Mesenteric ischemia pathophysiology
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The factors that regulate the intestinal blood flow play a vital role in the develpoment of mesenteric ischemia. Mucosa of the intestines has a high metabolic activity and accordingly a high blood flow requirement. The majority of blood supply of the intestine comes from the superior mesenteric artery, with a collateral blood supply from superior and inferior pancreaticoduodenal arteries (branches of the celiac artery) as well as the inferior mesenteric artery. The splanchnic circulation (arteries supplying the viscera) receives 15-35% of the cardiac output, making it sensitive to the effects of decreased perfusion. Mesenteric ischemia occurs when intestinal blood supply is compromised by more than 50% of the original blood flow without activation of adaptive responses. This can lead to disrutpion of mucosal barrier, allowing the release of bacterial toxins (present in the intestinal lumen) and vasoactive mediators which ultimately lead to complete necrosis (cell death) of the intestinal mucosa. This can further progress to depression in myocardial activity, sepsis, multiorgan failure, and without prompt intervention, even death.
Intestinal mucosal damage occurs in response to ischemic insult. In response to providing protection from ischemia, mesenteric vessels undergo intrinsic auto regulation, which is an adaptive response to ischemia. In order to compensate for the ischemia, there is vasoconstriction of mesenteric vessels resulting in increased tissue oxygen extraction along with vasodilation of the collateral vessels. Owing to this mechanism, intestine is able to compensate for around 75% reduction in blood flow.
Disturbance in the following factors play a role in the development of mesenteric ischemia:
(a) Mesenteric blood supply (General circulation)
(b) Collateral circulation
(c) Response of mesenteric vasculature to autonomic stimuli
(d) Vasoactive and humoral factors
|Box 1 in Row 1|
|Box 2 in Row 2|
|Box 3 in Row 3||Box 4 in Row 4|
(a) Blood supply of the mesentery:
- Superior mesenteric artery (SMA)
- Supplies the small intestine, proximal and mid colon upto the splenic flexure.
- Inferior mesenteric artery (IMA)
- Supplies hind gut starting from the splenic flexure to the rectum.
- Celiac artery (CA)
- Supplies the foregut, hepatobiliary system and spleen.
- The venous system parallels the arterial branches and drains into the portal venous system.
- The mesenteric circulation recevies approximately 25% of the resting and 35% of the postprandial cardiac output.
- Mucosal and submucosal layers of the intestine receive 70% of the mesenteric blood flow, with the rest supplying the muscularis and serosal layers.
Commonly affected arteries:
- Embolus can typically lodge into points of normal anatomic narrowing.
- This makes superior mesenteric artery the most vulnerable site because of its relatively larger diameter (more blood flow) and low take off angle (more likely to from the aorta.
- The majority of emboli lodge 3-10cm distal to the origin of superior mesenteric artery, classically sparing the proximal jejunum and colon.
(b) Collateral circulation:
- Intestines receive collateral blood supply at all levels from the superior and inferior pancreaticoduodenal arteries, branches of the celiac artery, which provides protection from ischemia.
- These arteries can compensate for 75% reduction in mesenteric blood flow for upto 12 hours, without substanial injury.
- An extensive collateral circulation protects the intestines from transient periods of inadequate perfusion. However, prolonged reduction in splanchnic blood flow leads to vasoconstriction in the affected vascular bed, and eventually reduces collateral blood flow.
- The SMA and IMA communicate via the marginal artery of Drummond and the meandering mesenteric artery.
- Collateralization between the IMA and systemic circulation occurs in the rectum as the superior rectal (hemorrhoidal) vessels merge with the middle rectal vessels from the internal iliac arteries.
- The areas lacking this collateralization are prone towards ischemia.
(c) Vasoactive and humoral factors regulating the mesenteric blood flow:
Mechansims that control the regulation of vascular tone of mesenteric circulation in response to periods of stress such as systemic hypotension or postprandial state.
- Intrinsic regulatory system that includes metabolic and myogenic factors.
- Extrinsic regulatory system that includes neural and humoral factors.
- Metabolic factors:
- Reduction in blood supply to the mesentery causes adaptive changes in the splanchnic circulation.
- A discrepancy between tissue oxyegn demand and supply raises the concentration of local metabolites such as hydrogen, potassium, carbon dioxide, and adenosine, resulting in vasodilation and hyperemia.
- Myogenic factors:
- Myogenic theory suggests that arteriolar tension receptors act to regulate vascular resistance in accordance with the transmural pressure.
- An acute decrease in perfusion pressure is compensated for by a reduction in arteriolar wall tension, thereby maintaining splanchnic blood flow.
- Neural component:
- The extrinsic neural component of splanchnic circulatory regulation comprises the alpha-activated vasoconstrictor fibers.
- Intense activation of vasoconstrictor fibers through alpha-adrenergic stimulation results in vasoconstriction of small vessels and a decrease in mesenteric blood flow.
- After periods of prolonged alpha-adrenergic vasoconstriction, blood flow increases, presumably through β-adrenergic stimulation, which acts as a protective response.
- Although numerous types of neural stimulation (e.g. vagal, cholinergic, histaminergic, and sympathetic) can affect the blood supply of the gut, the adrenergic limb of the autonomic nervous system is the predominant neural influence on splanchnic circulation.
- Humoral component:
- Numerous endogenous and exogenous humoral factors affect the splanchnic circulation.
- Norepinephrine and high doses of epinephrine produce intense vasoconstriction by stimulating the adrenergic receptors.
- Other pharmacologic compounds that decrease splanchnic blood flow include:
- Low-dose dopamine causes splanchnic vasodilation, whereas higher doses lead to vasoconstriction by stimulating alpha adrenergic receptors.
- Exogenous agents that increase mesenteric blood flow include:
- Sodium nitroprusside
- In addition, numerous natural neurotransmitters can serve as splanchnic vasodilators, including acetylcholine, histamine, nitric oxide, leukotrienes, thromboxane analogues, glucagon, and a couple of gastrointestinal hormones.
|Factors regulating mesenteric blood flow|
|Extrinsic reguatory system|
|Humoral (endogenous and exogenous)||Neural component|
|Decrease blood flow||Increase blood flow||Decrease blood flow||Increase blood flow|
|Intrinsic regulatory component|
|Decrease blood flow||Increase blood flow|
Areas prone to ischemia:
|Areas prone to ischemia||Blood supply|
||End arteries of superior mesenteric artery|
||End arteries of inferior mesenteric artery|
- The watershed areas that lack collateralization are as follows:
- Splenic flexure
- Supplied by the end arteries of SMA with no collateral circulation.
- Rectosigmoid junction
Supplied by the end arteries of IMA with no collateral circulation.
Mechanism of ischemia:
The sequence of events that take place in the small intestine subsequent to decreased blood flow:
|Decreased delivery of oxygen and nutrients|
|Disruption in cellular metabolism|
|Tissue injury due to hypoxia and reperfusion|
|Full thickness necrosis of the bowel|
|Perforation of the bowel wall|
- Mesenteric ischemia occurs when the blood supply to mesentery is reduced leading to disruption of cellular metabolism owing to oxygen and nutrient deficiency.
- In the first 4 hours following ischemia, necrosis of the mucosal villi occurs.
- Persistent ischemia for more than 6 hours results in transmural, mural or mucosal infarction, ultimately leading to bowel perforation.
- Prolonged ischemia leads to progressive vasoconstriction of the mesenetric vessels which raises the pressure in them resulting in lowering the collateral flow.
- This is followed by vasodilation, trying to restore blood flow to the area of ischemic insult.
|Time duration dince ischemia||Pathological changes in the small intestine|
|3-4 hours||Necrosis of the mucosal villi|
|6 hours||Transmural,mural or mucosal infarction|
|1-4 days||Bowel hemorrhage|
Response to ischemia:
- Restoration of blood flow to the area of ischemic insult resutls in reperfusion injury.
- This leads to release of oxygen free radicals, toxic byproducts of ischemic injury and neutrophil activation.
Post ischemic cellular changes:
Gross pathology shows following changes:
- Early stage of ischemia: Intestinal wall in congested.
- Late stage of ischemia: Edematous, friable and hemorrhagic bowel wall.
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