Acute Coronary Syndrome: Pathophysiology, 12-Lead Interpretation, and the Time Imperative

Acute coronary syndrome is a spectrum, not a single diagnosis. Understanding where a patient sits on that spectrum determines the urgency, the workup sequence, and the intervention pathway. The 12-lead ECG is the tool that answers the most important immediate question, and reading it correctly under pressure requires understanding what the waveform changes represent — not just which pattern maps to which territory.

The mechanism: plaque rupture and thrombus formation.

Atherosclerotic plaques build over years. The clinically dangerous plaque is not necessarily the most stenotic one — it is the one most vulnerable to rupture. Thin-capped plaques with large lipid cores are prone to sudden rupture, exposing subendothelial collagen and tissue factor to circulating blood. Platelet adhesion and aggregation follow within seconds. The thrombus that forms determines the clinical presentation.

A non-occlusive thrombus produces partial obstruction: unstable angina (no biomarker elevation) or NSTEMI (biomarker elevation). A fully occlusive thrombus cuts off flow to the territory distal to the blockage: STEMI. The distinction is physiologically important because the treatment pathways diverge at this point.

Myocardial necrosis begins within 20–30 minutes of complete occlusion. The wavefront of necrosis moves from the subendocardium outward. Time from occlusion to restoration of flow is the most modifiable determinant of infarct size — which is why door-to-balloon time targets exist and why they are not negotiable.

The 12-lead ECG: reading by territory.

The 12-lead ECG reflects myocardial injury in anatomically grouped leads. Understanding the coronary anatomy behind the lead groupings allows you to localize the culprit vessel and anticipate the complications that vessel’s territory produces.

Inferior wall (leads II, III, aVF): Right coronary artery (RCA) territory in 80% of patients. Inferior STEMI presents with ST elevation in II, III, and aVF with reciprocal depression in I and aVL. The RCA also supplies the right ventricle in most patients — inferior STEMI mandates right-sided leads (V4R at minimum) to evaluate for RV involvement. An RV MI changes management: these patients are preload-dependent and do not tolerate nitroglycerin or diuretics without adequate volume.

Anterior wall (leads V1–V4): Left anterior descending artery (LAD). Anterior STEMI carries the highest mortality of the STEMI patterns because the LAD supplies the largest myocardial territory. ST elevation in V1–V4 with reciprocal changes in inferior leads. A proximal LAD occlusion before the first septal and diagonal branches produces the largest infarcts.

Lateral wall (leads I, aVL, V5–V6): Left circumflex artery (LCx). Isolated lateral STEMI is less common; lateral changes more often accompany anterior or inferior patterns depending on the dominant artery anatomy.

Posterior wall: Not directly represented by a standard lead — the posterior wall is the far field for leads V1–V2. Posterior STEMI presents as ST depression with tall R waves and upright T waves in V1–V2 (mirror image of what you would see if the leads were looking directly at the posterior surface). Posterior leads (V7–V9) confirm the diagnosis. This pattern is frequently missed because the diagnosis requires knowing that depression in V1–V2 can represent ST elevation from the back.

STEMI criteria: ≥1 mm ST elevation in two or more contiguous limb leads, or ≥2 mm in two or more contiguous precordial leads. Note that leads V2–V3 have sex and age-adjusted thresholds per ACC/AHA guidelines: ≥2.5 mm in men under 40, ≥2 mm in men 40 and older, and ≥1.5 mm in women. A new LBBB in the setting of ischemic symptoms has traditionally been managed as, and may prompt emergent evaluation as, a STEMI equivalent in the setting of high clinical suspicion — though current guidance recognizes that LBBB alone is not diagnostic of AMI without additional findings. The Sgarbossa criteria provide a systematic approach to identifying superimposed ischemia in the setting of LBBB.

NSTEMI versus unstable angina: biomarkers make the call.

Both NSTEMI and unstable angina present without ST elevation. They are clinically indistinguishable at presentation. The differentiating factor is troponin elevation: elevated troponin = myocardial cell death = NSTEMI. Normal serial troponins = unstable angina. This distinction matters for risk stratification and the decision regarding timing of invasive evaluation.

High-sensitivity troponin assays can detect myocardial injury within 1–2 hours of onset. A negative troponin at presentation does not exclude NSTEMI if symptoms began within the last few hours — serial measurement at 1–3 hours is standard.

The management sequence.

For STEMI: the priority is restoring coronary flow as rapidly as possible. Dual antiplatelet therapy (aspirin + a P2Y12 inhibitor) and anticoagulation are initiated immediately. The reperfusion strategy — primary PCI or fibrinolysis — is determined by the time to PCI capability. Primary PCI is preferred if door-to-balloon time can be achieved within 90 minutes (60 minutes if transferred from another facility). Fibrinolysis is the alternative when PCI is not available within the window and contraindications are absent.

For NSTEMI/UA: risk stratification determines timing. High-risk features (elevated troponin, dynamic ECG changes, hemodynamic instability, signs of heart failure) warrant early invasive strategy within 24 hours. Intermediate and lower-risk patients may be managed with a more conservative approach. All receive dual antiplatelet therapy and anticoagulation.

In both cases: oxygen only if SpO&sub2; <90%. There is no benefit and potential harm from routine oxygen administration in normoxic patients with ACS. Nitroglycerin is contraindicated in RV MI (preload-dependent) and in patients who have taken a phosphodiesterase inhibitor within 24–48 hours.

The exam relevance.

CCRN and CEN ACS questions test three things: identifying the territory from the ECG pattern, anticipating the complication most likely from that territory (RV involvement in inferior MI, cardiogenic shock in anterior MI), and selecting the correct management step given the clinical context. A question about an inferior STEMI patient who develops hypotension after nitroglycerin is not testing pharmacology — it is testing whether you know that this patient is RV-dependent and that you should have checked the right-sided leads and avoided the nitroglycerin in the first place.

The mechanism explains the ECG. The ECG points to the territory. The territory predicts the complication. The complication determines the modification to standard management. That chain is the exam.