5.1 Acute Coronary Syndromes & Infarct Specific Complications

5.1 Acute Coronary Syndromes & Infarct Specific Complications

Acute Coronary Syndrome (ACS) represents a continuum of myocardial ischemia and necrosis driven by acute disruption of epicardial coronary artery blood flow. The underlying pathophysiology typically involves atherosclerosis, plaque rupture or erosion, platelet activation, and subsequent thrombus formation. ACS is clinically categorized into three distinct entities: Unstable Angina (UA), Non-ST-Segment Elevation Myocardial Infarction (NSTEMI), and ST-Segment Elevation Myocardial Infarction (STEMI). In critical care transport, rapid identification of coronary anatomy, infarct localization, hemodynamic destabilization, and acute post-MI mechanical complications is essential for guiding resuscitation, pharmacological management, and transport destination decisions.

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Pathophysiology & Coronary Vascular Anatomy

Myocardial perfusion is uniquely dependent on coronary blood flow during cardiac diastole, when ventricular contraction relaxes and epicardial vessel compression ceases. The two primary epicardial coronary arteries originate from the left and right sinuses of Valsalva above the aortic valve:

1. Left Main Coronary Artery (LMCA): Bifurcates into the Left Anterior Descending (LAD) and Left Circumflex (LCx) arteries.
   • Left Anterior Descending (LAD): Courses down the anterior interventricular sulcus. It supplies the anterior wall of the left ventricle, the anterior two-thirds of the interventricular septum, and the apex. Occlusion of the LAD causes anterior/septal infarctions, which carry high risks of cardiogenic shock, heart failure, and conductive disturbances (e.g., bundle branch blocks) due to extensive myocardial mass loss.
   • Left Circumflex (LCx): Courses around the left AV groove. It supplies the lateral wall of the left ventricle and, in left-dominant circulation (10-15% of individuals), the posterior wall and posterior descending artery.
2. Right Coronary Artery (RCA): Courses along the right AV groove, supplying the right atrium, right ventricle, and inferior wall of the left ventricle. In right-dominant vascular anatomy (85-90% of individuals), the RCA gives rise to the Posterior Descending Artery (PDA), which supplies the posterior wall and inferior one-third of the interventricular septum, as well as the AV nodal artery supplying the AV node.

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12-Lead ECG Localization & Reciprocal Changes

Standard 12-lead electrocardiography provides non-invasive spatial localization of acute myocardial injury by detecting ST-segment shifts relative to the baseline TP segment. ST-segment elevation $\ge 1\text{ mm}$ ($0.1\text{ mV}$) in two or more anatomically contiguous leads (or $\ge 1.5-2.0\text{ mm}$ in leads V2-V3 depending on age and sex) defines a STEMI. Reciprocal ST-segment depression in anatomically opposing leads strongly confirms acute transmural infarction.

Posterior Infarction Mechanics

Standard 12-lead ECGs do not directly view the posterior myocardium. Posterior wall infarction manifests in anterior leads (V1-V3) as mirror-image reciprocal changes: horizontal ST-segment depression, tall R waves ($R/S > 1.0$), and broad upright T waves. Placing posterior leads V7 (left posterior axillary line), V8 (left mid-scapular line), and V9 (left paraspinal line) reveals ST elevation $\ge 0.5\text{ mm}$, confirming acute transmural posterior STEMI requiring immediate reperfusion.

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Right Ventricular Infarction (RVI)

Right Ventricular Infarction occurs in up to 30-50% of inferior wall STEMIs due to proximal occlusion of the Right Coronary Artery (RCA) prior to the origin of the right ventricular marginal branches. Isolated RVI is rare.

Proximal RCA Occlusion
        │
        ├─► Inferior LV MI (Leads II, III, aVF)
        │
        └─► RV Marginal Branch Ischemia
                 │
                 ▼
   RV Dyskinesis & Dilation
                 │
                 ├─► Decreased RV Stroke Volume
                 │        │
                 │        ▼
                 │   Profound LV Underfilling (Preload Collapse)
                 │        │
                 │        ▼
                 │   Hypotension & Cardiogenic Shock
                 │
                 └─► Elevated Right Atrial Pressures (JVD + Clear Lungs)

Diagnostic & Hemodynamic Essentials of RVI

• Right-Sided ECG: A right-sided 12-lead ECG (leads V1R-V6R) must be performed on all patients presenting with inferior STEMI. ST-segment elevation $\ge 1\text{ mm}$ in lead V4R (placed at the 5th intercostal space, right mid-clavicular line) is the single most sensitive and specific marker of acute right ventricular injury.
• Triad of RVI: Hypotension, elevated Jugular Venous Pressure (JVD), and completely clear lung fields (absence of pulmonary rales/edema).
• Preload Dependence: The infarcted right ventricle loses active contractile power and functions as a passive conduit. Maintenance of Right Ventricular (and subsequently Left Ventricular) output relies entirely on elevated central venous pressure (filling pressure).
• Critical Contraindications: Administration of medications that reduce preload—such as Nitroglycerin, Morphine, Loop Diuretics (Furosemide), or ACE inhibitors—causes immediate, catastrophic drops in cardiac output and severe hypotension.
• Resuscitation Strategy: Treatment of RVI-induced hypotension requires aggressive volume expansion with continuous IV crystalloid boluses (e.g., 500-1000 mL normal saline or Lactated Ringer's) to raise CVP toward 14-18 mmHg. If hypotension persists despite volume loading, inotropic and vasopressor support with Norepinephrine (to maintain SBP/coronary perfusion) and Dobutamine (to enhance RV contractility) must be initiated.

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Cardiogenic Shock in ACS

Cardiogenic shock represents severe end-organ tissue hypoperfusion resulting from primary cardiac pump failure, most commonly caused by loss of $> 40%$ of functional left ventricular myocardium in acute transmural MI.

Clinical & Hemodynamic Diagnostic Criteria

1. Persistent Hypotension: Systolic Blood Pressure (SBP) $< 90\text{ mmHg}$ or Mean Arterial Pressure (MAP) $< 65\text{ mmHg}$ for $> 30\text{ minutes}$ (or requiring vasopressor support).
2. Reduced Cardiac Index: $\text{CI} < 2.2\text{ L/min/m}^2$ (or $< 2.0\text{ L/min/m}^2$ with mechanical support).
3. Elevated Ventricular Filling Pressure: Pulmonary Artery Wedge Pressure (PAWP) $> 15\text{ mmHg}$ or Right Atrial Pressure (CVP) $> 10-12\text{ mmHg}$.
4. End-Organ Hypoperfusion: Cool clammy extremities, oliguria ($< 0.5\text{ mL/kg/hr}$), altered sensorium, and arterial serum lactate $> 2.0\text{ mmol/L}$.

In cardiogenic shock, compensatory sympathetic reflex arc activation increases Systemic Vascular Resistance (SVR $> 1200\text{ dynes}\cdot\text{sec/cm}^{-5}$) to preserve MAP. However, this marked afterload surge increases left ventricular wall stress and myocardial oxygen demand, creating a lethal spiraling decline in cardiac output.

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Mechanical Post-MI Complications

Mechanical complications of acute myocardial infarction are life-threatening catastrophes that typically develop within 2 to 7 days post-MI during the tissue necrotic and macrophage-clearing phase. Critical care transport clinicians must immediately recognize their physical findings and hemodynamic signatures.

1. Papillary Muscle Rupture

• Pathophysiology: Infarction of the papillary muscle causes acute loss of mitral valve leaflet tethering, resulting in catastrophic, massive acute mitral regurgitation.
• Anatomical Vulnerability: The posteromedial papillary muscle is affected 6 to 10 times more frequently than the anterolateral papillary muscle. The posteromedial muscle has a single blood supply derived exclusively from the Posterior Descending Artery (RCA/LCx). In contrast, the anterolateral papillary muscle enjoys dual blood supply from both the LAD and LCx.
• Clinical Presentation: Sudden onset of acute respiratory distress, flash pulmonary edema, severe cardiogenic shock, and a new harsh holosystolic murmur at the cardiac apex radiating to the axilla. Note: In hyperacute severe mitral regurgitation, atrial-ventricular pressure equalization can render the murmur faint or silent.
• Management: Immediate afterload reduction with continuous Sodium Nitroprusside or Intra-Aortic Balloon Pump (IABP) counterpulsation to facilitate forward flow, alongside emergency surgical mitral valve repair/replacement.

2. Ventricular Septal Defect (VSD)

• Pathophysiology: Transmural necrosis of the interventricular septum leads to wall rupture and the formation of an internal shunt.
• Anatomical Distribution: Occurs in anterior MIs (apical septal rupture via LAD) or inferior MIs (basal septal rupture via RCA).
• Clinical Presentation: Rapid biventricular failure, cardiogenic shock, and a new, harsh holosystolic murmur heard best at the left lower sternal border, frequently accompanied by a palpable parasternal thrill.
• Hemodynamic Confirmation: Pulmonary Artery Catheter (PAC) sampling demonstrates an oxygen saturation step-up ($> 8-10%$) between the Right Atrium and Right Ventricle due to left-to-right shunting of oxygenated blood.
• Management: Vasodilator/IABP afterload reduction and urgent surgical or transcatheter septal closure.

3. Ventricular Free Wall Rupture & Cardiac Tamponade

• Pathophysiology: Complete transmural rupture of the left ventricular free wall into the pericardial sac, resulting in massive hemopericardium and acute cardiac tamponade.
• Clinical Presentation: Sudden electromechanical dissociation (PEA arrest), profound hypotension, and classic Beck's Triad:
  1. Hypotension with narrow pulse pressure
  2. Distended neck veins (elevated CVP)
  3. Muffled/distant heart sounds
• Diagnostic Signs: Pulsus paradoxus ($> 10\text{ mmHg}$ drop in SBP during inspiratory phase) and electrical alternans on ECG.
• Management: Immediate pericardiocentesis for temporary hemodynamic stabilization followed by emergency sternotomy and surgical repair.

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Pharmacological Management of ACS

Acute medical therapy focuses on halting thrombus propagation, reducing myocardial oxygen demand, restoring coronary arterial flow, and stabilizing plaque.

Antiplatelet & Anticoagulant Regimens

• Aspirin: Cyclooxygenase-1 (COX-1) inhibitor preventing thromboxane A2 production. Dose: 162 - 325 mg non-enteric coated tablet, chewed immediately.
• P2Y12 Inhibitors: ADP receptor antagonists administered as dual antiplatelet therapy (DAPT).
  • Ticagrelor (Brilinta): 180 mg loading dose, then 90 mg BID (reversible binding).
  • Prasugrel (Effient): 60 mg loading dose, then 10 mg daily (contraindicated in prior stroke/TIA).
  • Clopidogrel (Plavix): 300 - 600 mg loading dose, then 75 mg daily.
• Anticoagulation:
  • Unfractionated Heparin (UFH): IV bolus 60 units/kg (maximum 4000 units), followed by infusion at 12 units/kg/hr (maximum 1000 units/hr), titrated to maintain anti-Xa or aPTT 1.5 - 2.0 times control (typically 50-70 seconds).
  • Enoxaparin (Lovenox): 1 mg/kg SC q12h (or 0.75 mg/kg for severe renal impairment).

Antianginal & Hemodynamic Titration

• Nitroglycerin: Direct venous and arterial vasodilator. Administered sublingually (0.4 mg q5min x 3) or IV infusion (5 - 200 mcg/min). Reduces LV preload and PAWP. Absolute contraindications: Right Ventricular Infarction, SBP $< 90\text{ mmHg}$, bradycardia $< 50\text{ bpm}$, or recent phosphodiesterase-5 inhibitor use (sildenafil within 24h, tadalafil within 48h).
• Beta-Blockers: Oral Metoprolol (25-50 mg q6h) initiated within 24 hours reduces ventricular dysrhythmias and infarct size by decreasing HR and contractility. Absolute contraindications: Acute heart failure, cardiogenic shock, severe bradycardia, or second/third-degree AV block.