Ashley Hardy, MD and Marie Crandall, MD, MPH
You are called to the surgical floor to evaluate a 65-year-old female who underwent a right hemicolectomy for colon cancer four days prior. For the last few hours the patient has had multiple episodes of nausea with vomiting and has yet to have flatus or a bowel movement since her procedure. Her vital signs are normal, her abdominal exam is notable only for distension, and labs obtained earlier that morning are unremarkable. Of note, the patient is still requiring use of her morphine PCA for postoperative analgesia. You order a KUB, as seen in Figure 37-1.
Figure 37-1. Small bowel obstruction. Supine film showing dilated loops of small bowel and no gas in the colon. (Reproduced, with permission, from Doherty GM. Current Diagnosis & Treatment: Surgery. 13th ed. New York: McGraw-Hill; 2010. Figure 29-5.)
1. For this patient, name two key findings you should be looking for on the KUB.
2. How can you distinguish the small intestine from the colon on a KUB?
3. Given the patient’s nausea and vomiting, you elect to place a nasogastric tube (NGT). What radiographic landmarks would you use to ensure the tube is properly positioned?
4. What findings on a KUB would warrant emergent surgical intervention?
READING AND USING A KUB
1. Although they stand for kidneys, ureters, and bladder, KUBs are more commonly utilized to assess for abnormal conditions of the gastrointestinal tract and to determine the position of various indwelling devices, including NGTs, Dobhoff (feeding) tubes, and ureteral stents. In a patient such as this one, with a history of recent abdominal surgery and several bouts of nausea and vomiting, it is important to assess for the presence of obstruction or evidence of anastomotic breakdown (as indicated by the presence of free intraperitoneal air). A KUB is quick, relatively inexpensive, and has a lower radiation dose than CT, making it a common initial diagnostic study.
When encountering any type of film, including a KUB, it is important to take a systematic approach to interpretation. Doing so ensures that key findings pertinent to making appropriate decisions regarding a patient’s care are not missed. If previous films are available, it is helpful to compare the findings with those of the current study. After ensuring that you’re viewing the film for the correct patient, determine the orientation (right vs left as indicated by a marker or using the gastric air bubble in the LUQ as a guide). Also determine if you’re looking at a film that was obtained while the patient was supine versus erect as this will influence whether or not you’re able to visualize the presence of air–fluid levels and free air under the diaphragm. Keep in mind that on plain radiographs high-density structures (generally those that contain calcium such as bone, gallstones, and kidney stones) are white. Similarly, soft tissue and fluid are light gray, while gas is black.
After orienting yourself to the image, be sure to look for the presence of extraluminal air. Then, turn your focus to the hollow organs. First try to locate the stomach within the LUQ. Note that its visibility is influenced by the presence or absence of gastric air and whether or not the film is erect versus supine—in a supine film the meniscus between the gas bubble and gastric contents will not be visible.
Next, locate the small intestine and the colon. You should assess the entire bowel for evidence of intestinal dilation, keeping in mind the “3/6/9 rule”: the accepted upper limit of normal for the diameter of small bowel is 3 cm, for the colon 6 cm, and for the cecum 9 cm. Dilation of the small or large bowel usually represents either a functional obstruction (ileus) or a mechanical obstruction. The image in the case (Figure 37-2, copied from Figure 37-1again here, with labels) is an excellent example of a KUB that demonstrates the uniform dilation of the small bowel often seen with either type of obstruction. Note there are no cutoff points or “coffee bean”–shaped loops of bowel that could represent a closed loop obstruction, something that would require immediate operative intervention. Furthermore, note that because it is supine, there are no air–fluid levels, although the boundaries between the loops of bowel are prominent and likely represent intra-abdominal fluid.
Figure 37-2. The same supine film as presented in the case. Note the dilated loops of small bowel as well as what appears to be fluid between the small bowel walls. (Reproduced, with permission, from Doherty GM. Current Diagnosis & Treatment: Surgery. 13th ed. New York: McGraw-Hill; 2010.)
Figure 37-3, in contrast, is an upright film that shows air–fluid levels. You should note the very large gastric air bubble and the paucity of colonic air (normally seen deep in the pelvis, below the pelvic brim). Both of these elements suggest an obstruction. If you know approximately where the small and large bowels lie within the abdomen, the pattern of air–fluid levels can also suggest the location of the obstruction. For example, in the case of a small bowel obstruction, multiple air–fluid levels may be seen involving the small intestine only, whereas a large bowel obstruction may present with air–fluid levels within both the small and large intestines.
Figure 37-3. Upright film shows a distended stomach with a large gastric bubble. It also shows slightly dilated loops of small bowel with air–fluid levels and a paucity of colonic gas, both consistent with a small bowel obstruction. (Courtesy of Deborah Levine, MD.)
2. On a KUB, the small and large intestines are most easily distinguished from each other on the basis of position, diameter, mucosal markings, and gas patterns. As previously mentioned, the small intestine tends to lie within the midabdomen and has a normal diameter of less than 3 cm, whereas the colon usually courses along the periphery and is normally less than 6 cm in diameter. In terms of bowel markings, the circular mucosal folds (plicae circulares or valvulae conniventes) of the small bowel span the entire circumference of the wall. They are also closer in proximity to each other than are the haustra of the large intestine, which appear to cross only part of the large intestinal lumen (see Figure 37-2).
3. Using a KUB, one can follow the entire gastrointestinal course of an NGT and ensure its proper positioning beneath the left hemidiaphragm and well within the lumen of the stomach (see Figure 37-4).
Figure 37-4. A feeding tube located below the diaphragm—note that in this image the tube is postpyloric. (Reproduced, with permission, from McKean SC, Ross JJ, Dressler DD, Brotman DJ, Ginsberg JS, eds. Principles and Practice of Hospital Medicine. Figure 119-4.)
Postplacement imaging with a KUB can ensure that the tube is not erroneously placed within the patient’s airway or that it ends too proximally within the esophagus.
4. The presence of extraluminal intraperitoneal gas or a pneumoperitoneum is a worrisome finding on a KUB and usually warrants emergent surgical intervention. Note that some extraluminal gas is to be expected after open and laparoscopic surgery, although it typically resolves over a period of 3 to 6 days and is not accompanied by concerning physical exam findings. The most common place to look for extraluminal gas is underneath the diaphragm. Because air is less dense than the intra-abdominal contents, it tends to rise, so extraluminal gas is most easily visualized on an erect or upright film. Note that it’s best to look for air underneath the right hemidiaphragm as it may be difficult to distinguish the extraluminal air due to a perforation from the gastric air bubble on the left. Other potential locations for extraluminal gas include the bowel wall (pneumatosis intestinalis, which creates a “bubble wrap” appearance), the portal venous system (usually as a result of severe bowel ischemia), and within the biliary tree (usually following instrumentation or as a result of a biliary-enteric fistula).
TIPS TO REMEMBER
Always take a systematic approach when interpreting a KUB.
Dilated loops of bowel, a prominent gastric bubble or dilated stomach, air–fluid levels, and a paucity of colonic gas all suggest an obstructive process (either ileus or a mechanical obstruction).
When trying to determine if bowel is dilated or not, remember the “3/6/9 rule” (small bowel, colon, cecum).
The small and large intestines can be discriminated from each other by position (small intestine—central, large intestine—peripheral) and bowel markings (small intestine—markings span the entire wall circumference, large intestine—markings span only part of the wall circumference).
Except in the acute postoperative period, the presence of free extraluminal intraperitoneal air usually warrants emergent intervention.
1. Which radiographic finding would most likely require emergent abdominal exploration?
A. Cecum dilated to 8 cm
B. Air–fluid levels within the small intestine
C. Air underneath the right hemidiaphragm
D. Radiopaque densities in the RUQ
2. Match each term with its radiographic characteristic:
1. C. Air underneath the right hemidiaphragm is an abnormal radiographic finding that suggests an intra-abdominal perforation, warranting emergent abdominal exploration. The upper limit of normal for cecal diameter is 9 cm, air–fluid levels within the small intestine suggest a bowel obstruction (which can often be treated nonoperatively), and radiopaque densities in the RUQ likely represent gallstones.
2. A and D—b. The colon tends to lie in a peripheral location within the abdomen and has haustra (bowel markings that cross only part of the intestinal lumen). B and C—a. The small bowel is identified radiographically by its central position in the abdomen and by the plicae circulares that span the entire circumference of the small intestinal wall.