Small-Bowel Obstruction
Francis K. Lee
George Chapman is a 42-year-old man who presents to the emergency department complaining of 3 days of crampy abdominal pain, nausea, and vomiting. Prior to the onset, he has had occasional abdominal colic that was relieved by intermittent bowel movement. However, for the past 3 days he has not had a bowel movement, and his abdominal pain has become diffuse and crampy, relieved only by emesis. He states that his vomitus is somewhat foul-smelling and dark green. His last meal was 3 days ago, and he has been having difficulty tolerating even liquids for the past 12 hours. His medical history is significant for appendectomy for ruptured appendicitis followed by intraabdominal abscess treated by percutaneous drainage tube approximately 5 years ago.
What is the differential diagnosis?
View Answer
The patient’s history of crampy abdominal pain relieved by emesis and a previous abdominal surgery suggests that this patient has small-bowel obstruction (SBO) until proven otherwise. Other possibilities include adynamic ileus, large-bowel obstruction (LBO), volvulus, gastroenteritis, pancreatitis, and mesenteric vascular occlusion.
What are the most common causes of SBO?
View Answer
In adults, postoperative adhesions are the most common cause of SBO, producing up to two thirds of cases (26% to 64%); next are incarcerated hernia (6% to 21%) and neoplasms. Inflammatory bowel disease, diverticulitis, gallstone ileus, and bezoars are other less common causes of SBO (1).
What are the three most salient features of SBO on history, and why is it important to recognize them?
View Answer
A history of abdominal pain, obstipation, and emesis typify SBO. More frequently than not the diagnosis must be made by history alone because the physical examination and laboratory tests are not diagnostic. A delay in diagnosing SBO can lead to catastrophe.
On physical examination, Mr. Chapman’s vital signs are as follows: blood pressure, 140/92 mm Hg; heart rate, 100 beats per minute; respirations, 18 per minute; temperature, 98.2°F. Cardiopulmonary findings are within normal limits. His abdomen is noteworthy for a well-healed 10-cm scar in the right lower quadrant, moderate distention, and somewhat hyperactive bowel sounds. Except for some discomfort upon palpation, there is no significant tenderness or abnormal mass. The rectal examination is normal and heme negative.
Why is testing for blood important?
View Answer
Heme-positive stool may be an early indication of ischemic bowel. The mucosal layer of the bowel wall is most susceptible to ischemia and may bleed before full-thickness bowel injury occurs. Also, in elderly patients, one should keep in mind cancer as a possible cause of bowel obstruction.
What is the significance of the quality of the bowel sounds?
View Answer
In the early period of obstruction, peristalsis is increased, and hyperactive bowel sounds can be heard as the intestines attempt to overcome the obstruction. As the bowel distends, reflex inhibition of bowel motility results in a quiet abdomen (2). The quality of the bowel sounds does not help in differentiating between a partial and a complete obstruction.
What are the most common physical findings associated with SBO?
View Answer
The most common physical abnormalities found in a patient with SBO are those associated with dehydration (e.g., low-grade fever, dry skin turgor or mucosa, tachycardia). Other than distention, the abdominal examination is most commonly equivocal (i.e., moderate subjective discomfort upon palpation without a bona fide tenderness or rebound).
Why does SBO not usually produce significant abdominal tenderness? How is this related to the pathophysiology of SBO?
View Answer
Regardless of the cause, SBO results in either (a) simple distal intestinal obstruction with proximal distention and no vascular compromise or (b) strangulation of bowel or closed-loop intestinal obstruction. The consequences of intestinal obstruction and distention are decreased luminal fluid resorption and increased intraluminal fluid secretion (e.g., intestinal distention, nausea, emesis), overall fluid and electrolyte abnormality from fluid shifting into the interstitial third space, and dehydration, and luminal fluid stasis and overgrowth of bacteria (i.e., foul-smelling vomitus). Intestinal obstruction and proximal distention alone cause crampy abdominal pain. Mesenteric vascular occlusion due to strangulation or closed-loop bowel ischemia causes significant pain but not the same degree of tenderness. It is critically important that severe and increasing abdominal pain, even without significant tenderness, is treated surgically. Waiting for signs of peritonitis such as abdominal tenderness, rebound tenderness, or systemic signs of inflammation (e.g., fever or increased leukocyte count) leads to catastrophic delay, as these signs portend that the ischemic bowel may have already progressed to necrosis or perforation.
The emergency department physician reports that the patient’s laboratory test and abdominal radiograph results are pending.
Which laboratory abnormalities are anticipated?
View Answer
Signs of dehydration from emesis and shifting of fluid into the third space are common, such as mildly elevated hematocrit and normal or upper normal white blood cell count. Blood chemistries may show prerenal azotemia (blood urea nitrogen-creatinine ratio [BUN-Cr] above 20) and hypochloremic, hypokalemic metabolic alkalosis from ongoing emesis of acidic gastric juice.
What are the salient features of SBO on the kidney, ureter, and bladder and upright abdominal radiographs?
View Answer
Radiographic examination of a patient with SBO indicates (a) multiple loops of distended small bowel in a stepladder pattern, which (b) layer out on the upright film showing air-fluid levels, and (c) absence of colonic air or stool. Other findings, such as free air under the right diaphragm from bowel perforation, are looked for as part of the routine radiographic evaluation.
Is it possible for the abdominal radiographs of SBO not to show any air-fluid levels?
View Answer
Yes. Sometimes early in the course of SBO, effective emesis and relief of intraluminal fluid can give a normal-appearing bowel gas pattern. Also, completely fluid-filled loops of bowel can give a ground glass appearance on radiograph without the air-fluid levels.
What is the significance of loops of distended small bowel and air in the colon and rectum if they are visible on abdominal radiographs?
If the abdominal radiographs show loops of distended small bowel, distended cecum, and colon up to the descending colon, yet no rectal air or stool, what is the significance?
View Answer
The possibility of LBO increases in the differential diagnosis. However, colonic air does not rule out SBO because colonic air is sometimes seen in the early phase of SBO.
What causes LBO, how does one work it up, and what are the treatments?
Causes of Large-Bowel Obstruction
Causes of LBO include sigmoid or cecal volvulus, obstructive colon cancer, obstruction from inflammatory reactions to diverticulitis or ulcerative colitis, fecal impaction or foreign body obstruction, sliding hernia, intraperitoneal adhesions (e.g., postsurgical adhesions, endometriosis). Children face a host of congenital problems, such as Hirschsprung’s disease, imperforate anus, and meconium ileus.
Workup for Large-Bowel Obstruction
In addition to review of history, workup includes rectal examination and proctosigmoidoscopy to look for distal rectosigmoid disease. If the distal segment is normal in the presence of LBO, barium enema or colonoscopy is helpful to determine the more proximal colonic lesions.
Treatments for Large-Bowel Obstruction
The treatments must achieve two therapeutic goals: relieving the obstruction and addressing the underlying problem. For example, in the case of a septic and metabolically deranged patient with obstructive colon cancer, the obstruction can be dealt with by placing a diverting colostomy proximal to the point of obstruction, and the definitive resection may be performed as a second procedure. Or, if the bowel preparation status and the patient’s physiologic state are optimal, the cancerous segment may be resected and a primary anastomosis performed. A complete knowledge of the entire colon is necessary before beginning any definitive treatment for obstructive colon cancer. In cecal volvulus, on the other hand, the bowel may simply be untwisted and cecopexy performed. The management of LBO is very different from that of SBO, thus it is important to distinguish the two.
The abdominal radiographs show distended loops of small bowel with air-fluid levels. There is some air, although scant, in the colon. There is no free air under the diaphragm, and the chest radiograph is unremarkable. The laboratory test results are still pending. The working diagnosis is now SBO.
What is the next step?
View Answer
The patient’s bowel must be decompressed with a nasogastric tube (NGT).
The patient is most likely dehydrated and needs aggressive intravenous (IV) fluid resuscitation. A urinary catheter is inserted to monitor urine output. Boluses of IV fluid are administered until the urine output becomes adequate (0.5 to 1 mL per kg per hour). If the patient is frail and elderly with a complicating cardiac disease, fluid resuscitation is done with central venous pressure monitoring.
Most importantly, a decision must be made as to whether this patient requires surgery.
How does a physician decide whether the patient is a candidate for surgery?
View Answer
The decision to operate depends on the index of suspicion for bowel strangulation, closed-loop obstruction, and ischemic bowel. It is difficult to make the diagnosis of strangulation and ischemic bowel with just history and physical examination.
Clearly, abdominal pain associated with fever, leukocytosis, acidosis, peritoneal sign, and shock are all indications of bowel necrosis and necessitate surgical exploration. Generally speaking, even in the absence of these physical signs, unrelenting and increasing abdominal pain with obstipation and radiographic signs of SBO indicate surgery. Unless there are extenuating circumstances to the contrary, complete bowel obstruction should not be dismissed without exploratory surgery.
What is the difference between complete and partial SBO?
View Answer
Partial SBO is distinguished from complete SBO by passage of flatus through the rectum and radiographic presence of air or stool in the colon despite the loops of distended small bowel. These indicate partial blockage of the intestines, allowing distal passage of some air and fluid.
Complete SBO is associated with a significant risk of strangulation and bowel ischemia. The incidence of necrotic bowel in patients with complete SBO has been reported to be as high as 30% (3). Patients with partial obstruction have a much lower incidence of ischemic complications. Accordingly, partial SBO may be treated conservatively, whereas a complete SBO requires timely operative intervention.
Does complete SBO always require operation?
View Answer
Complete SBO does not always require operation. For patients who have had multiple episodes of SBO and who have been successfully managed without operation, it may be worthwhile to try an initial period of conservative management with nasogastric decompression. In such patients, proceed to surgery if pain becomes worse or if obstipation continues for 3 or 4 days without clinical progress. Generally speaking, however, the maxim “do not let the sun set or rise on bowel obstruction” holds true for complete SBO.
The patient undergoes insertion of the NGT and Foley catheter. He receives approximately 1 L of IV lactated Ringer’s solution, and he produces 80 mL of urine in the subsequent hour. When the patient is interviewed again, he reveals that he has just passed a large amount of flatus following the NGT insertion. In addition, the radiologist confirms that the patient has some air and stool in the colon. The radiologist asks whether he should perform a contrast study of the upper gastrointestinal tract with small bowel follow through to confirm the presumptive diagnosis.
What is the role of contrast studies in the diagnosis of SBO?
View Answer
Contrast studies, which take up to 6 hours of bowel transit time, should never substitute for clinical judgment and should not delay timely operation. However, contrast studies do play a role in patients who have been on NGT decompression for a trial period without significant pain yet have not resolved their obstructive symptoms. These studies also play a role in patients who have been admitted repeatedly for the same obstructive symptoms, even if the patient’s symptoms resolve. Overall, however, the role of contrast-enhanced radiographic studies in acute presentation of SBO is limited.
When told that a contrast radiologic study would not be necessary, the radiologist suggests that an abdominal computed tomography (CT) scan be considered because it is accurate for diagnosing SBO as well as many other intraabdominal abnormalities in cases in which the diagnosis is not absolutely clear.
What is the role of abdominal CT scan in the evaluation of SBO?
View Answer
It is reasonable to obtain a CT scan if there is sufficient reason to suspect specific diagnoses other than SBO.
How accurate are the various radiographic studies in diagnosing SBO?
View Answer
The overall sensitivity of plain abdominal radiograph is only 66%. Low-grade SBO can be interpreted as normal 21% of the time. Only 13% of complete SBO is interpreted as “definite SBO.”
Barium enteroclysis is very accurate. Enteroclysis has a sensitivity of 100% and specificity of 88%. It can also predict the distance and etiology of obstruction in 86% to 88% of patients.
Abdominal CT scan appears to be more inconsistent in terms of accuracy. There have been reports touting that its accuracy is as high as 90%. For high-grade or complete SBO, the sensitivity may be as high as 88%. But for a low-grade SBO, the sensitivity is as low as 48%. Overall, the accuracy has been found to be as low as 66%, with sensitivity of 68% and specificity of 78%.
Considering such inconsistent accuracy with abdominal CT alone, some have advocated the use of CT enteroclysis with the oral contrast infused directly into the intestines through a long NGT. CT enteroclysis is suggested to have an accuracy as high as the barium enteroclysis with the added benefits of visualizing closed loop obstructions and localizing the lesions in a three-dimensional map, a piece of information helpful to a laparoscopic surgeon (4).
The presumptive diagnosis of partial SBO is made and Mr. Chapman is admitted for nasogastric decompression and observation. The nurse asks what IV fluids you would like to use.
How does one determine the IV fluid requirement for a 70-kg patient?
View Answer
Fluid volume, sodium, and potassium requirements are determined separately. The following are rough estimations (5):
Daily Fluid Volume Requirement
Adults: 35 mL per kg per day × 70 kg = 2450 mL per day
Children: 100 mL per kg per day for first 10 kg body weight (0 to 10 kg)
+50 mL per kg per day for second 10 kg body weight (10 to 20 kg)
+20 mL per kg per day for each kg greater than 20 kg body weight (>20 kg)
Daily Sodium Requirement
Adults: 1.5 to 2 mEq per kg per day × 70 kg = 100 to 140 mEq per day
Children: 3.5 mEq per kg per day
Daily Potassium Requirement
Adults: 0.5 mEq per kg per day × 70 kg = 35 mEq per day
Children: 2 to 3 mEq per kg per day
Caloric Replacement Through Peripheral IV
Adults: 100 g glucose per day produces protein-sparing effects; that is, it minimizes endogenous muscle breakdown so that the body can generate glucose (gluconeogenesis) for the brain during the first few days of starvation (6).
The usual IV solution is 5% dextrose in water with 0.45% normal saline (NS) and 20 mEq potassium chloride at 100 mL per hour (D5 ½ NS + 20 KCl at 100 mL per hour). This order will provide the following:
TABLE 10.1. Normal Content of Gastrointestinal Secretions | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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120 g dextrose per day, which is presumably adequate for protein sparing and minimizing the nitrogen loss and muscle breakdown
2400 mL fluid per day, which is adequate volume replacement
184 mEq sodium per day, which is more than adequate for sodium replacement (some physicians prefer to give 25% NS instead, which would provide 92 mEq of sodium per day)
48 mEq potassium per day, which is more than adequate for potassium replacement
Thus, D5 ½ NS + 20 KCl at 100 mL per hour has become the usual IV order for a healthy 70-kg adult patient. However, each component of the IV therapy must change according to the following additional clinical factors:
Third-space losses (increase fluid and electrolyte requirements)
Operative blood and fluid losses (increase fluid and electrolyte requirements)
Specific body secretory losses (increase fluid and electrolyte requirements)
Table 10.1 lists the specific secretory losses that must be replaced with supplemental IV therapy (7).
It is likely that this patient has had a period of gastric emesis resulting in severe salt and water deficit. Depending on the electrolyte profile, the IV order must be modified accordingly from the standard solution. Table 10.2 lists commonly used IV fluid preparations. The nurse also informs the physician that the laboratory has called her to report the following serum test results:
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