21.3 Abdominal Compartment Syndrome & Acute Mesenteric Ischemia

ABIM Critical Care Medicine Study Guide Exam

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1 Chapter 1: Exam Orientation: Format, Blueprint & Study Strategy

1.1 ABIM Critical Care Medicine Exam: Format, Blueprint & Study Strategy

2 Chapter 2: Acute Respiratory Failure & ARDS

2.1 ARDS Definition, Pathophysiology & Phenotypes 2.2 Lung-Protective Mechanical Ventilation 2.3 Refractory ARDS: Prone Positioning, Neuromuscular Blockade & VV-ECMO

3 Chapter 3: Obstructive Airways, Pulmonary Vascular & Pleural Emergencies

3.1 Severe Asthma & COPD Exacerbations in the ICU 3.2 Massive & Submassive Pulmonary Embolism 3.3 Pulmonary Hypertension & Right Ventricular Failure in the ICU 3.4 Massive Hemoptysis, Complex Pleural Effusions & Thoracic Drainage

4 Chapter 4: Airway, Parenchymal & Nonthrombotic Pulmonary Emergencies

4.1 Airway Management, Upper-Airway Obstruction & Artificial-Airway Complications 4.2 Diffuse Alveolar Hemorrhage, Interstitial Lung Disease, Pulmonary Vasculitis & Noncardiogenic Edema 4.3 Severe Community-Acquired & Viral Pneumonia 4.4 Nonthrombotic Embolism (Air, Fat, Amniotic Fluid, Tumor, Septic) & Acute Chest Syndrome

5 Chapter 5: Mechanical Ventilation Modes, Dyssynchrony & Liberation

5.1 Mechanical Ventilator Modes & Advanced Breath Delivery 5.2 Patient-Ventilator Dyssynchrony & Waveform Analysis 5.3 Ventilator Liberation, Spontaneous Breathing Trials & Post-Extubation Support

6 Chapter 6: Shock Syndromes & Hemodynamic Principles

6.1 Shock Classification, Cellular Pathophysiology & Biomarkers 6.2 Hemodynamic Monitoring: Arterial Lines, CVP & Pulmonary Artery Catheter 6.3 Dynamic Assessment of Fluid Responsiveness & Resuscitation Targets

7 Chapter 7: Vasoactive Therapies & Mechanical Circulatory Support

7.1 Vasopressors & Inotropes: Pharmacology & Clinical Titration 7.2 Temporary Mechanical Circulatory Support: IABP, Impella & TandemHeart 7.3 Venoarterial ECMO (VA-ECMO) & Differential Hypoxemia 7.4 Post-Cardiac Arrest Care, Targeted Temperature Management & Neuroprognostication

8 Chapter 8: Acute Kidney Injury & Renal Replacement Therapy

8.1 KDIGO AKI Classification, Etiologies & Diagnostic Evaluation 8.2 Continuous Renal Replacement Therapy (CRRT) vs Intermittent Hemodialysis 8.3 Indications & Timing for RRT in Critical Illness: AKIKI & STARRT-AKI

9 Chapter 9: Electrolyte, Acid-Base & Endocrine Emergencies

9.1 Severe Dysnatremias: Hyponatremia, Hypernatremia & Osmotic Demyelination 9.2 Potassium, Calcium, Magnesium & Phosphate Disorders 9.3 Complex & Mixed Acid-Base Disorders: Boston vs Copenhagen vs Stewart 9.4 Endocrine Crises: DKA, HHS, Thyroid Storm, Myxedema Coma & Adrenal Crisis

10 Chapter 10: Glycemic Control, Hyperammonemia, Pituitary & Adrenal Disorders

10.1 ICU Glycemic Control & Severe Hypoglycemia 10.2 Non-Hepatic Hyperammonemia & Metabolic Encephalopathy 10.3 Pituitary Emergencies, Adrenal Excess & Critical Illness-Related Corticosteroid Insufficiency

11 Chapter 11: Acute Coronary Syndromes & Cardiac Arrhythmias

11.1 Acute Coronary Syndromes & Mechanical Complications of Myocardial Infarction 11.2 Critical Tachyarrhythmias: Atrial Fibrillation with RVR, VT & VF 11.3 Critical Bradyarrhythmias, Conduction Blocks & Temporary Cardiac Pacing

12 Chapter 12: Acute Heart Failure, Valvular & Pericardial Emergencies

12.1 Acute Decompensated Heart Failure & Cardiogenic Pulmonary Edema 12.2 Acute Valvular Regurgitation & Prosthetic Valve Emergencies 12.3 Pericardial Tamponade & Acute Aortic Syndromes

13 Chapter 13: Myocardial Disease, Hypertensive & Cardiac Device Emergencies

13.1 Fulminant Myocarditis, Stress & Peripartum Cardiomyopathy, and Hypertrophic Cardiomyopathy 13.2 Hypertensive Emergencies & Ruptured Abdominal Aortic Aneurysm 13.3 Implanted Cardiac Devices, LVAD Emergencies & Electrical Storm

14 Chapter 14: Sepsis & Systemic Infectious Diseases

14.1 Sepsis-3 Definitions & Surviving Sepsis Campaign 2026 Guidelines 14.2 Refractory Septic Shock & Adjunctive Therapies 14.3 Multidrug-Resistant Pathogens & Antimicrobial Stewardship in the ICU

15 Chapter 15: ICU-Acquired & Severe Focal Infections

15.1 Hospital-Acquired & Ventilator-Associated Pneumonia 15.2 Central Line-Associated Bloodstream Infections & Catheter-Associated UTIs 15.3 Intra-Abdominal Sepsis, Necrotizing Soft Tissue Infections & CNS Infections

16 Chapter 16: Focal, Special-Host & Systemic Infections

16.1 CNS Infections Beyond Bacterial Meningitis: Encephalitis, Brain Abscess & Epidural Abscess 16.2 Infections in the Immunocompromised Host, Tropical & Bioterror Infections 16.3 Infective Endocarditis, Device Infections & Other Focal Cardiovascular Infections 16.4 Antimicrobial Pharmacokinetics, Allergy, Toxicity & Infection Prevention 16.5 Gastrointestinal & Hepatobiliary Infections: Fulminant C. difficile, Typhlitis, CMV Colitis & Abscesses

17 Chapter 17: Neurocritical Care & Coma

17.1 Coma, Altered Mental Status & Acute Delirium in the ICU 17.2 Status Epilepticus & Non-Convulsive Status Epilepticus 17.3 Acute Ischemic Stroke & Intracranial Hemorrhage

18 Chapter 18: Cerebrovascular Syndromes, Encephalopathy & Withdrawal States

18.1 Cerebral Venous Sinus Thrombosis, PRES & Reversible Cerebral Vasoconstriction Syndrome 18.2 Alcohol & Sedative Withdrawal, Wernicke Encephalopathy & Drug-Induced Encephalopathy

19 Chapter 19: Intracranial Hypertension & Neuromuscular Weakness

19.1 Intracranial Pressure Monitoring, Cerebral Perfusion & Tiered ICP Management 19.2 Acute Neuromuscular Respiratory Failure, ICU-Acquired Weakness & Brain Death Determination

20 Chapter 20: Traumatic Brain Injury, Spinal Cord Injury & Toxin-Mediated Paralysis

20.1 Traumatic Brain Injury Management & Paroxysmal Sympathetic Hyperactivity 20.2 Acute Spinal Cord Injury, Neurogenic Shock & Autonomic Dysreflexia 20.3 Toxin-Mediated Paralysis & Neuromuscular Crises: Botulism, Tetanus, Tick Paralysis & Myasthenic Crisis

21 Chapter 21: Gastrointestinal, Hepatic & Abdominal Emergencies

21.1 Acute Upper & Lower Gastrointestinal Bleeding in the ICU 21.2 Acute Liver Failure & Severe Acute Pancreatitis 21.3 Abdominal Compartment Syndrome & Acute Mesenteric Ischemia

22 Chapter 22: Luminal GI Emergencies & Complications of Cirrhosis

22.1 Esophageal, Gastric & Bowel Emergencies: Perforation, Obstruction, Ogilvie Syndrome & Colonic Ischemia 22.2 Complications of Cirrhosis: Alcohol-Associated Hepatitis, Hepatorenal Syndrome & Pulmonary Vascular Syndromes

23 Chapter 23: Trauma, Burns & Perioperative Critical Care

23.1 Polytrauma Resuscitation, Damage Control & Massive Transfusion 23.2 Severe Burn Injury, Inhalation Injury & Environmental Emergencies 23.3 Perioperative Critical Care for High-Risk Surgical Patients

24 Chapter 24: Thoracic Trauma, Environmental Injury & Obstetric Critical Care

24.1 Thoracic Trauma: Flail Chest, Pulmonary Contusion, Great Vessel & Tracheobronchial Injury 24.2 Environmental Injury: Drowning, Diving, Altitude, Heat, Cold, Electrical & Radiation 24.3 Obstetric & Genitourinary Emergencies and the Physiology of Pregnancy

25 Chapter 25: Transplantation, Donor Management & Post-Cardiac Surgery Care

25.1 Solid Organ Transplant Recipients & Deceased Donor Management 25.2 Post-Cardiac Surgery Critical Care: Bleeding, Tamponade, Low Output & Vasoplegia

26 Chapter 26: Hematologic Emergencies, Toxicology, Critical Ultrasound & ICU Liberation

26.1 Hematologic Emergencies in the ICU: HIT, DIC, TMA & Anticoagulant Reversal 26.2 Critical Care Toxicology, Toxidromes & Specific Antidotes 26.3 Critical Care Point-of-Care Ultrasound (POCUS) 26.4 ICU Liberation, Analgosedation, Goals of Care & Critical Care Ethics

27 Chapter 27: Transfusion Medicine, Hematologic & Oncologic Disorders

27.1 Transfusion Medicine: Products, Thresholds, Reactions & Apheresis 27.2 Red and White Cell Disorders, Hemoglobinopathies, Factor Deficiencies & Hypercoagulable States 27.3 Oncologic Emergencies, Hematopoietic Cell Transplant Complications & Cellular Therapy Toxicity

28 Chapter 28: Clinical Pharmacology, Adverse Drug Reactions & Specific Poisonings

28.1 Pharmacokinetic Principles, Dose Adjustment & Drug Interactions in Critical Illness 28.2 Adverse Drug Reactions: Anaphylaxis, Severe Cutaneous Reactions & Hyperthermic Syndromes 28.3 Specific Poisonings: Sodium-Channel Blockers, Lithium, Valproate, Theophylline, Iron, Sulfonylureas & Methemoglobinemia

29 Chapter 29: Ethics, Research Methods, Administration & Abdominal Ultrasound

29.1 Advance Care Planning, Nonbeneficial Treatment, Informed Consent & Confidentiality 29.2 Critical Care Research Methods, Statistics, IRB Oversight & Quality Improvement 29.3 Abdominal, Vascular & Procedural Point-of-Care Ultrasound

21.3 Abdominal Compartment Syndrome & Acute Mesenteric Ischemia

Key Takeaways Explain This Topic AI

Intra-Abdominal Hypertension (IAH) is defined by a sustained intra-abdominal pressure (IAP) ≥ 12 mmHg \ge 12\text{ mmHg} ≥ 12 mmHg , whereas Abdominal Compartment Syndrome (ACS) is defined as a sustained IAP > 20 mmHg > 20\text{ mmHg} > 20 mmHg (with or without abdominal perfusion pressure APP < 60 mmHg \text{APP} < 60\text{ mmHg} APP < 60 mmHg ) associated with new-onset organ dysfunction or failure.

Transvesical bladder pressure measurement serves as the gold-standard diagnostic tool: it must be performed with the patient strictly supine, instilling no more than 20 − 25 mL 20-25\text{ mL} 20 − 25 mL of sterile saline, zeroing the transducer at the mid-axillary line at the level of the iliac crest, and recording pressure at end-expiration.

Uncontrolled intra-abdominal hypertension induces profound multisystem organ failure: diminished venous return and increased afterload impair cardiac output, diaphragmatic elevation precipitates restrictive lung mechanics and hypercapnia, renal vein compression drives oliguric acute kidney injury, and impaired jugular venous outflow elevates intracranial pressure.

Management follows the WSACS tiered medical escalation protocol—incorporating gastrointestinal decompression, fluid optimization with diuresis or CRRT, percutaneous drainage of collections, and neuromuscular blockade—with decompressive laparotomy and temporary open abdomen negative pressure wound therapy reserved for refractory ACS.

Acute mesenteric ischemia (AMI) encompasses four distinct pathophysiological subtypes—SMA embolism, SMA thrombosis, non-occlusive mesenteric ischemia (NOMI), and mesenteric venous thrombosis (MVT)—requiring urgent biphasic CTA of the abdomen/pelvis without oral contrast for prompt diagnosis, followed by subtype-specific revascularization, resection of infarcted bowel, intra-arterial papaverine for NOMI, or systemic anticoagulation for MVT.

Last updated: October 2026

Intra-Abdominal Hypertension and Abdominal Compartment Syndrome: Definitions and Pathophysiology

The World Society of the Abdominal Compartment Syndrome (WSACS) establishes consensus definitions and clinical management guidelines for intra-abdominal pressure (IAP) derangements in critically ill patients.

WSACS Definitions and Grading Criteria

Normal Physiological IAP : In healthy, non-critically ill adults, baseline resting IAP ranges from 0 0 0 to 5 mmHg 5\text{ mmHg} 5 mmHg . In critically ill, mechanically ventilated adult ICU patients, baseline IAP typically measures between 5 5 5 and 7 mmHg 7\text{ mmHg} 7 mmHg .

Intra-Abdominal Hypertension (IAH) : Defined as a sustained or repeated pathological elevation of IAP ≥ 12 mmHg \ge 12\text{ mmHg} ≥ 12 mmHg .

• Grade I IAH : IAP 12 − 15 mmHg 12 - 15\text{ mmHg} 12 − 15 mmHg
• Grade II IAH : IAP 16 − 20 mmHg 16 - 20\text{ mmHg} 16 − 20 mmHg
• Grade III IAH : IAP 21 − 25 mmHg 21 - 25\text{ mmHg} 21 − 25 mmHg
• Grade IV IAH : IAP > 25 mmHg > 25\text{ mmHg} > 25 mmHg

Abdominal Compartment Syndrome (ACS) : Defined as a sustained IAP > 20 mmHg > 20\text{ mmHg} > 20 mmHg (with or without an Abdominal Perfusion Pressure < 60 mmHg <60\text{ mmHg} < 60 mmHg ) that is associated with new-onset organ dysfunction or failure (e.g., oliguria, respiratory failure, hemodynamic collapse, elevated ICP).

Abdominal Perfusion Pressure (APP) : Analogous to cerebral perfusion pressure, APP quantifies the net vascular perfusion driving pressure across intra-abdominal organs: APP = MAP − IAP \text{APP} = \text{MAP} - \text{IAP} APP = MAP − IAP A target APP ≥ 60 mmHg \text{APP} \ge 60\text{ mmHg} APP ≥ 60 mmHg has been demonstrated to correlate with visceral perfusion, intestinal mucosal blood flow, and survival, serving as a superior prognostic resuscitation endpoint compared to mean arterial pressure (MAP) alone. [Intra-Abdominal Pressure Spectrum (mmHg)]

• 0 ─────────── 5 ─────────── 7 ─────────── 12 ─────────── 15 ─────────── 20 ─────────── >25
• Normal Non-ICU Normal ICU Grade I Grade II Grade III Grade IV
• └─────────────── Intra-Abdominal ───────────────┘
• Hypertension (IAH)
• │
• IAP > 20 mmHg + New Organ Failure
• ▼
• ABDOMINAL COMPARTMENT SYNDROME
• Standardized Technique for Transvesical Bladder Pressure Monitoring

The gold-standard bedside method for monitoring intra-abdominal pressure is indirect transvesical bladder pressure measurement through an indwelling urinary catheter. Standardized WSACS Transvesical Bladder Pressure Measurement:

• 1. Position Patient Strictly Flat (Complete Supine Position)
• 2. Connect Pressure Transducer to Urinary Foley Catheter Drainage Port
• 3. Zero Transducer at Mid-Axillary Line at Level of Iliac Crest
• 4. Clamp Distal Drainage Bag Tubing
• 5. Instill MAXIMUM 20 to 25 mL Sterile Saline into Bladder
• 6. Ensure Absence of Active Abdominal Muscle Contraction
• 7. Record Pressure in mmHg at End-Expiration
• 8. Unclamp Foley Catheter Drainage Tubing Immediately After Reading

Caution

Common Technical Pitfalls in Bladder Pressure Measurement :

Excessive Instillation Volume : Historical protocols instilled 50 − 100 mL 50-100\text{ mL} 50 − 100 mL of saline. Volumes exceeding 25 mL 25\text{ mL} 25 mL induce detrusor muscle stretching and involuntary bladder smooth muscle contraction, falsely elevating measured pressure by 5 − 10 mmHg 5-10\text{ mmHg} 5 − 10 mmHg . Instill strictly 20 − 25 mL 20-25\text{ mL} 20 − 25 mL .

Patient Positioning : Elevating the head of the bed to 30 − 45 ∘ 30-45^\circ 30 − 4 5 ∘ mechanically shifts heavy visceral contents caudally onto the bladder, falsely elevating IAP by 4 − 8 mmHg 4-8\text{ mmHg} 4 − 8 mmHg . The patient must be placed completely supine ( 0 ∘ 0^\circ 0 ∘ ) for measurements.

Zero Reference Landmark : The transducer must be zeroed at the intersection of the mid-axillary line and the iliac crest . Zeroing at the phlebostatic axis or symphysis pubis introduces significant hydrostatic artifact.

Respiratory Cycle Timing : Positive pressure mechanical ventilation increases intrathoracic pressure transmitted across the diaphragm. Pressures must be recorded precisely at end-expiration .

Multi-System Organ Consequences of Elevated Intra-Abdominal Pressure

The abdominal cavity functions as a closed, semi-rigid anatomical compartment bounded by the diaphragm, spine, pelvis, and muscular abdominal wall. Unconstrained pressure elevation compromises virtually every major organ system: ┌──────────────────────────┐

• │ Sustained IAP > 20 mmHg │
• └────────────┬─────────────┘
• ┌─────────────────────────────┼─────────────────────────────┐
• ▼ ▼ ▼
• [Cardiovascular] [Respiratory] [Renal]
• IVC / Portal Compression • Diaphragm Displaced Upward • Renal Vein Compression

• Decreased Venous Return • Thoracic Compliance Falls • Elevated Renal Venous Press.

• Decreased Cardiac Output • Peak & Plateau Pressures Rise • RAPP = MAP - IAP Collapses
• Falsely Elevated CVP/PAWP • Hypercapnia & Hypoxemia • Oliguria / Rapid AKI

• ┌─────────────────────────────┴─────────────────────────────┐
• ▼ ▼
• [Neurologic] [Gastrointestinal]
• Impaired Jugular Venous Drainage • Splanchnic Hypoperfusion
• Elevated Intracranial Pressure (ICP) • Mucosal Ischemia & Acidosis
• CPP = MAP - ICP Collapses • Bacterial Translocation / SIRS

Cardiovascular Dysfunction :

Cephalad displacement of the diaphragm and direct extrinsic compression collapse the thin-walled inferior vena cava (IVC) and portal vein, markedly reducing venous return (cardiac preload) and reducing cardiac output.

Transmission of elevated IAP into the thoracic compartment increases intrathoracic pressure, increasing left ventricular afterload (elevated systemic vascular resistance, SVR).

Falsely Elevated Filling Pressures : Central venous pressure (CVP) and pulmonary artery wedge pressure (PAWP) are elevated due to transmitted intrathoracic pressure, misleading clinicians into diagnosing hypervolemia when effective circulating preload is actually severely depleted.

Respiratory Dysfunction :

The diaphragm is displaced cephalad into the thoracic cavity, compressing the lower lung lobes and causing extensive compressive basal atelectasis.

Total respiratory system and chest wall compliance plunge, resulting in marked increases in peak inspiratory pressures ( P peak P_{\text{peak}} P peak ​ ) and plateau airway pressures ( P plat P_{\text{plat}} P plat ​ ).

Severe ventilation-perfusion mismatching and intrapulmonary shunting induce refractory hypoxemia and dead-space hypercapnic respiratory failure.

Renal Dysfunction (The Most Sensitive Organ System) :

Oliguria ( < 0.5 mL/kg/h <0.5\text{ mL/kg/h} < 0.5 mL/kg/h ) is characteristically the earliest clinical herald of evolving ACS.

Renal parenchymal and direct renal vein compression cause dramatic increases in renal venous pressure ( > 20 − 30 mmHg >20-30\text{ mmHg} > 20 − 30 mmHg ). Concurrently, renal arterial perfusion pressure ( R A P P = M A P − I A P RAPP = MAP - IAP R A PP = M A P − I A P ) collapses, dropping glomerular filtration rate (GFR).

Renal cortical hypoperfusion stimulates intense activation of the renin-angiotensin-aldosterone system (RAAS), driving avid tubular sodium and water retention and compounding the volume overload cycle.

Neurological Dysfunction :

Transmission of intra-abdominal and intrathoracic pressures impairs cerebral venous drainage through the internal jugular veins.

Increased cerebral venous outflow resistance causes cerebral venous congestion, elevating intracranial pressure (ICP) and reducing cerebral perfusion pressure ( C P P = M A P − I C P CPP = MAP - ICP CPP = M A P − I CP ).

Gastrointestinal and Splanchnic Dysfunction :

Direct compression of the celiac axis and mesenteric microcirculation produces gut mucosal ischemia, loss of intestinal tight junction integrity, bacterial translocation, and systemic endotoxemia, reinforcing secondary multi-organ dysfunction syndrome (MODS).

Tiered Medical Management and Surgical Decompression

The WSACS defines a structured, tiered medical management protocol to reverse intra-abdominal hypertension before irreversible organ damage occurs:

Management Tier Therapeutic Target Specific Interventions

Tier 1: Initial Medical Measures Evacuate intraluminal contents and relieve external constriction Insert nasogastric tube and rectal tube to low continuous suction; administer prokinetic agents (metoclopramide 10 mg 10\text{ mg} 10 mg IV, erythromycin 250 mg 250\text{ mg} 250 mg IV); temporarily hold enteral nutrition; remove constricting abdominal binders and dressings; optimize analgesia to minimize abdominal wall splinting.

Tier 2: Advanced Medical Measures Evacuate space-occupying lesions and optimize fluid balance Perform bedside ultrasound to identify ascites, hemoperitoneum, or peripancreatic fluid; perform percutaneous catheter drainage (PCD); avoid crystalloid over-resuscitation; initiate diuretic therapy (furosemide infusion) or early Continuous Renal Replacement Therapy (CRRT) with ultrafiltration to achieve neutral or negative fluid balance.

Tier 3: Neuromuscular Blockade Maximize abdominal wall compliance Administer a brief trial of intravenous sedation deepening combined with a neuromuscular blocking agent (NMBA) bolus (e.g., cisatracurium or rocuronium); chemical paralysis eliminates voluntary and involuntary abdominal wall muscle tone, lowering IAP by 4 − 8 mmHg 4-8\text{ mmHg} 4 − 8 mmHg .

Tier 4: Surgical Intervention Definitive mechanical decompression Emergent Decompressive Laparotomy with temporary abdominal closure (open abdomen with negative pressure wound therapy) for refractory ACS (sustained IAP > 20 mmHg >20\text{ mmHg} > 20 mmHg with progressive new organ failure unresponsive to medical tiers).

Temporary Open Abdomen and Negative Pressure Wound Therapy

Following surgical decompressive laparotomy, the abdominal fascia must never be closed primarily , as closure under tension causes immediate recurrent ACS. The abdomen is managed as a temporary open abdomen utilizing negative pressure wound therapy (NPWT) with continuous dynamic fascial traction.

A fenestrated visceral non-adherent protective barrier is tucked deeply into the paracolic gutters over the small bowel and omentum to prevent adhesions to the abdominal wall.

Polyurethane sponge and continuous subatmospheric suction ( − 75 -75 − 75 to − 125 mmHg -125\text{ mmHg} − 125 mmHg ) evacuate third-spaced peritoneal fluid, control abdominal domain loss, and maintain medial dynamic fascial traction, facilitating delayed primary fascial closure within 7 to 10 days.

Acute Mesenteric Ischemia (AMI): Subtype Etiologies and Pathophysiology

Acute mesenteric ischemia (AMI) is a catastrophic vascular emergency characterized by sudden, critical reduction in splanchnic blood supply to the bowel, resulting in mucosal ischemia that rapidly progresses to transmural infarction, bowel gangrene, perforation, and fatal septic shock.

• Four Distinct Subtypes of AMI [Acute Mesenteric Ischemia (AMI)]
• │
• ┌───────────┼───────────┬───────────┐
• ▼ ▼ ▼ ▼
• SMA Embolism SMA Thrombus NOMI MVT
• (~50%) (~25%) (~20%) (~5-10%)

Cardioembolic Atheroscler. Low-Flow Hypercoag.

• Atrial Fib Postprandial Sepsis/Shock Polycythemia
• Mid-SMA Ostial Origin Vasopressors SMV / Portal

Feature SMA Embolism (SMAE) SMA Thrombosis (SMAT) Non-Occlusive Mesenteric Ischemia (NOMI) Mesenteric Venous Thrombosis (MVT)

Incidence ∼ 50 % \sim 50\% ∼ 50% of cases ∼ 25 % \sim 25\% ∼ 25% of cases ∼ 20 % \sim 20\% ∼ 20% of cases ∼ 5 − 10 % \sim 5 - 10\% ∼ 5 − 10% of cases

Primary Etiology Dislodged cardiac mural thrombus (atrial fibrillation, post-MI, severe cardiomyopathy, endocarditis) Acute thrombosis over severe pre-existing atherosclerotic plaques Severe diffuse splanchnic arterial vasoconstriction in systemic low-flow shock states Thrombosis of superior mesenteric vein (SMV) driven by hypercoagulable states, cirrhosis, or local pancreatitis

Anatomical Site Impacts 3 − 8 cm 3-8\text{ cm} 3 − 8 cm distal to SMA origin (at taper point), sparing proximal jejunal branches Origin / ostium of the SMA directly off the abdominal aorta Diffuse, discontinuous vasoconstriction across entire SMA and IMA branches Superior mesenteric vein, splenic vein, and portal venous confluence

Clinical Presentation Classic "Pain out of proportion to exam" : sudden, excruciating periumbilical pain; soft abdomen initially Acute severe pain superimposed on chronic "intestinal angina" (postprandial pain, food fear, weight loss) Insidious, vague abdominal distension, discomfort, or unexplained lactic acidosis in critically ill shock patients Subacute, persistent, dull crampy periumbilical abdominal pain, ascites, and bloody diarrhea

CTA Imaging Findings Discrete, round, filling defect with sharp cutoff in mid-SMA; normal aortic origin Occlusion at the SMA ostium / origin; heavy calcification of aorta and celiac axis Widely patent proximal SMA trunk with diffuse, segmental narrowing ("string-of-sausages" spasm) Thrombus filling defect within SMV/portal vein; marked bowel wall thickening and mesenteric stranding

Primary Management Urgent surgical embolectomy with or without bowel resection; mandatory second-look laparotomy Urgent surgical bypass (aorto-mesenteric) or retrograde open mesenteric stenting (ROMS) with or without resection Hemodynamic resuscitation, wean alpha-vasopressors, continuous intra-arterial Papaverine infusion ( 30 − 60 mg/h 30-60\text{ mg/h} 30 − 60 mg/h ) Systemic therapeutic anticoagulation with unfractionated heparin; surgery reserved strictly for peritonitis

• Diagnostic Algorithm: Urgent Biphasic CTA and Laboratory Pitfalls
• Biphasic Multidetector CT Angiography (CTA)

Urgent biphasic CT angiography of the abdomen and pelvis without oral contrast is the diagnostic gold standard for suspected acute mesenteric ischemia, providing sensitivity and specificity > 95 % >95\% > 95% .

Timing and Protocol : Thin-slice multidetector CT with both an arterial phase (visualizing the celiac axis, SMA, and IMA from origin to periphery) and a portal venous phase (evaluating the portal vein, SMV, bowel wall hypoenhancement or hyperenhancement, and mesenteric edema).

Important

Strict Avoidance of Oral Contrast :

Oral contrast is strictly contraindicated in suspected acute mesenteric ischemia. Intraluminal positive oral contrast obscures mucosal enhancement patterns, conceals mural ischemia, masks mucosal arterial extravasation, causes motion artifact, delays time-sensitive vascular imaging by hours, and induces severe pulmonary aspiration in critically ill, nauseated patients.

Laboratory Markers and Clinical Pitfalls

Serum Lactate : Elevated arterial or venous lactate is a late, ominous finding indicating established, irreversible transmural bowel infarction and gangrene. A normal serum lactate level does not exclude early, reversible mesenteric ischemia! Waiting for lactate elevation before ordering imaging or intervening guarantees bowel necrosis and high mortality.

Leukocytosis and D-dimer : Leukocytosis ( > 15 , 000 − 20 , 000 / μ L >15,000-20,000/\mu\text{L} > 15 , 000 − 20 , 000/ μ L ) with a left shift is common. D-dimer is sensitive ( > 95 % >95\% > 95% ) but non-specific; a normal D-dimer can assist in ruling out acute mesenteric vascular occlusion in low-probability presentations.

Therapeutic Strategies: Revascularization, Bowel Viability, and Second-Look Laparotomy

Therapeutic management centers on immediate revascularization, excision of non-viable bowel, and preservation of borderline-ischemic intestine. [Confirmed Acute Mesenteric Ischemia on CTA]

• │
• Immediate Systemic Anticoagulation
• (Unfractionated Heparin IV Bolus + Infusion)
• │
• ┌───────────────┴───────────────┐
• ▼ ▼
• Occlusive Arterial (SMAE / SMAT) Non-Occlusive (NOMI)
• │ │
• ├─ Immediate Revascularization ├─ Optimize Cardiac Output / Hemodynamics
• │ (Embolectomy / Bypass / ROMS)├─ Wean Splanchnic Vasoconstrictors
• ├─ Resect Frankly Necrotic Bowel├─ Selective SMA Catheter Angiography
• ├─ Leave Questionable Bowel └─ Intra-Arterial Papaverine (30-60 mg/h)
• ├─ Temporary Open Abdomen
• └─ Mandatory 24-48h Second-Look
• 1. Initial Systemic Anticoagulation

Unless active gastrointestinal bleeding or an absolute contraindication exists, immediately administer an intravenous bolus of unfractionated heparin ( 80 units/kg 80\text{ units/kg} 80 units/kg ) followed by continuous infusion ( 18 units/kg/h 18\text{ units/kg/h} 18 units/kg/h ). Heparin prevents downstream microvascular propagation of thrombus and preserves collateral capillary beds.

2. Surgical Revascularization and Bowel Resection

In occlusive mesenteric arterial ischemia (SMAE, SMAT), emergent operating room transfer is mandatory:

Revascularization First : In the absence of gross peritonitis, arterial revascularization (surgical embolectomy via transverse arteriotomy, or prosthetic/saphenous vein bypass) should precede definitive bowel resection. Restoring pulsatile arterial flow often revives large segments of dusky, stunned intestine, preserving critical bowel length and preventing short gut syndrome.

Bowel Viability Assessment : Following revascularization, bowel viability is evaluated using clinical inspection (color, active peristalsis, mesenteric arterial pulsation), handheld Doppler ultrasound over the antimesenteric border, and intravenous fluorescein dye injection examined under a Wood's ultraviolet lamp (patchy or non-fluorescent areas correlate with non-viability).

Damage Control Principles : Resect only frankly gangrenous or perforated bowel. Do not perform primary anastomoses during the initial emergency operation; leave bowel in discontinuity using surgical staples, place a temporary abdominal closure NPWT device, and transfer to the ICU for aggressive physiological resuscitation.

3. Mandatory Second-Look Laparotomy

In any patient undergoing surgical intervention for mesenteric ischemia where questionable or borderline bowel was preserved, a mandatory planned "second-look" laparotomy must be scheduled within 24 24 24 to 48 hours 48\text{ hours} 48 hours . Clinical abdominal examination and laboratory markers are entirely unreliable in evaluating the ongoing viability of compromised bowel in the sedated, critically ill patient. The second look allows reassessment after 24 to 48 hours of restored perfusion and ICU resuscitation, enabling definitive resection of demarcated necrotic bowel or safe primary bowel reconstruction.