14.1 Acute Coronary Syndromes & Myocardial Infarction

Spectrum of Acute Coronary Syndromes (ACS)

Acute coronary syndromes encompass a clinical and pathophysiological continuum triggered by an abrupt, critical imbalance between myocardial oxygen demand and coronary blood supply. In the overwhelming majority of cases, ACS is initiated by the rupture or erosion of a vulnerable atherosclerotic plaque within an epicardial coronary artery.

Pathophysiologic Cascade

1. Plaque Disruption: Fibrous cap rupture (most common, driven by macrophage matrix metalloproteinases) or superficial endothelial plaque erosion exposes underlying thrombogenic collagen and subendothelial tissue factor to circulating blood.
2. Platelet Adhesion & Activation: Platelets adhere to exposed von Willebrand factor via glycoprotein Ib (GPIb) surface receptors. Platelet activation ensues, causing the degranulation and release of adenosine diphosphate (ADP), thromboxane A2 (TxA2), and serotonin, alongside surface expression of active glycoprotein IIb/IIIa (GPIIb/IIIa) receptors.
3. Thrombus Propagation: Conformational activation of GPIIb/IIIa receptors binds circulating fibrinogen, cross-linking adjacent platelets to form a primary platelet plug. Concurrently, the coagulation cascade is triggered by tissue factor, generating thrombin, which converts soluble fibrinogen into an insoluble fibrin meshwork, stabilizing the intra-luminal thrombus.

                                  ACUTE CORONARY SYNDROMES SPECTRUM
                                  
  ┌────────────────────────────────────────────────────────────────────────────────────────┐
  │                               VULNERABLE PLAQUE DISRUPTION                             │
  │                      (Rupture or Erosion + Platelet Activation)                         │
  └───────────────────────────────────────────┬────────────────────────────────────────────┘
                                              │
             ┌────────────────────────────────┴────────────────────────────────┐
             ▼                                                                 ▼
┌─────────────────────────────────────────┐       ┌─────────────────────────────────────────┐
│ TOTAL OCCLUSIVE THROMBUS                │       │ SUB-TOTAL / INTERMITTENT OCCLUSION      │
│ • Fibrin- and red-cell-rich "red" clot  │       │ • Platelet-rich "white" clot            │
│ • Transmural myocardial ischemia        │       │ • Subendocardial myocardial ischemia    │
│ • Persistent ST elevation on 12-lead    │       │ • ST depression, T inversion, or normal │
│ • Troponin positive (extensive injury)  │       │ • Troponin positive vs. negative        │
└────────────────────┬────────────────────┘       └────────────────────┬────────────────────┘
                     │                                                 │
                     ▼                                                 ▼
┌─────────────────────────────────────────┐       ┌─────────────────────────────────────────┐
│ STEMI (ST-Elevation MI)                 │       │ NSTE-ACS (Non-ST-Elevation ACS)         │
│ • Immediate emergency reperfusion       │       │ • Troponin Elevated -> NSTEMI           │
│ • Target door-to-balloon <= 90 min       │       │ • Troponin Negative -> Unstable Angina  │
│ • High early transmural necrosis risk   │       │ • Medical stabilization + risk score    │
└─────────────────────────────────────────┘       └─────────────────────────────────────────┘

Clinical Classification

• ST-Segment Elevation Myocardial Infarction (STEMI): Complete, persistent epicardial coronary artery occlusion by an occlusive thrombus resulting in transmural myocardial ischemia and necrosis. Characterized by acute ST-segment elevation or equivalent repolarization abnormalities on a 12-lead electrocardiogram (ECG) and subsequent elevation of cardiac biomarkers.
• Non-ST-Segment Elevation Myocardial Infarction (NSTEMI): Subtotal, intermittent, or micro-embolizing coronary occlusion resulting in subendocardial myocardial necrosis. Diagnosed by objective biomarker evidence of myocardial cell injury (elevated cardiac troponin) in the absence of acute persistent ST-segment elevation.
• Unstable Angina (UA): Myocardial ischemia at rest, new-onset severe angina (CCS Class III or IV), or accelerating/crescendo angina in the absence of detectable myocardial necrosis (persistently normal serial high-sensitivity cardiac troponins). Pathophysiologically identical to NSTEMI, but representing an earlier or less severe ischemic insult without irreversible myocyte death.

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12-Lead Electrocardiographic Diagnostic Criteria

Prompt acquisition and physician interpretation of a standard 12-lead ECG is the cornerstone of ACS evaluation and must occur within 10 minutes of first medical contact (FMC).

Standard STEMI Diagnostic Cutoffs

According to the Fourth Universal Definition of Myocardial Infarction, STEMI requires new or presumed-new ST-segment elevation measured at the J-point in at least two anatomically contiguous leads:

• Standard Contiguous Leads (I, aVL, II, III, aVF, V4, V5, V6): ST elevation >= 1.0 mm (0.1 mV at standard calibration of 10 mm = 1 mV).
• Precordial Leads V2 and V3: Because healthy individuals (particularly young men) demonstrate physiological J-point elevation, age- and sex-specific criteria are mandatory:
  • Women (any age): >= 1.5 mm (0.15 mV).
  • Men >= 40 years: >= 2.0 mm (0.2 mV).
  • Men < 40 years: >= 2.5 mm (0.25 mV).

Anatomical Lead Groupings & Culprit Coronary Vessels

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Left Bundle Branch Block & The Modified Sgarbossa Criteria

Baseline left bundle branch block (LBBB) alters early ventricular septal depolarization (from right to left) and delays left ventricular activation, producing deep QS complexes in V1–V3 and broad, notched R waves in I, aVL, and V6. Under normal physiological conditions, repolarization is discordant with depolarization (i.e., ST segments and T waves are deflected in the direction opposite to the main QRS deflection). This baseline discordance obscures typical ischemic ST elevation.

While historic guidelines considered any "new or presumed new LBBB" as an automatic STEMI equivalent, contemporary data reveal that <10% of patients with new LBBB have acute coronary occlusion. Instead, clinicians must apply the validated Modified Smith-Sgarbossa Criteria to diagnose acute occlusive myocardial infarction in the presence of LBBB or ventricular paced rhythms:

                                  MODIFIED SGARBOSSA CRITERIA
                                  
  ┌────────────────────────────────────────────────────────────────────────────────────────┐
  │ 1. CONCORDANT ST ELEVATION (5 Points)                                                  │
  │ • ST elevation >= 1.0 mm in >= 1 lead with a predominantly positive QRS complex        │
  │ • Highest specificity (>98%, positive likelihood ratio ~25); STEMI equivalent          │
  └────────────────────────────────────────────────────────────────────────────────────────┘
                                              │
  ┌───────────────────────────────────────────┴────────────────────────────────────────────┐
  │ 2. CONCORDANT ST DEPRESSION (3 Points)                                                 │
  │ • ST depression >= 1.0 mm in leads V1, V2, or V3                                       │
  │ • Reflects posterior/septal transmural injury; specificity >95% (LR+ ~11)              │
  └────────────────────────────────────────────────────────────────────────────────────────┘
                                              │
  ┌───────────────────────────────────────────┴────────────────────────────────────────────┐
  │ 3. MODIFIED SMITH CRITERION: EXCESSIVELY DISCORDANT ST ELEVATION                       │
  │ • Discordant ST elevation >= 1.0 mm with an ST / S ratio <= -0.25 (i.e., |ST/S| >= 25%)│
  │ • Replaces original arbitrary >=5 mm rule; boosts sensitivity from 52% to 91% (spec 90%)│
  └────────────────────────────────────────────────────────────────────────────────────────┘

• Clinical Application: A Sgarbossa score >= 3 or a positive Modified Smith criterion (|ST/S| >= 25%) warrants immediate emergency activation of the cardiac catheterization laboratory for primary PCI.

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Posterior Myocardial Infarction: Recognition & Verification

Occlusion of the left circumflex artery or a dominant distal right coronary artery frequently results in isolated transmural necrosis of the basal and posterior walls of the left ventricle. Because standard 12-lead ECG electrodes are placed anteriorly, posterior ST elevation is not directly recorded; instead, anterior precordial leads (V1–V3) record reciprocal mirror-image changes:

• Lead V1–V3 Hallmarks:
  • Horizontal or downsloping ST-segment depression (reciprocal to posterior ST elevation).
  • Prominent, tall, broad R waves with an R/S ratio > 1.0 or R-wave duration >= 40 ms (reciprocal to posterior Q waves).
  • Upright, symmetrical T waves (reciprocal to posterior T-wave inversions).
• Confirmatory Posterior Lead Placement (V7–V9):
  • Whenever unexplained ST depression is noted in V1–V3, posterior leads must be placed immediately:
    • V7: Posterior axillary line in the 5th intercostal space.
    • V8: Mid-scapular line at the inferior angle of the left scapula in the 5th intercostal space.
    • V9: Left paraspinal border in the 5th intercostal space.
  • Diagnostic Threshold: ST-segment elevation >= 0.5 mm (0.05 mV) in at least one posterior lead (or >= 1.0 mm in men < 40 years) confirms posterior STEMI and mandates immediate emergent reperfusion.

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Right Ventricular Myocardial Infarction (RVMI)

Right ventricular infarction complicates approximately 30% to 50% of acute inferior STEMIs and results from proximal occlusion of the right coronary artery upstream of the acute marginal branches.

Clinical Presentation & Hemodynamics

• The Classic Triad: Hypotension, elevated jugular venous pressure (often with Kussmaul sign: an abnormal inspiratory rise in JVP), and completely clear lung fields (absence of left ventricular pulmonary edema).
• Preload Dependence: The ischemic right ventricle is acutely dilated, hypokinetic, and non-compliant. Because the RV cannot generate adequate forward force to overcome normal pulmonary vascular resistance, right ventricular end-diastolic volume and forward output depend almost entirely on elevated right atrial venous filling pressures (preload).

Electrocardiographic Recognition

• Standard ECG Suspicion: Inferior ST elevation where lead III elevation is greater than lead II (ST elevation in III > II), accompanied by ST elevation in V1 with reciprocal ST depression in V2.
• Right-Sided ECG Confirmation: Obtain a right-sided precordial ECG immediately. ST elevation >= 1.0 mm in lead V4R (placed in the 5th intercostal space in the right midclavicular line) is the most sensitive and specific marker of acute RV involvement.

Fatal Pharmacologic Pitfalls & Hemodynamic Management

• ABSOLUTE PHARMACOLOGIC CONTRAINDICATION: Do NOT administer sublingual or intravenous nitroglycerin, morphine, or diuretics. Venodilation and volume depletion cause a sudden, precipitous drop in right ventricular preload. Deprived of filling pressure, the infarcted RV ceases forward flow, triggering catastrophic cardiovascular collapse, refractory shock, and electromechanical arrest.
• Emergency Resuscitation Protocol:
  1. Discontinue all nitrates and preload-reducing agents immediately.
  2. Administer rapid intravenous boluses of isotonic crystalloids (500 to 1,000 mL of 0.9% normal saline or lactated Ringer's) to augment right-sided filling pressure and drive forward flow into the left atrium.
  3. If mean arterial pressure remains depressed despite 1 to 2 liters of fluid resuscitation, initiate inotropic support with dobutamine or vasopressor support with norepinephrine.
  4. Expedite immediate mechanical revascularization via primary PCI to restore right coronary perfusion.

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High-Sensitivity Cardiac Troponin (hs-cTn) & Rapid Algorithms

High-sensitivity cardiac troponins (hs-cTnT and hs-cTnI) quantify myocardial necrosis with extraordinary precision, detecting circulating troponin concentrations at nanogram-per-liter (ng/L) thresholds with an analytical coefficient of variation <= 10% at the 99th percentile upper reference limit (URL).

Rapid 0/1-Hour and 0/2-Hour Triage Protocols (ESC/ACC)

Rather than waiting 6 to 12 hours for conventional troponin elevations, high-sensitivity assays evaluate baseline concentration and absolute dynamic delta (Delta) changes over 1 or 2 hours:

                                  HIGH-SENSITIVITY TROPONIN ALGORITHM
                                  
  ┌────────────────────────────────────────────────────────────────────────────────────────┐
  │ PRESENTATION WITH ACUTE CHEST PAIN (hs-cTn drawn at 0 hours)                           │
  └───────────────────────────────────────────┬────────────────────────────────────────────┘
                                              │
             ┌────────────────────────────────┼────────────────────────────────┐
             ▼                                ▼                                ▼
┌─────────────────────────┐      ┌─────────────────────────┐      ┌─────────────────────────┐
│ RULE-OUT PATHWAY        │      │ OBSERVE / INCONCLUSIVE  │      │ RULE-IN PATHWAY         │
│ • Baseline hs-cTn very  │      │ • Intermediate baseline │      │ • Baseline hs-cTn >3-5x │
│   low (if pain >3 hrs)  │      │   concentration         │      │   99th percentile URL   │
│ • OR low baseline with  │      │ • OR non-significant    │      │ • OR absolute 1h/2h     │
│   no delta at 1h or 2h  │      │   delta change          │      │   delta meets rule-in   │
│ • NPV > 99% for MI      │      │ • Action: 3-hour hs-cTn │      │ • PPV 75-85% for Type 1 │
│ • Safe early discharge  │      │   + bedside echo / CCTA │      │   MI; urgent angiography│
└─────────────────────────┘      └─────────────────────────┘      └─────────────────────────┘

Differential Diagnosis of Troponin Elevation

Elevated cardiac troponin indicates myocardial injury, but is not synonymous with an acute atherothrombotic plaque rupture (Type 1 MI). Clinicians must discern between:

• Type 1 Myocardial Infarction: Plaque disruption with occlusive or subocclusive coronary thrombus.
• Type 2 Myocardial Infarction: Myocardial necrosis resulting from supply/demand mismatch without acute plaque disruption (e.g., severe tachyarrhythmias, hypertensive emergency, severe hypoxemia, acute blood loss anemia, septic shock).
• Non-Ischemic Myocardial Injury: Acute myocarditis, stress (Takotsubo) cardiomyopathy, cardiac contusion, acute heart failure exacerbation.
• Extracardiac / Systemic Pathology: Acute pulmonary embolism (acute right ventricular strain), severe sepsis, acute aortic dissection, end-stage renal disease (reduced clearance combined with chronic subclinical micro-injury).

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Immediate Pharmacotherapy for Acute Coronary Syndromes

1. Dual Antiplatelet Therapy (DAPT)

• Aspirin: 162 to 325 mg non-enteric coated, chewed immediately upon presentation to ensure rapid buccal and gastric absorption (inhibits platelet cyclooxygenase-1 within 15–30 minutes). Maintenance: 81 mg daily indefinitely.
• P2Y12 Receptor Antagonists:
  • Ticagrelor (Brilinta): Direct-acting, reversible P2Y12 inhibitor. Loading dose: 180 mg orally, followed by 90 mg twice daily. Preferred over clopidogrel for primary PCI and NSTEMI due to superior mortality reduction without requiring hepatic CYP2C19 bioactivation. Common adverse effect: self-limiting central dyspnea.
  • Prasugrel (Effient): Potent, irreversible thienopyridine. Loading dose: 60 mg orally, followed by 10 mg daily (reduced to 5 mg daily if body weight < 60 kg or age >= 75 years). Preferred in primary PCI. BLACK BOX CONTRAINDICATION: Prior history of stroke or transient ischemic attack (TIA) due to an unacceptable risk of fatal intracranial hemorrhage.
  • Clopidogrel (Plavix): Irreversible prodrug requiring hepatic CYP2C19 activation. Loading dose: 600 mg orally for primary PCI, or 300 mg orally for patients receiving fibrinolytic therapy (withhold loading dose and administer 75 mg if age >= 75 years). Indicated when ticagrelor or prasugrel is contraindicated or unavailable.

2. Parenteral Anticoagulation

• Unfractionated Heparin (UFH): Initial IV bolus 60 units/kg (maximum 4,000 units), followed by continuous infusion at 12 units/kg/h (maximum 1,000 units/h), titrated to a therapeutic aPTT of 50 to 70 seconds. First-line agent during primary PCI and preferred in severe renal insufficiency (eGFR < 30 mL/min). Reversible with protamine sulfate.
• Enoxaparin (LMWH): 1 mg/kg subcutaneously every 12 hours (reduce to 1 mg/kg every 24 hours if CrCl < 30 mL/min). In patients receiving fibrinolysis: initial IV bolus 30 mg followed by 1 mg/kg SC (omit IV bolus if age >= 75).
• Bivalirudin: Direct thrombin inhibitor; alternative to heparin in patients with heparin-induced thrombocytopenia (HIT) or high bleeding risk.

3. Anti-Ischemic & Hemodynamic Agents

• Nitroglycerin: Sublingual 0.4 mg every 5 minutes up to 3 doses; continuous IV infusion (5 to 10 mcg/min titrated up to 200 mcg/min) for persistent chest pain, hypertension, or acute pulmonary edema.
  • CONTRAINDICATIONS: Systolic BP < 90 mm Hg, bradycardia (<50 bpm), severe tachycardia (>100 bpm), suspected right ventricular infarction, or phosphodiesterase-5 (PDE-5) inhibitor use within 24 hours (sildenafil, vardenafil) or 48 hours (tadalafil) due to catastrophic vasodilation and death.
• Oral Beta-Blockers: Initiate oral cardioselective beta-blockade (e.g., metoprolol tartrate 25–50 mg orally every 6 hours or metoprolol succinate) within the first 24 hours in hemodynamically stable patients.
  • CONTRAINDICATIONS: Signs of acute decompensated heart failure (pulmonary crackles > 1/3 lung bases, peripheral hypoperfusion), high risk for cardiogenic shock (age > 70, SBP < 120 mm Hg, heart rate > 110 or < 60 bpm), PR interval > 0.24 seconds, second- or third-degree heart block, or cocaine-induced chest pain (risk of unopposed alpha-1 adrenergic stimulation exacerbating coronary spasm and hypertension).
• High-Intensity Statin: Atorvastatin 80 mg or rosuvastatin 40 mg initiated immediately prior to or upon admission, regardless of baseline LDL-C levels, for plaque stabilization and pleiotropic anti-inflammatory effects.
• Supplemental Oxygen: Indicated ONLY if ambient pulse oximetry SpO2 < 90% or the patient is in overt respiratory distress. Hyperoxia triggers coronary vasoconstriction, enhances free radical generation, and increases infarct size and mortality in normoxic patients.

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Reperfusion Strategies & Timelines for STEMI

                                  STEMI REPERFUSION DECISION TIMELINE
                                  
  ┌────────────────────────────────────────────────────────────────────────────────────────┐
  │                   PATIENT ARRIVAL WITH CONFIRMED 12-LEAD ECG STEMI                     │
  └───────────────────────────────────────────┬────────────────────────────────────────────┘
                                              │
             ┌────────────────────────────────┴────────────────────────────────┐
             ▼                                                                 ▼
┌─────────────────────────────────────────┐       ┌─────────────────────────────────────────┐
│ PCI-CAPABLE FACILITY                    │       │ NON-PCI-CAPABLE FACILITY                │
│ • Primary PCI preferred strategy        │       │ • Assess anticipated transfer delay     │
│ • Benchmark: Door-to-Balloon <= 90 min  │       │ • FMC-to-Device <= 120 min achievable?  │
└─────────────────────────────────────────┘       └────────────────────┬────────────────────┘
                                                                       │
                                      ┌────────────────────────────────┴────────────────┐
                                      ▼                                                 ▼
                         ┌─────────────────────────┐                       ┌─────────────────────────┐
                         │ YES: FMC-to-Device <=120│                       │ NO: Delay > 120 min     │
                         │ • Transfer for primary  │                       │ • FIBRINOLYTIC STRATEGY │
                         │   PCI immediately       │                       │ • Door-to-Needle <= 30m │
                         │ • Door-In-Door-Out <=30m│                       │ • Check Contraindication│
                         └─────────────────────────┘                       └─────────────────────────┘

Benchmarks & Decision Pathways

1. Primary PCI: Gold-standard mechanical reperfusion for all patients presenting within 12 hours of symptom onset.
   • Door-to-Balloon Time: <= 90 minutes for patients presenting directly to a PCI-capable facility.
   • First Medical Contact (FMC) to Balloon Time: <= 120 minutes for patients presenting to a non-PCI facility who require transfer. Door-in-door-out (DIDO) time at the transferring facility should be <= 30 minutes.
2. Fibrinolytic Therapy: Indicated when primary PCI cannot be accomplished within 120 minutes of FMC, provided symptom onset is < 12 hours and no absolute contraindications exist.
   • Door-to-Needle Time: Administer weight-adjusted IV fibrinolytic within <= 30 minutes of arrival.
   • Preferred Agent: Tenecteplase (TNK-tPA) administered as a single weight-based IV bolus over 5 seconds (simplifies dosing and avoids pump infusion delays compared to alteplase). Reduce dose by 50% in patients >= 75 years.

Absolute Contraindications to Fibrinolytic Therapy (Must Memorize)

• Any prior intracranial hemorrhage (ICH) at any time.
• Known structural cerebral vascular lesion (e.g., arteriovenous malformation, aneurysm).
• Known malignant intracranial neoplasm (primary or metastatic).
• Ischemic stroke within the past 3 months (excluding acute ischemic stroke within 4.5 hours being actively treated).
• Suspected acute aortic dissection.
• Active internal bleeding or bleeding diathesis (excluding menses).
• Significant closed-head trauma or facial trauma within the past 3 months.
• Intracranial or intraspinal surgery within the past 2 months.
• Severe uncontrolled hypertension refractory to emergency therapy (systolic BP > 180 mm Hg or diastolic BP > 110 mm Hg).

Post-Fibrinolysis Pharmacoinvasive Strategy

Following fibrinolysis, transfer the patient immediately to a PCI-capable center:

• Failed Fibrinolysis (< 50% ST resolution at 60–90 minutes post-bolus or persistent hemodynamic instability): Perform emergency rescue PCI.
• Successful Fibrinolysis (>= 50% ST resolution and pain relief): Perform routine scheduled coronary angiography and stenting between 2 and 24 hours post-fibrinolysis.