12.2 Mesenteric Ischemia and Abdominal Compartment Syndrome (01.S.6–7)
Key Takeaways
- CTA with intravenous contrast is the first-line test for suspected acute mesenteric ischemia; a normal lactate does not exclude the diagnosis because lactate rises late.
- The four mechanisms — arterial embolus, arterial thrombosis, mesenteric venous thrombosis, and nonocclusive mesenteric ischemia — have different vessels, tempos, and treatments.
- World Society of the Abdominal Compartment Syndrome (WSACS) ACS is a sustained intra-abdominal pressure >20 mm Hg with new organ dysfunction; IAH is IAP ≥12 mm Hg.
- Measure bladder pressure with a 25 mL saline instill, fully supine, at end-expiration, transducer zeroed at the mid-axillary line; 50–100 mL instills falsely raise the reading.
- High intra-abdominal pressure raises intrathoracic pressure, impedes cerebral venous return, increases ICP, and can drop cerebral perfusion pressure; abdominal perfusion pressure is MAP minus IAP.
Two abdominal catastrophes share a neuro ICU habit of presenting without a helpful examination: acute mesenteric ischemia (AMI) and abdominal compartment syndrome (ACS). Both raise lactate late. Both can look like “just ileus” or “just ventilator asynchrony.” Both interact with ICP. Independent OpenExamPrep teaching for this examination follows the mesenteric-ischemia and ACS topics in the ABPN Content Specifications and is not a surgical-society product.
Why the gut infarcts quietly here
Pain out of proportion to physical findings is the classic arterial-embolic phrase. In a sedated SAH or TBI patient there may be no pain report at all. Clues become unexplained lactic acidosis after ICP is controlled, a sudden vasopressor spike, bloody diarrhea, abdominal distention, and feeding failure. By the time the abdomen is rigid, transmural infarction and perforation are already present and mortality is high. The examination item is therefore early CTA, not waiting for a surgical abdomen.
Four mechanisms, four plans
Rough epidemiologic shares in mixed series are arterial embolus ~40–50%, arterial thrombosis ~20–30%, nonocclusive mesenteric ischemia (NOMI) ~20%, and mesenteric venous thrombosis ~5–15%. Those percentages are orientation, not a scoring system. What you must separate is anatomy and treatment.
Arterial embolus usually lodges in the SMA distal to the origin of the middle colic artery. Atrial fibrillation, a ventricular thrombus after myocardial infarction, and endocarditis are the usual sources — all common after stroke or after anticoagulation was stopped for intracranial hemorrhage. Onset is abrupt. Proximal jejunum and the transverse colon may be relatively spared because the embolus sits beyond those branches. The patient may have other emboli (limb, spleen, kidney).
Arterial thrombosis occurs at an atherosclerotic SMA origin. There is often a prodrome of postprandial pain, sitophobia, and weight loss (chronic mesenteric ischemia) then an acute occlusion. Because the ostium is lost, a greater length of midgut is at risk and collaterals from the celiac axis or IMA may already be tenuous. Endovascular stenting or surgical revascularization plus resection of dead bowel is the language of this disease; anticoagulation alone is not.
Mesenteric venous thrombosis (usually superior mesenteric vein) is slower: days of pain in a younger or hypercoagulable patient, or after abdominal surgery, pancreatitis, or cirrhosis with portal hypertension. CTA in the portal venous phase shows the clot. If there is no peritonitis, first-line therapy is anticoagulation (unfractionated heparin in the ICU so it can be stopped for a craniotomy or EVD revision). Operate for dead bowel; second-look laparotomy is common; anticoagulation continues afterward unless a neurosurgical constraint is absolute and then the conversation is about IVC filters and timing, not about ignoring the vein clot.
NOMI is low-flow splanchnic vasoconstriction with patent proximal arteries. The usual patient is already in your unit: cardiogenic shock, hypovolemia, high-dose norepinephrine or vasopressin, cocaine, or ergot. CTA shows bowel ischemia — wall hypoenhancement, pneumatosis, fluid — with open SMA/IMA trunks. Treatment is restore perfusion: volume, inotropes if the problem is cardiac output, and weaning vasoconstrictors. Catheter-directed papaverine or other vasodilators appear in older angiographic algorithms when a catheter is already in the SMA. Laparotomy is for perforation or clearly dead bowel, not for every patent-vessel NOMI.
| Type | Typical patient | Vessel finding | First specific therapy |
|---|---|---|---|
| Arterial embolus | AF, sudden pain, soft belly | SMA cutoff beyond middle colic origin | Resuscitate, heparin if the brain allows, embolectomy/endovascular plus resect dead bowel |
| Arterial thrombosis | Atherosclerosis, postprandial prodrome | Ostial SMA occlusion | Revascularize; resect infarct |
| Venous thrombosis | Hypercoagulable, slower tempo | SMV/portal thrombus on venous-phase CTA | Anticoagulation if no peritonitis; resect only necrotic bowel |
| NOMI | Shock, high-dose vasopressors | Patent origins, ischemic bowel | Reverse low flow; vasodilator infusion in selected cases; operate for dead bowel |
Lactate is late; CTA is the test
Do not wait for lactate. A normal lactate does not exclude AMI; an elevated lactate does not prove it and may reflect seizure, hypoperfusion, or metformin. Leukocytosis, metabolic acidosis, and phosphate elevation are likewise late. D-dimer is sensitive and hopelessly nonspecific. The first-line imaging study is multiphasic CT angiography of the abdomen and pelvis with intravenous contrast: noncontrast (if looking for bleed or hyperdense thrombus), arterial, and portal venous phases. Oral contrast delays care and can hide mural enhancement. Catheter angiography is now mostly a treatment platform (embolectomy, stent, papaverine) rather than the screening test. Plain films are for free air, not for ruling out ischemia.
If CTA shows occlusion and the abdomen is surgical, the patient goes to the operating room (often with a hybrid endovascular option). Damage-control resection of frankly necrotic bowel, temporary abdominal closure, and a second look at 24–48 hours are standard because bowel of uncertain viability can recover after flow is restored. Do not perform a massive primary anastomosis in a vasopressor-dependent, acidotic patient.
Worked example: a 78-year-old whose anticoagulation was held after lobar ICH develops sudden abdominal pain. The belly is soft. Lactate is 1.4 mmol/L. The wrong answer is reassurance because lactate is normal. The right answer is CTA now, looking for an SMA embolus from atrial fibrillation. If ICH still prohibits full anticoagulation, that is a documented risk–benefit problem after the diagnosis is made — it is not a reason to skip the scan.
Abdominal compartment syndrome
Intra-abdominal pressure (IAP) is the steady pressure in the abdominal cavity. The WSACS 2013 consensus, still the definition set examinations use, is precise:
- Normal IAP in a critically ill adult is about 5–7 mm Hg.
- Intra-abdominal hypertension (IAH) is a sustained or repeated IAP ≥12 mm Hg.
- IAH grades: I 12–15, II 16–20, III 21–25, IV >25 mm Hg.
- ACS is a sustained IAP >20 mm Hg (with or without abdominal perfusion pressure <60 mm Hg) plus new organ dysfunction or failure.
Abdominal perfusion pressure (APP) = mean arterial pressure (MAP) − IAP. Analogous to cerebral perfusion pressure, APP conceptually tracks whether the gut and kidneys are actually perfused. A MAP of 80 mm Hg with an IAP of 24 mm Hg is an APP of 56 mm Hg — not a comfortable number.
Primary ACS starts in the abdominopelvic compartment (hemorrhage, pancreatitis, bowel edema after ischemia, packing). Secondary ACS starts outside it: massive crystalloid resuscitation, burns, sepsis capillary leak, and the neuro ICU pattern of liters of fluid after SAH, TBI, or spinal shock. Recurrent ACS returns after a prior decompression, including after a tight abdominal closure or a binder.
How to measure bladder pressure
The reference intermittent method is transvesical. Instill a maximum of 25 mL of sterile saline into the empty bladder (larger instills — 50 to 100 mL — falsely elevate IAP). Place the patient fully supine, wait until abdominal muscles are quiet (sedate or paralyze if the patient is straining), zero the transducer at the mid-axillary line, and read at end-expiration. Report mm Hg, not cm H2O (1 mm Hg ≈ 1.36 cm H2O). Head-of-bed elevation, a full bladder, and coughing all artifactually raise the number; they do not mean you should skip measurement in a patient who cannot lie flat — document the conditions and trend.
Clinical clues that should trigger a measurement: oliguria not explained by hypovolemia, high peak inspiratory pressures, a tight abdomen, new vasopressor need, and a rising ICP without a new cranial mass. The abdomen is a compartment; treat it like one.
Medical decompression, then the knife
ACS is not an automatic trip to laparotomy if you can drop IAP in minutes, but it is also not a “watch the number overnight” disease once organs are failing. A WSACS-style medical bundle:
- Evacuate intraluminal contents — nasogastric tube, rectal tube, treat ileus, consider colonoscopy only if it will actually decompress a mega-colon.
- Evacuate extra-luminal contents — paracentesis for tense ascites (common in cirrhosis and in some SAH patients who received volume).
- Improve abdominal wall compliance — adequate sedation, neuromuscular blockade if the wall is rigid from fighting the ventilator, reverse Trendelenburg or head-up if ICP allows a compromise, remove constricting binders.
- Limit further volume injury — stop reflexive crystalloid, use blood products for hemorrhage, consider hypertonic fluids or continuous renal replacement for overload.
- Support perfusion — raise MAP if APP is low, remembering that extra norepinephrine can worsen NOMI.
- Decompressive laparotomy if ACS persists with organ failure. Leave the abdomen open with a temporary closure; a tight primary fascia can recreate ACS in an hour.
Worked example: a 52-year-old after aneurysm clipping received 8 L of crystalloid and 4 units of red cells. Peak airway pressure is 38 cm H2O, urine output 10 mL/h, bladder pressure 23 mm Hg, ICP 24 mm Hg, MAP 80 mm Hg. APP = 80 − 23 = 57 mm Hg. CPP = 80 − 24 = 56 mm Hg. The same number that is failing the kidney is failing the brain. NGT, paralysis if fighting, stop the fluid, drain ascites if present — and if those do not drop IAP, open the abdomen. Raising PEEP to “treat” the high airway pressure makes venous return and ICP worse.
Interaction with ICP and CPP
The abdomen and the skull share a venous system. Elevated IAP is transmitted through the diaphragm to intrathoracic pressure, which raises central venous pressure and impedes jugular and vertebral venous outflow. Cerebral venous congestion raises cerebral blood volume and therefore ICP. MAP may fall at the same time because venous return to the heart falls, afterload on the right ventricle rises, and high airway pressures worsen dead space. CPP = MAP − ICP therefore drops from both ends. High PEEP, abdominal binders, Trendelenburg, and ACS are additive. In a monitored TBI patient, a sudden ICP spike with oliguria and a rock-hard abdomen is ACS until measured.
Conversely, decompressive laparotomy in true ACS can lower ICP. That is not a license to open every tense belly; it is the reason a neurointensivist measures bladder pressure instead of stacking osmotic therapy for an ICP crisis that originated below the diaphragm. After a hemicraniectomy the skull is no longer a closed box, but venous obstruction from ACS still harms remaining brain and the opposite hemisphere.
| Pressure relationship | Formula or threshold | Neuro ICU implication |
|---|---|---|
| CPP | MAP − ICP | Falls if ACS drops MAP or raises ICP |
| APP | MAP − IAP | Gut/kidney analog; ACS often APP <60 mm Hg |
| ACS | IAP >20 mm Hg + new organ failure | Laparotomy if medical bundle fails |
| IAH | IAP ≥12 mm Hg | Start the medical bundle before organs fail |
| Bladder method | 25 mL, supine, end-expiration, mid-axillary zero | Do not instill 70 mL “to get a waveform” |
Exam traps: waiting for lactate before CTA; treating NOMI with SMA embolectomy; treating venous thrombosis with immediate extensive resection and no heparin; diagnosing ACS from a grade-II IAP of 18 mm Hg without organ failure; calling ACS “ruled out” because the abdomen is not rigid in a paralyzed patient; and treating ACS-related high airway pressures with more PEEP while ignoring IAP.
Anticoagulation was held after ICH. The patient, who has atrial fibrillation, develops sudden severe abdominal pain. The abdomen is soft and lactate is 1.2 mmol/L. Which statement is most accurate?
After massive crystalloid resuscitation, bladder pressure is 24 mm Hg, urine output is 8 mL/h, and peak airway pressures have risen. MAP is 85 mm Hg. How should this be classified and treated?
In a TBI patient with a tight abdomen, how does high intra-abdominal pressure affect the brain?
A patient in cardiogenic shock on high-dose norepinephrine has a rising lactate and bloody diarrhea. CTA shows patent celiac, SMA, and IMA origins with hypoenhancing small-bowel mucosa. What is the best initial vascular plan?