Ischemic Bowel Disease

What is ischemic bowel disease?

Ischemic bowel disease is caused by tissue hypoxia and ischemic injury of the small or large intestine as a result of a persistent decrease in mesenteric blood flow, decreased oxygen content of red blood cells, or mesenteric venous stasis. Ischemic bowel disease can manifest in numerous ways, such as acute or chronic midabdominal pain (meal-induced), vomiting, sitophobia (fear of eating), weight loss, diarrhea, ileus, gastrointestinal bleeding, intestinal infarction, peritonitis, or fibrotic strictures.

Describe the gross anatomy of the mesenteric vascular system

Three major arteries and two major veins compose the mesenteric circulation.

The connection of major arteries and veins via capillaries, arterioles, and venules is known as the splanchnic circulation ( Figure 57-1 ).

The celiac artery provides blood to the stomach, proximal duodenum, part of the pancreas, spleen, liver, gallbladder, and biliary tree. The SMA provides blood to the rest of the duodenum and pancreas, the entire small intestine, and the large intestine up to the splenic flexure. The IMA supplies the remainder of the colon and rectum, with the latter receiving dual blood supply from internal iliac arteries as well. The IMV drains into the splenic vein, and the SMV and splenic vein anastomose to form the portal vein. Mirroring the arterial blood supply, there is dual venous drainage of the rectum into the systemic system through the inferior vena cava via the internal iliac veins and through the IMV to the portal circulation.

An extensive collateral circulatory system exists between the systemic and splanchnic vascular networks. Describe this system.

The several systemic-splanchnic and intersplanchnic collateral channels that connect the three major mesenteric arteries and their branches become apparent in the event of occlusion of one of the major branches ( Figure 57-2 ):

• Pancreaticoduodenal arcade provides collateral channels between the celiac axis and SMA (the superior pancreaticoduodenal arteries of the celiac axis collateralize with the inferior pancreaticoduodenal arteries of the SMA).

• Marginal artery of Drummond, composed of branches of the SMA and IMA, is a continuous arterial pathway that runs parallel to the entire colon.

• The middle colic branch of the SMA and the left colic branch of the IMA are connected by the arc of Riolan.

• The IMA connects with the systemic circulation via the iliac artery by the ileomesenteric arcade.

• A slowly developing occlusion promotes the opening of these collateral channels; thus chronic mesenteric arterial insufficiency (e.g., abdominal angina) is unusual unless there is virtually complete occlusion of two of the three major mesenteric arteries, including the SMA.

What is meant by autoregulation?

Autoregulation is the concept by which blood flow remains relatively constant via the response of arterioles and venules to changes in perfusion. A steep gradient of pressure exists between the artery and proximal portion of the arteriole. If there is a decrease in arterial perfusion or an increase in oxygen demand (as in the postprandial state), arterioles dilate and additional capillaries are recruited to prevent tissue hypoxia. Additionally, adjustments in the resistance of the venous system are employed to maintain adequate cardiac output. For example, an increase in tone occurs in the setting of hypotension to enhance venous blood return to the heart ( Figure 57-3 ).

What are the different varieties of ischemic bowel disease?

Ischemic bowel disease can be sorted into various categories based on the vascular component affected (arterial or venous), the duration of reduction of blood flow through the vessel (acute or chronic), and the cause of the reduction in flow (occlusive or nonocclusive).

It can also be sorted into clinical entities:

• Acute mesenteric ischemia (AMI), usually due to emboli, thrombi, or vasoconstriction

• Chronic mesenteric ischemia, usually a result of atherosclerotic disease

• Colonic ischemia, most often secondary to transient hypoperfusion ( Figure 57-4 )

  

What clinical circumstances predispose to ischemic bowel disease?