Diseksi Aorta

01Pathophysiology

Aortic dissection begins with a tear in the tunica intima of the aortic wall. Once the intima is breached, the force of arterial blood under high pressure enters the tunica media and splits it longitudinally, creating two channels: the true lumen (the original aortic channel) and the false lumen (the newly formed space within the media). The dissection can propagate either anterograde (distally) or retrograde (proximally) along the length of the aorta.

The most important predisposing factor is chronic hypertension, which causes long-standing shear stress on the aortic wall, leading to cystic medial degeneration (previously called cystic medial necrosis). This is the histopathological hallmark: degeneration and fragmentation of elastic fibers and smooth muscle cells in the media, with accumulation of mucoid (basophilic) ground substance. In connective tissue disorders such as Marfan syndrome (fibrillin-1 mutation) and Ehlers-Danlos type IV (type III collagen defect), the aortic wall is inherently weak, predisposing to dissection even at younger ages and lower blood pressures.

The reason the pain is described as "tearing" and "migratory" is directly tied to the pathophysiology: as the dissection flap extends along the aorta, it stretches adventitial nerve fibers sequentially. A dissection beginning in the ascending aorta causes anterior chest pain; as it propagates into the descending aorta, the pain migrates to the interscapular region and even the abdomen.

Clinical consequences depend on which branch vessels are compromised by the dissection flap:

• Coronary arteries (most commonly the right coronary artery): acute MI, typically inferior STEMI

• Brachiocephalic/subclavian arteries: blood pressure discrepancy between arms, upper limb ischemia

• Carotid arteries: stroke or altered mental status

• Spinal arteries: paraplegia (anterior spinal artery syndrome)

• Renal arteries: acute renal failure, refractory hypertension

• Mesenteric arteries: bowel ischemia

• Iliac arteries: lower extremity ischemia

If the dissection extends retrograde into the aortic root, it can disrupt the aortic valve apparatus, causing acute aortic regurgitation (the new early diastolic murmur in the vignette). It can also rupture into the pericardial sac, resulting in cardiac tamponade (Beck's triad: hypotension, distended neck veins, muffled heart sounds), which is the leading cause of death in proximal dissections.

02Classification and Clinical Manifestation

Stanford Classification (Most Tested)

DeBakey Classification (Less Commonly Tested)

Clinical Manifestation by Malperfusion Syndrome

Temporal Classification

The distinction between acute and chronic matters because mortality is highest in the first 48 hours (approximately 1-2% per hour in untreated Type A), making rapid diagnosis and treatment the central exam concept.

03Diagnostic Workup

The first imaging study a patient often receives is a portable chest X-ray, which can be done at the bedside. The hallmark finding is a widened mediastinum (>8 cm at the level of the aortic knob on a PA film). However, up to 10-20% of dissections have a normal CXR, so a normal film does not rule out the diagnosis. If clinical suspicion is high, you must proceed to advanced imaging regardless of the CXR result.

CT angiography (CTA) is the best initial test in a hemodynamically stable patient. It is fast (seconds to acquire), widely available, and has sensitivity and specificity both exceeding 95%. The diagnostic finding is the intimal flap with contrast opacification of both the true and false lumen. CTA also maps the full extent of the dissection and identifies involvement of branch vessels, which guides surgical planning.

TEE is the preferred test when the patient is too unstable to leave the emergency department or ICU for the CT scanner. It can be performed at the bedside or in the operating room and provides real-time assessment of the ascending aorta, aortic valve function, and pericardial space. Its limitation is a "blind spot" in the distal ascending aorta and proximal arch due to tracheal/bronchial air interference.

MRI/MRA has the highest combined sensitivity and specificity (approaching 98-100%) and is therefore the most accurate test overall. However, it is impractical in the acute setting because it requires prolonged scan times, limits monitoring access, and is often unavailable on an emergent basis. It is most useful for follow-up of chronic or stable dissections.

D-dimer is worth understanding for exam purposes. It is elevated in aortic dissection because of the activation of the coagulation cascade within the false lumen. Its primary utility is its negative predictive value: in patients with a low pretest probability (Aortic Dissection Detection risk score of 0), a normal D-dimer effectively excludes dissection. However, a positive D-dimer is nondiagnostic and cannot confirm dissection on its own.

An ECG must always be obtained because the differential includes acute MI. However, be careful: if the dissection involves the right coronary ostium, the ECG may show inferior STEMI. The critical teaching point is that thrombolytics and anticoagulation are absolutely contraindicated in dissection. This is why you must consider dissection before treating an apparent MI, especially if the chest pain is described as tearing, there is a BP discrepancy, or the patient has a widened mediastinum.

04Management & Treatment

Step 1: Acute Stabilization ("HR Before BP")

The single most important initial intervention is heart rate control with IV beta-blockers. This is the "next best step" in virtually every exam vignette presenting an acute dissection. The rationale is that reducing the heart rate decreases the rate of rise of aortic pressure ($dP\mathrm{/}dtdP/dt$), which is the primary force propagating the dissection. Simply lowering blood pressure without first controlling heart rate (for example, using a vasodilator alone) causes reflex tachycardia, which paradoxically increases $dP\mathrm{/}dtdP/dt$ and worsens the dissection.

• Esmolol is the preferred agent because of its ultra-short half-life (~9 minutes), allowing rapid titration.

• Labetalol is an acceptable alternative (combined alpha-beta blockade).

• Target: HR <60 bpm, then SBP 100-120 mmHg.

Step 2: Add Vasodilator If Needed

If blood pressure remains elevated despite adequate beta-blockade, sodium nitroprusside can be added. The critical rule: never give nitroprusside before establishing beta-blockade, because nitroprusside alone causes reflex tachycardia. Nitroprusside toxicity (cyanide accumulation) is a concern with prolonged infusions (>48-72 hours).

An alternative if nitroprusside is contraindicated (e.g., renal insufficiency, prolonged use) is IV nicardipine (5-15 mg/hr), which does not cause reflex tachycardia to the same degree.

Step 3: Pain Control

IV morphine is used for pain management. Pain relief is not merely comfort care; uncontrolled pain triggers sympathetic activation, raising heart rate and blood pressure, which worsens the dissection. Adequate analgesia is part of hemodynamic management.

Step 4: Definitive Treatment Based on Classification

• Type A: Proceed directly to emergent open surgical repair. There is no role for "watchful waiting" in Type A; without surgery, mortality approaches 50% at 48 hours. Surgery involves replacing the affected ascending aorta with a synthetic graft (Dacron). If the aortic root is involved, a composite valve-graft (Bentall procedure) may be required.

• Uncomplicated Type B: Continue medical therapy. These patients are monitored with serial imaging. The false lumen may thrombose and remodel over time. Elective TEVAR may be considered in subacute/chronic settings to promote favorable remodeling.

• Complicated Type B (defined by malperfusion of any end-organ, rupture or impending rupture, refractory hypertension, persistent/recurrent pain, or rapid aortic expansion): TEVAR is the preferred intervention. TEVAR involves endovascular placement of a covered stent-graft over the primary intimal tear to redirect flow into the true lumen.

Step 5: Long-Term Management

All survivors require lifelong oral beta-blocker therapy, aggressive blood pressure control (goal SBP <130 mmHg), and surveillance imaging with CTA or MRA. Patients with Marfan syndrome may benefit from losartan (ARB) in addition to beta-blockers, as evidence suggests it slows aortic root dilation through TGF-beta pathway modulation. Patients should avoid isometric exercise, heavy lifting, and stimulant drugs.

Contraindications and Special Populations:

• Pregnancy: Dissection occurs most often in the third trimester (hormonally mediated changes in the aortic wall). Management principles are the same; labetalol is preferred over esmolol. Nitroprusside should be avoided due to fetal cyanide toxicity. Emergent delivery (usually cesarean) may be needed before or concurrent with aortic repair.

• Cocaine-associated dissection: Avoid pure beta-blockers initially (risk of unopposed alpha stimulation); use labetalol (alpha-beta blocker) or a combination of phentolamine (alpha-blocker) plus a beta-blocker. Benzodiazepines are also used.

05Differential Diagnosis & Distractors

06Traps and High-Yield Pearls

The single most dangerous trap on exam questions about aortic dissection is giving thrombolytics or heparin to a patient who appears to be having an acute MI but is actually having a dissection. The vignette will describe a patient, often younger than the typical MI demographic, with "chest pain" and inferior ST elevation on ECG. Embedded in the stem will be subtle clues: the pain is tearing, started suddenly at full intensity, radiates to the back, or there is a blood pressure difference between arms. If you miss these clues and select "administer tPA" or "give heparin and proceed to catheterization," the result is catastrophic hemorrhage into the false lumen or pericardium. Always scan for dissection red flags before choosing thrombolysis.

A second common trap involves the sequencing of antihypertensive therapy. Students frequently select nitroprusside or another vasodilator as the first-line agent because they focus on the dangerously high blood pressure. The correct answer is always beta-blocker first (esmolol or labetalol). The reasoning is physiological: you must reduce $dP\mathrm{/}dtdP/dt$ by lowering heart rate before you lower afterload with a vasodilator. Giving a vasodilator first causes reflex tachycardia, increases shear stress, and propagates the dissection.

A third trap is confusing the indication for surgery versus medical management. Remember: Type A = Always surgery. Type B = Blood pressure management (unless complicated). If the vignette describes an ascending aortic dissection and asks for the "next best step in management," the answer is emergent surgical referral, even if the patient looks stable. Conversely, do not select surgery for an uncomplicated Type B dissection; medical therapy is the standard of care.

Finally, watch for vignettes featuring young patients with connective tissue disorders (tall, thin habitus, arm span exceeding height, pectus excavatum, lens subluxation = Marfan syndrome) or pregnant women in the third trimester presenting with acute-onset back pain. These are designed to test whether you can recognize dissection in atypical demographics where hypertension may not be the primary risk factor.

The core competency being tested is pattern recognition under pressure: identifying the tearing, migratory pain with pulse deficits that points to dissection, avoiding the MI mimic trap, sequencing beta-blockers before vasodilators, and routing Type A to surgery while managing uncomplicated Type B medically.