I taught the sophomore class about peritoneal detail, effusion and pneumoperitoneum yesterday. They came up with some great questions after the lecture. Here are some of the questions and answers:
What technique can be used in place of an erect or cross-table study to diagnose free air in the peritoneum in a practice where a horizontal beam isn’t available?
If you don’t have a horizontal beam option, there are a couple of things to try. First of all, I would take a radiograph with the animal in left lateral recumbency. Since fluid in the stomach will gravitate towards the table, and gas will rise, the fundus will be filled with fluid and will not mimic gas under the diaphragm. I would also place some towels or sandbags under the thorax and cranial abdomen to make sure that the diaphragm was the highest point. If you let the animal lay in that position for 10 minutes or so before taking the radiograph, you will maximize the chance of gas rising to the top.
The other option is to look for it with ultrasound. I think this is a much more difficult technique than using radiographs, but can be sensitive. You would be looking for the “dirty shadowing” of gas reflecting the ultrasound beam, that are outside of the bowel loops. These are also usually at the highest point of the abdomen.
Either way, a fluid sample might help to diagnose a peritonitis from a wound or bowel rupture. There is often fluid as well as gas in peritonitis, and you may not even need ultrasound to get a sample for cytology.
How does FIP change peritoneal detail?
The wet form of FIP causes a large amount of effusion, but the dry form also causes some granulomatous disease. My impression is that in the wet form, the effusion is just a uniform decrease in opacity, especially since there is usually a large amount of fluid. If there is a smaller effusion and/or a granulomatous component, it tends too look more streaky or mottled. I think this is because the mesentery and omentum become thickened and edematous. That said, I don’t think we can diagnose wet or dry from radiographs, but you should definitely have FIP on your differential list in either case.
Is there a way to radiographically determine what type of free fluid is in the abdomen…(ie blood vs urine, vs, intestinal fluid, vs transudate etc.)?
Radiographically, we can’t tell what type of effusion is in the abdomen. Blood, urine, transudate and exudate are all the same soft tissue opacity. Free gas might give you a clue that it was an exudate, or the fact that you can’t see the bladder might point you towards urine. But visually they look the same.
On ultrasound, you can look at the echogenicity of the effusion which helps a bit. Transudate and urine are anechoic, while blood and exudate are more echogenic. The cells within these two fluids produce hyperechoic material within the fluid.
Regardless of the imaging method, you will still need a sample of the effusion to make the diagnosis.