Mesenteric Ischemia

Acute Mesenteric Ischemia (AMI)

1. rare but mortality 25-80% even in modern era

2. related to challenges & delay in diagnosis

3. anatomy & physiology review

   • SMA is primary small bowel supply (some collateral flow from celiac & IMA)

   • splanchnic circulation receives 20-35% of CO, depending on feeding state

     1. 70% of this circulation is directed to bowel mucosa & submucosa

     2. splanchnic O2 extraction is low → requires 50% reduction before small bowel ischemia

     3. studies show that mesenteric ischemia does not occur until MAP ≤ 45

     4. bowel can compensate for up to 75% reduction in blood flow for up to 12h

   • intrinsic & extrinsic regulatory mechanism of mesenteric blood flow

     1. intrinsic

        • hypoxic vasodilation

        • reactive hyperemia

        • pressure-flow autoregulation

        • myogenic factors (ie ↓perfusion w/reflexive vasoconstriction)

        • metabolic factors (ie O2 supply/demand imbalance, ↑K, acidosis)

     2. extrinsic

        • mural (autonomics → alpha-adrenergic fibers → vasoconstriction → ↑splanchnic flow)

        • humural (ie norepinephrine, epinephrine, dobutamine, dopamine

4. pathogenesis: sustained bowel hypoperfusion → ischemic mucosa & submucosa injury

   • ↑permeability/disruption → bacterial translocation

   • sloughing → diarrhea w/wo blood

   • infarction & necrosis → perforation → peritonitis & sepsis

   • +/- reperfusion injury → organ dysfunction s/p tx

5. etiology

   • Embolism of the SMA

     1. classically reported to account for 40-50% of causes

     2. majority 2/2 cardiac mural thrombus (uncommonly 2/2 coronary/cerebral angiography)

     3. majority lodge in SMA 2/2 oblique angle from which it arises from aorta

     4. 50% lodge 3-6 cm distal to middle colic artery (ie 1st major branch of SMA)

     5. typically less severe vs thrombosis (classically spares proximal jejunum and colon)

   • Thrombosis of the SMA

     1. historically 25-30% of cases; modern cases series suggest now leading cause

     2. virtually all occur 2/2 severe atherosclerotic disease (most commonly at origin of SMA)

     3. slow progressive nature of atherosclerosis allows collateral flow development

     4. typically more severe vs embolism (can extend duodenum to transverse colon)

   • Mesenteric Vein Thrombosis

     1. least common, reported in up to 10% of cases

     2. 2/2 intra-abdominal pathology (ie malignancy, pancreatitis) or hypercoagulability

     3. usually results in a more segmental ischemia, often involves PV or SMV or both

     4. transition from viable/non-viable bowel is gradual

   • Non-Occlusive Mesenteric Ischemia (NOMI) aka low-flow state

     1. accounts for 20% of cases

     2. classically incl.

        • CKD pt on iHD who becomes hypotensive and develops diffuse abdominal pain

        • critically ill pts on multiple vasopressors

     3. tends to affect watershed areas

        • SMA-IMA (Griffiths point or splenic flexure)

        • SRA-MRA (Sudeck point or rectosigmoid junction)

6. clinical

   • severe abd pain out of proportion to a soft abd exam

     1. need to have a high index of suspicion

   • n/v w/diarrhea +/- blood in stools

   • 33% present w/triad of abd pain, fever, hemoccult+ stool

   • arterial embolism: acute severe pain, hx AFib, PO intolerance

   • arterial thrombosis: post-prandial pain, unintentional wt loss

   • venous thrombosis: subacute pain, anorexia, hypovolemia

   • NOMI: critical illness, multiple pressors, abd distension, sepsis

7. workup

   • vitals may normal-to-deranged

     1. severe hypovolemia can result 2/2 bowel edema

   • ↑WBC, abnormal lytes, ↑amylase, lactic acidosis are non-specific

     1. a normal lactate does not r/o AMI

   • obtain ECG w/troponins to r/o arrhythmia and recent MI

   • CT abdomen/pelvis w/contrast

     1. may be non-diagnostic or show evidence of occlusion vs non-occlusion

   • be cautious: the following imaging may delay treatment

     1. MRA obviates contrast but requires time and produces lower resolution images

        • risks of delaying correct diagnosis outweigh risks of contrast (get the CT)

     2. plain films can show pneumoperitoneum or PVG but do not identify cause

     3. duplex US is not usually useful in AMI 2/2 overlying bowel edema

     4. TTE can delineate cardiac source of emboli but should not delay revascularization

   • when diagnosed and treated w/in 24h of sx onset, survival is still only 50%

     1. drops sharply to < 30% w/delays in diagnosis

8. diagnosis: mesenteric CTA w/arterial and venous phase

   • arterial embolism: meniscus sign w/abrupt occlusion of SMA several centimeters from origin

   • arterial thrombosis: SMA occlusion at origin often w/enlarged collaterals

     1. take note of calcifications on pre-contrast images esp. SMA os in the saggital plane

   • venous thrombosis: splenomegaly & ascites

   • other: abnormal bowel wall enhancement, intestinal dilation & thickening, pneumatosis intestinalis, portal venous gas, splenic or kidney infarction, free abdominal fluid, aortic dissection

9. therapy

   • early & aggressive resuscitation (fluids, lytes, etc.)

   • +/- vasopressors (only if needed)

   • broad spectrum abx if concern for bowel infarction

   • NGT (decompress bowel) & Foley (monitor UOP)

   • systemic heparinization

   • indications for emergent surgery

     1. peritonitis

     2. pneumoperitoneum

     3. pneumatosis

     4. HDUS

   • goals of emergent surgery

     1. establish goals of care

     2. reestablish blood flow to ischemic bowel

        • SMA embolus: embolectomy w/primary closure or patch angioplasty

        • SMA thrombosis: vein graft bypass (ex. retrograde iliac to distal SMA)

        • venous thrombosis & NOMI: supportive +/- IR systemic vs directed thrombolysis

     3. resect all non-viable segments of bowel

     4. preserve all potentially viable bowel (place temporary abd closure and go to ICU)

   • determining bowel viability (probably best for 2nd-look)

     1. palpation of SMA pulse

     2. anti-mesenteric Doppler signal

     3. fluorescein injection f/b Wood's lamp exam

     4. intentional enterotomy to assess for bleeding

     5. evidence of peristalsis

     6. indocyanine green (ICG) w/SPY portable handheld imager (SPY-PHI)

   • options for pts w/o emergent indications but w/significant occlusion

     1. endovascular embolectomy

     2. thrombolysis w/wo stenting

10. prognosis

    • peri-op mortality for revascularization ranges 40-90%

    • subset of pts who survive may develop short gut syndrome

Chronic Mesenteric Ischemia

1. TBD