structures. In following the above sequence, you will identify a safe pocket without overlying vessels.
Fuhrman pigtail catheter
The Cook Medical website has a useful video to understand insertion of the Fuhrman (8.5 Fr) pigtail catheter. This catheter is often used for thoracic, pericardial and abdominal drainage. Insertion is performed via the traditional Seldinger technique.
This video by "carefusion" describes the use of the Safe-T-Centesis pleural-peritoneal drainage system.
Brown, G. M., Otremba, M., Devine, L. A., Gray, C., Millington, S. J., & Ma, I. W. Y. (2016). Defining Competencies for Ultrasound-Guided Bedside Procedures: Consensus Opinions From Canadian Physicians. Journal of Ultrasound in Medicine : Official Journal of the American Institute of Ultrasound in Medicine, 35(1), 129–141. https://doi.org/10.7863/ultra.15.01063
Millington, S. J., & Koenig, S. (2018). Better with Ultrasound: Paracentesis. Chest. https://doi.org/10.1016/j.chest.2018.03.034
Nazeer, S. R., Dewbre, H., & Miller, A. H. (2005). Ultrasound-assisted paracentesis performed by emergency physicians vs the traditional technique: A prospective, randomized study. American Journal of Emergency Medicine, 23(3), 363–367. https://doi.org/10.1016/j.ajem.2004.11.001
Sekiguchi, H., Suzuki, J., & Daniels, C. E. (2013). Making paracentesis safer: A proposal for the use of bedside abdominal and vascular ultrasonography to prevent a fatal complication. Chest, 143(4), 1136–1139. https://doi.org/10.1378/chest.12-0871