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ABDOMEN AND PELVIS
INTRAPERITONEAL FREE FLUID
The main pathology we look for in the abdomen of the trauma patient is intraperitoneal fluid. Fluid can be difficult to differentiate from fluid filled organs and vessels. The key to remember is that fluid infiltrates potential space. So the edges of a fluid collection will have acute angles and spiky corners. Comparitively, organs and vessels have rounded edges. Secondly, except for known fascial layers, fluid does not have echogenic walls like vessels and hollow organs.
FF superior to bladder: note: the FF has bowel floating in it and has spiky corners. The bladder has a rounded border.
DETECTING FREE FLUID
Branney et al (1) showed that doctors with an undifferentiated US skill set, who only ultrasounded Morison's pouch, could detect a mean volume of 619ml The minimum volume of fluid detected was 225ml and 97% of participants detected free fluid when >1L was infused into the abdomen. This is an important study because it shows that the sensitivity of US for FF is low at small volumes (but with experience you can detect small volumes!) and also importantly. when it matters (ie the hypotensive patient with a litre of blood in their belly), US is very sensitive.
So trust what you see. If you see free fluid, it probably is there. Also, go looking for small fluid collections because this puts the patient in a higher injury category and it's good to know that at the bedside than getting a call from radiology.
AREAS OF FLUID COLLECTION
In the supine patient, it collects first in the paracolic gutter and then Morison's pouch (2). In the upright patient, it will collect in the pelvis. The location of the haemoperitoneum does not correlate wih the organ which has been injured except in the LUQ (2). Localised perisplenic haemoperitoneum is associtaed with splenic injury (3,4). In the LUQ: make sure you look carefully between the spleen and the diaphragm, because fluid often collects here.
Anechoic FF around the paracolic gutter (note: Morison's is empty)
Detecting free fluid in Morison's pouch is relatively simple. The LUQ and pelvis are a bit trickier.
anechoic FF all around the spleen
In the pelvis, a fluid collection can be mistaken for a full bladder. Looking in both transverse and longitudinal will help. Look for bowel floating in free fluid. The bladder wall is also thick and muscular.
Free fluid posterior to the bladder and superior to the uterus. Note the acute angled edges, bowel floating in the FF and the thick muscular wall of the bladder.
Baque et al (5) showed that you can estimate the volume of fluid in the abdomen most accurately by measuring the cranio-caudal lenth of the fluid in Morison's pouch. The formula for this is vol (ml) = 33x + 470 where x is the distance in mm.
As a rule of thumb, about 1cm in Morison's pouch would equate to about 1L of fluid and 5mm about 500ml.
Haemoperitoneum vol (ml) = 33x (cm) + 470
APPEARANCE OF HAEMOPERITONEUM ON US
Unclotted blood appears anechoic or black on US (6). However, with time, the blood clots and becomes isoechoic or hyperechoic in comparison with solid organs like the liver or spleen. Thus in delayed trauma presentations/ transfers look for an abnormal contour or thickening of solid organs to discover a surrounding haematoma.
RUQ Paracolic gutter showing isoechoic haematoma above the liver
Sometimes in the case of severe lacerations to an organ, it may be difficult to discern normal organ architecture. This is particularly true of the kidney where the bleeding is usually contained in the retroperitoneum by Gerota's fascia.
MIMICS
FAT
Intra-abdominal fat can look like echogenic haematoma. However, it is more heterogenous (strandy). Also, free fluid is never seen between the kidney and the spine, whereas fat usually sits anterior and posterior to the kidney. Sometimes, the fat may only reach below the superior pole of the kidney. Fluid is unable to do this due to Gerota's fascia.
Heterogenous fat surrounding the kidney
Heterogenous fat lifting the superior pole of the kidney
Fat is heterogenous and echogenic and is seen posterior to kidney
Free fluid prevented from going posterior to the kidney by Gerota's fascia
Haematoma sits anterior to kidney
EDGE ARTEFACT
Sometimes when US encounters a curved structure, it becomes refracted laterally. When this occurs, information deep to the curved structure is usually lost as the US wave does not return to the same US crystal on the machine. This creates a drop-out (anechoic shadowing-like area) in a wedge shape adjacent to the curved edge.
The edge artefact may appear like free fluid. Important factors that help with differentiation are: edge artefact is attached to the contour of the kidney, is seen all the way down the screen and creates drop out so that the spine is not visualised deep to this and edge artefact is not seen in other views.
ASCITES
Ascites is hard to differentiate from acute haemoperitoneum. Some things may help:
1. Usually ascites is a large volume. So if the patient is haemodynamically stable stable, this is not going to be haemoperitoneum.
2. Ascites due to liver failure may show a shrunken liver with irregular edges due to cirrhosis.
Ascites and cirrhosis: Note the superior liver edge is scalloped and irregular
3. Ascites due to right heart failure may show a dilated RV on echo, dilated IVC and clinical peripheral oedema
4. Ascites due to renal failure or peritoneal dialysis: the patient may have a Hx of renal failure and have tiny shrunken kidneys on US.
In the unstable patient, with no other cause for hypotension, assume that the free fluid is haemoperitoneum. Especially if the IVC is small and collapsing.
GALL BLADDER
The GB can sometime sit between the liver and kidney and be confused for free fluid. However, remember fluid has acute angles at the edges as it infiltrates any available space where as the GB has rounded edges. Also, the GB has a nice bright white Gb wall. If in doubt image the same area from a different plane.
A distended GB looks like FF in Morison's pouch
IVC
You can sometimes image the IVC posterior to the liver and it can look like FF. But it will vary in size with respiration and have bright white walls. You may also see the hepatic vein opening into it.
RETROPERITONEAL HAEMORRHAGE
When you get good at US, you will be able to differentiate retroperitoneal bleeding from intraperitoneal haemorrhage. Ma et al (8) showed that emergency physicians examining for retroperitoneal blood had a specificity of 100% and accuracy of 99%.
Differentiating retroperitoneal from intraperitoneal bleeding is important because retroperitoneal bleeding from vessels or renal laceration may do well with angioembolisation (7) or conservative management rather than laparotomy.
Gerota's fascia is the key to differentiating the two. Gerota's fascia is the bright white line anterior to the kidney on US. Retroperitoneal bleeding will be posterior to this, creating an anechoic layer between the bright white fascia and the renal parenchyma.
Anechoic retroperitoneal blood posterior to Gerota's Fascia
By comparison, fluid in Morison's and the splenorenal recess is anterior to Gerota's fascia so you will see the bright white line immediately adjacent to the renal parenchyma.
INTRAPERITONEAL FREE AIR
eFAST isn't supposed to be able to pick up bowel injury, but you can if you try. Studies have shown that emergency physicians can detect free air with a high degree of accuracy (9). Looking for free gas specifically with eFAST will waste time. The main role of the eFAST is to detect immediate life threats. Bowel perforation is no an immediate life threat. However, if you see free gas, it increases your worry about the patient.
Bowel injury may cause free fluid and/ or free gas. Free gas floats to the surface and is easiest seen anterior to the liver where it will form a curtain of A lines which obscure the liver. But remember, as with lung US, to see A lines well, the transducer needs to be perpendicular with the air. So you may need to angle or title to discern whether a hyperechoic area is just gas within bowel or free gas.
To image free gas begin in the midaxillary line at the right hypochondrium, transducer vertically oriented, probe marker cranial. Translate the transducer from here towards the midline and any free gas will show up. Free gas abuts the peritoneum so the peritoneal line will be bright white like the pleural line in a pneumothorax.
Free gas: note the bright white peritoneal line with A lines immediately posterior to this. A bit more angling would have helped define the A lines better
Sometimes large bowel gas can look like free gas in the same position. The difference is there will be an anechoic layer (bowel wall) between the peritoneum and the bowel gas.
Note how when the gas is within the bowel wall, there is still a bright white line and A lines posteriorly. However, there will always be an anechoic layer of bowel wall between this white line and the parietal peritoneum
SUBCUTANEOUS EMPHYSEMA
Subcutaneous emphysema can interfere with your imaging. Pressing down hard against ribs may move some of the gas out of the way giving you a better view. However, sometimes there's not much you can do because US will not penetrate through air. Because subcut emphysema is loculated within the soft tissue, it may look like hyperechoic points with dirty shadowing posteriorly or like A lines.
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REFERENCES
1. Branney SW, Wolfe RE, Moore EE, Albert NP, Heinig M, Mestek M, Eule J. Quantitative sensitivity of ultrasound in detecting free intraperitoneal fluid. J Trauma. 1995 Aug;39(2):375-80
2. Rozycki GS, Ochsner MG., Feliciano DV, Thomas B, Boul- anger BR, Davis FE, Falcone RE, Schmidt JA .(1998) Early detection of hemoperitoneum by ultrasound examination of the right upper quadrant: a multicentric study. J Trauma 45:878– 883
3. McKenney KL. Ultrasound of blunt abdominal trauma. Radiol Clin North Am. 1999 Sep;37(5):879-93
4. Richards JR, McGahan PJ, Jewell MG, Fukushima LC, McGahan JP. Sonographic patterns of intraperitoneal hemorrhage associated with blunt splenic injury. J Ultrasound Med. 2004 Mar;23(3):387-94,
5. Baqué P, Iannelli A, Dausse F, de Peretti F, Bourgeon A. A new method to approach exact hemoperitoneum volume in a splenic trauma model using ultrasonography. Surg Radiol Anat. 2005 Aug;27(3):249-53
6. Richards JR, McGahan JP. Focused Assessment with Sonography in Trauma (FAST) in 2017: What Radiologists Can Learn. Radiology. 2017 Apr;283(1):30-48
7. Schlegel RN, Fitzgerald M, O'Reilly G, Clements W, Goh GS, Groombridge C, Johnny C, Noonan M, Ban J, Mathew J. The injury patterns, management and outcomes of retroperitoneal haemorrhage caused by lumbar arterial bleeding at a Level-1 Trauma Centre: A 10-year retrospective review. Injury. 2023 Jan;54(1):145-149.
8. Ma OJ, Mateer JR, Ogata M, Kefer MP, Wittmann D, Aprahamian C. Prospective analysis of a rapid trauma ultrasound examination performed by emergency physicians. J Trauma. 1995 Jun;38(6):879-85
9. Moriwaki Y, Sugiyama M, Toyoda H, Kosuge T, Arata S, Iwashita M, Tahara Y, Suzuki N. Ultrasonography for the diagnosis of intraperitoneal free air in chest-abdominal-pelvic blunt trauma and critical acute abdominal pain. Arch Surg. 2009 Feb;144(2):137-41