Acute Coronary Syndromes

Historical Context & Motivation

The concept of acute coronary syndromes (ACS) has evolved dramatically over the past century, transforming from a largely fatal condition to one amenable to aggressive intervention. In the early 1900s, physicians recognized myocardial infarction as a distinct clinical entity, yet treatment options were limited to bed rest and morphine. The subsequent decades brought electrocardiography, coronary angiography, and ultimately percutaneous coronary intervention — each milestone reshaping how we diagnose and treat acute ischemic events. Today, ACS remains the leading cause of morbidity and mortality worldwide, but timely recognition and evidence-based management can profoundly improve patient outcomes.

The central question that drives the study of ACS is deceptively simple: when a patient presents with chest pain, how do you rapidly distinguish between stable angina, unstable angina, NSTEMI, and STEMI — and how does that classification dictate the urgency and nature of your intervention? This lesson addresses the pathophysiology underlying plaque rupture, the diagnostic criteria that differentiate ACS subtypes, and the evidence-based treatment algorithms you will encounter on Step 2 and in clinical rotations.

Core Principles & Definitions

Acute coronary syndromes represent a spectrum of clinical presentations caused by acute myocardial ischemia, almost always resulting from atherosclerotic plaque disruption with superimposed thrombus formation. The unifying pathophysiological mechanism is a sudden reduction in coronary blood flow sufficient to cause myocardial injury. ACS is categorized based on ECG findings and biomarker evidence of myocardial necrosis into three entities: unstable angina (UA), non–ST-elevation myocardial infarction (NSTEMI), and ST-elevation myocardial infarction (STEMI). Distinguishing among these entities is the critical first step in management.

Visual Explanation — The ACS Spectrum

As the diagram illustrates, the pathophysiological continuum begins with a vulnerable atherosclerotic plaque characterized by a thin fibrous cap and a large, lipid-rich necrotic core. When this cap ruptures or erodes, the exposure of subendothelial collagen and tissue factor triggers platelet adhesion, activation, and aggregation, followed by the coagulation cascade and thrombus formation. The clinical presentation depends on whether the thrombus is partially or completely occlusive and whether sufficient ischemia occurs to produce cardiomyocyte death detectable by troponin. The 12-lead ECG serves as the initial branching point: ST-segment elevation in a territorial distribution immediately classifies the patient as STEMI and triggers emergent reperfusion therapy, while its absence directs the patient into the NSTE-ACS pathway where serial troponin measurements become the key differentiator.

Pathophysiological Mechanisms & Diagnostic Markers

The Ischemic Cascade

When coronary blood flow is abruptly compromised, the myocardium undergoes a predictable sequence of events known as the ischemic cascade. This concept is clinically important because different diagnostic modalities detect abnormalities at different stages of the cascade. The earliest changes are biochemical — depletion of ATP and accumulation of lactate — followed by diastolic dysfunction (detectable by echocardiography), then systolic wall motion abnormalities, and subsequently ECG changes (ST-segment deviation). Chest pain is actually a relatively late manifestation. This explains why some patients develop silent ischemia with ECG or biomarker evidence of injury before symptom onset, a phenomenon particularly common in diabetic patients with autonomic neuropathy.

Troponin Kinetics

Cardiac troponin I (cTnI) and troponin T (cTnT) are structural proteins in the contractile apparatus of cardiomyocytes. When myocardial necrosis occurs, these proteins are released into the circulation. With conventional assays, troponin becomes detectable 3–6 hours after the onset of ischemia, peaks at 12–24 hours, and remains elevated for 7–14 days. High-sensitivity troponin (hs-cTn) assays detect much lower concentrations and may become positive within 1–3 hours, enabling rapid rule-in or rule-out protocols. The 2020 ESC guidelines endorse a 0/1-hour or 0/2-hour algorithm: if the initial hs-cTnT or hs-cTnI is below the limit of detection and the delta at 1–2 hours is minimal, MI can be ruled out with a negative predictive value exceeding 99%.

ECG Localization of Ischemia

Classification & Risk Stratification

Once a patient is categorized into the NSTE-ACS pathway (no persistent ST elevation), clinicians must stratify the risk of adverse outcomes to guide the urgency of invasive management. Two validated scoring systems dominate clinical practice and board examinations: the TIMI risk score and the GRACE score. The TIMI score for NSTE-ACS assigns one point each for seven variables and is straightforward to calculate at the bedside. The GRACE score is more complex but provides a more precise estimate of in-hospital and 6-month mortality. Both inform decisions regarding early invasive strategy versus initial conservative management.

TIMI Risk Score for NSTE-ACS

• Age ≥ 65 years (1 point)
• ≥ 3 CAD risk factors — hypertension, diabetes, dyslipidemia, smoking, family history (1 point)
• Known coronary stenosis ≥ 50% (1 point)
• ASA use within the past 7 days (1 point)
• ≥ 2 anginal episodes within 24 hours (1 point)
• ST deviation ≥ 0.5 mm on presenting ECG (1 point)
• Elevated cardiac biomarkers (1 point)

Each variable contributes one point, yielding a total score of 0–7. Patients with a TIMI score of 0–2 are considered low risk (< 8% risk of death, MI, or urgent revascularization at 14 days), while a score of 5–7 identifies high risk patients (> 26% event rate). The simplicity of TIMI makes it ideal for rapid bedside use, though the GRACE score is considered more accurate for mortality prediction.

Worked Clinical Example

Comparing ACS Subtypes & Treatment Strategies

Reperfusion Strategy Selection in STEMI

The choice between primary PCI and fibrinolysis in STEMI depends primarily on the availability of a PCI-capable facility within a timely window. Current ACC/AHA guidelines recommend primary PCI as the preferred reperfusion strategy when it can be performed within 90 minutes of first medical contact at a PCI-capable hospital, or within 120 minutes if transfer from a non–PCI-capable hospital is required. If these time targets cannot be met, fibrinolysis should be administered within 30 minutes of hospital arrival (door-to-needle time), provided the patient presents within 12 hours of symptom onset and has no absolute contraindications. After fibrinolysis, patients should be transferred for routine angiography within 3–24 hours (pharmacoinvasive strategy).

Complications & Advanced Considerations

Understanding the complications of acute myocardial infarction is essential for Step 2 and clinical practice. Complications are broadly categorized by their temporal relationship to the index event and by mechanism — mechanical, arrhythmic, inflammatory, or thromboembolic. Recognizing these entities early is critical because many are rapidly fatal without intervention.

Looking forward, ongoing research into personalized antiplatelet therapy guided by platelet function testing and pharmacogenomics (e.g., CYP2C19 polymorphisms affecting clopidogrel metabolism) may refine DAPT strategies. Additionally, advances in intracoronary imaging with optical coherence tomography (OCT) and intravascular ultrasound (IVUS) are improving our understanding of plaque morphology and stent optimization. These technologies represent the frontier of ACS management, bridging the gap between population-level guidelines and truly individualized care.

Practice Problems

Acute Coronary Syndromes — Summary