Can you confidently tell which patient has an adynamic ileus versus a small bowel obstruction? Let’s learn to read bowel gas patterns like a radiologist.  There are two things to consider when evaluating any gas pattern. One, are there any dilated loops? And two, what’s the distribution of the air? In a normal abdominal radiograph, you can use the 3-6-9 rule to identify any abnormally dilated loops.

The small bowel should be no more than 3 centimeters in diameter. The transverse colon should be no more than 6 centimeters in diameter. And the cecum should be no more than nine centimeters in diameter. Also, if contents is moving through the bowel and colon normally, there should be more gas in the colon than the small bowel.

And typically there’ll be at least a little bit of gas in the rectum. Let’s look at our two abnormal cases.  This patient has bowel loops that are dilated well past three centimeters. We can be confident that there’s small bowel loops because of the valvulae conniventes extending across the lumen.  We talked about these in our last video.

Also notice that there’s very little gas in the colon compared to the many dilated small bowel loops. The reason for this is that contents are getting stuck in the small bowel and bacteria are forming gas, which is getting trapped in these loops. Additionally, despite some gas filled loops of small bowel in the pelvis, there’s very little gas in the rectum, again, a vote for abnormal transit.

All these findings are concerning for a small bowel obstruction. What about this patient? We can see that there’s both dilated colon as well as small bowel loops, and they’re fairly evenly distributed throughout the whole abdomen.  Also, again, aside from some scattered small bowel loops, there’s very little gas in the actual rectum, again, favoring lack of normal transit.

These findings favor an adynamic ileus. Let’s review. You can use the 3-6-9 rule to find any dilated small bowel or colon loops. Normally, there should be more gas in the colon than in the small bowel, and in normal forward transit, there should be at least some air in the rectum. We also discussed two abnormal cases.

In small bowel obstruction, there’ll be very little gas in the colon, with most of the gas in dilated small bowel loops, with no air in the rectum. In adynamic ileus, there’ll be dilated colon and small bowel. Which will be fairly evenly distributed in the abdomen and again probably not very much air at the rectum due to abnormal forward transit.

Now you can assess bowel gas patterns like a radiologist.

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Bowel Gas Patterns

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