2.4 Complications of Acute Myocardial Infarction

Mechanical Complications of Acute Myocardial Infarction

Mechanical complications represent catastrophic structural ruptures occurring primarily between days 2 and 7 post-MI (the period of peak macrophage infiltration and tissue softening during necrosis clearance). Although less common in the primary PCI era, they carry acute mortality rates exceeding 50%.

1. Acute Mitral Regurgitation Secondary to Papillary Muscle Rupture

• Pathophysiology: Rupture of a necrotic papillary muscle or its chordae tendineae causes flail mitral valve leaflets and massive systolic regurgitation into the left atrium.
• Vascular Vulnerability: Most commonly complicates inferior or posterolateral MIs. The posteromedial papillary muscle has a single blood supply from the posterolateral branch of the RCA or LCx, making it 6 to 10 times more prone to rupture than the anterolateral papillary muscle (which receives dual blood supply from both LAD and LCx).
• Clinical Presentation: Sudden clinical deterioration with flash pulmonary edema, severe dyspnea, cardiogenic shock, and a new harsh, high-pitched holosystolic murmur heard best at the apex radiating to the axilla.
• Pulmonary Artery Catheter (PAC) Finding: Demonstrates large, prominent v-waves on the pulmonary artery wedge pressure (PAWP) tracing, reflecting massive blood regurgitation into the left atrium during ventricular systole.
• Management: Urgent echocardiography, intra-aortic balloon pump (IABP) or Impella placement for left ventricular unloading, IV vasodilators (nitroprusside) if SBP permits, and immediate emergency surgical valve repair/replacement.

2. Ventricular Septal Defect (VSD / Septal Rupture)

• Pathophysiology: Coagulative necrosis of the interventricular septum creates a transmural tear connecting the high-pressure left ventricle to the low-pressure right ventricle.
• Associated MI Territory: Occurs in both anterior MIs (apical septal rupture via LAD) and inferior MIs (basal septal rupture via RCA).
• Clinical Presentation: Sudden biventricular failure, severe hypotension, cardiogenic shock, and a new, loud, harsh, holosystolic murmur heard best at the left lower sternal border (LLSB), frequently accompanied by a palpable thrill.
• PAC Finding: Demonstrates an oxygen saturation step-up (≥ 5% at the ventricular level, the classic Antman criterion; some sources use a more conservative ≥ 10%) between blood drawn from the Right Atrium (RA) and blood drawn from the Right Ventricle (RV) or Pulmonary Artery (PA), caused by oxygenated left ventricular blood shunting rightward across the defect.
• Management: Emergency surgical or percutaneous device closure; hemodynamic stabilization with IABP and inotropes.

3. Left Ventricular Free Wall Rupture (FWR)

• Pathophysiology: Full-thickness tear of the necrotic ventricular free wall into the pericardial sac, resulting in rapid hemopericardium.
• Clinical Presentation: Typically presents as sudden electromechanical dissociation (PEA), acute cardiac tamponade (Beck's Triad: hypotension, muffled heart sounds, JVD), pulsus paradoxus, and rapid cardiovascular collapse.
• Management: Immediate emergency pericardiocentesis to relieve tamponade, accompanied by emergency open cardiac surgery.

Electrical Complications: Arrhythmias & Conduction Blocks

Ischemia disrupts cardiac cell membrane resting potentials, slows conduction velocity, and promotes re-entry pathways.

Ventricular Arrhythmias

• Early Ventricular Fibrillation (VF) / Ventricular Tachycardia (VT): Occurs within the first 24 to 48 hours post-MI due to ischemic cellular instability ("primary VF"). Managed with immediate defibrillation/cardioversion and IV Amiodarone or Lidocaine. Primary VF does not adversely alter long-term prognosis if successfully resuscitated.
• Late VF / VT (>48 Hours Post-MI): Indicates established myocardial scar tissue and carries a grave long-term prognosis, requiring electrophysiology consultation and evaluation for an Implantable Cardioverter-Defibrillator (ICD).

Bradyarrhythmias and Conduction Blocks by MI Location

Conduction disturbances differ radically based on coronary anatomy:

1. Inferior MI (RCA Occlusion):
   • Mechanism: The RCA supplies the AV node via the AV nodal artery in 90% of individuals. Ischemia induces hypervagotonia and AV nodal ischemia.
   • Conduction Block Type: Sinus Bradycardia and Mobitz Type I (Wenckebach) 2nd-Degree AV Block.
   • Characteristics: Nodal (narrow QRS), hemodynamically stable or transient, and highly responsive to Atropine (1 mg IV, repeated every 3–5 minutes to a maximum of 3 mg). Rarely requires permanent pacing.
2. Anterior MI (LAD Occlusion):
   • Mechanism: The septal perforators of the LAD supply the bundle of His and bundle branches below the AV node.
   • Conduction Block Type: Mobitz Type II 2nd-Degree AV Block or Complete (3rd-Degree) Heart Block.
   • Characteristics: Infranodal block featuring a wide QRS escape rhythm, marked hemodynamic instability, and high risk of sudden ventricular asystole. Destruction of the conduction system is extensive.
   • Nursing Management: Atropine is ineffective and contraindicated for infranodal blocks because it accelerates the sinus node without improving infranodal conduction, worsening block. Immediate transcutaneous pacing (TCP) must be initiated, followed by transvenous pacemaker insertion.

Hemodynamic Complications: RV Infarction & Cardiogenic Shock

Right Ventricular Infarction Management

Right ventricular infarction accompanies inferior STEMI in 30-50% of cases. Because the right ventricle is thin-walled, acute ischemic necrosis impairs RV contractile force, reducing forward stroke volume into the pulmonary circulation.

• Clinical Triad: Hypotension, clear lung fields on auscultation, and marked jugular venous distention (JVD).
• PAC Hemodynamics: Elevated Right Atrial Pressure (RAP/CVP ≥ 10-15 mmHg) with normal or low Pulmonary Artery Wedge Pressure (PAWP <12 mmHg) and low Cardiac Index (CI <2.0 L/min/m²).
• Treatment Principles:
  • Volume Loading: Administer IV isotonic saline boluses to raise RV preload, driving blood passively across the pulmonary bed into the left atrium.
  • Strict Contraindications: Preload-reducing drugs are strictly prohibited! Avoid Nitrates, Morphine, Diuretics, and ACE Inhibitors. Reducing preload in RV MI destroys forward cardiac output, precipitating profound cardiac arrest.
  • Inotropic Support: Administer Dobutamine if fluid administration fails to restore mean arterial pressure.

Cardiogenic Shock (CS)

Cardiogenic shock occurs when >40% of left ventricular mass undergoes acute necrosis, leading to severe end-organ hypoperfusion.

• Hemodynamic Parameters: Systolic BP <90 mmHg, Cardiac Index <2.2 L/min/m², PAWP >15 mmHg, and Systemic Vascular Resistance (SVR >1500 dyn·s·cm⁻⁵).
• Inotropic & Vasopressor Management: Norepinephrine is the first-line vasopressor for severe hypotension. Dobutamine or Milrinone provides inotropic support to augment contractility.
• Mechanical Circulatory Support (MCS): Escalation to Intra-Aortic Balloon Pump (IABP), Impella (percutaneous microaxial flow pump), or VA-ECMO to reduce LV wall stress and restore systemic perfusion.

Pericardial Complications: Early Pericarditis vs. Dressler Syndrome