21.3 Abdominal Compartment Syndrome & Acute Mesenteric Ischemia

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 $0$ to $5\text{ mmHg}$. In critically ill, mechanically ventilated adult ICU patients, baseline IAP typically measures between $5$ and $7\text{ mmHg}$.
• Intra-Abdominal Hypertension (IAH): Defined as a sustained or repeated pathological elevation of IAP $\ge 12\text{ mmHg}$.
  • Grade I IAH: IAP $12 - 15\text{ mmHg}$
  • Grade II IAH: IAP $16 - 20\text{ mmHg}$
  • Grade III IAH: IAP $21 - 25\text{ mmHg}$
  • Grade IV IAH: IAP $> 25\text{ mmHg}$
• Abdominal Compartment Syndrome (ACS): Defined as a sustained IAP $> 20\text{ mmHg}$ (with or without an Abdominal Perfusion Pressure $<60\text{ mmHg}$) 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: $\text{APP} = \text{MAP} - \text{IAP}$ A target $\text{APP} \ge 60\text{ mmHg}$ 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

---

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

1. 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.
2. 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 ($P_{\text{peak}}$) and plateau airway pressures ($P_{\text{plat}}$).
   • Severe ventilation-perfusion mismatching and intrapulmonary shunting induce refractory hypoxemia and dead-space hypercapnic respiratory failure.
3. Renal Dysfunction (The Most Sensitive Organ System):
   • Oliguria ($<0.5\text{ mL/kg/h}$) is characteristically the earliest clinical herald of evolving ACS.
   • Renal parenchymal and direct renal vein compression cause dramatic increases in renal venous pressure ($>20-30\text{ mmHg}$). Concurrently, renal arterial perfusion pressure ($RAPP = MAP - IAP$) 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.
4. 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 ($CPP = MAP - ICP$).
5. 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:

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 ($-75$ to $-125\text{ mmHg}$) 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

---

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 $>95\%$.

• 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).

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 ($>15,000-20,000/\mu\text{L}$) with a left shift is common. D-dimer is sensitive ($>95\%$) 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 ($80\text{ units/kg}$) followed by continuous infusion ($18\text{ units/kg/h}$). 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 $24$ to $48\text{ hours}$. 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.