8.2 Acute ischemia, injury & infarction (STEMI criteria)

The three tissue states of an evolving MI

An acute coronary event unfolds as three overlapping zones of injured tissue, and each prints a signature on the ECG. From least to most severe:

1. Ischemia — oxygen supply falls short of demand, but the cells are still viable. Repolarization is disturbed, producing T-wave inversion and/or horizontal or down-sloping ST depression. These changes are reversible if flow is restored.
2. Injury — a more severe, ongoing lack of perfusion. The hallmark is ST-segment elevation, the finding that defines a STEMI and signals an artery that is acutely, and usually completely, occluded right now.
3. Infarction — actual muscle death. Over hours to days a pathologic Q wave develops, evidence that a region of myocardium is electrically dead. Q waves are frequently permanent and mark an old or completed infarct.

Reading these in sequence — inverted T, elevated ST, new Q — lets a technician estimate not just that an event occurred but roughly when.

How the picture evolves over time

The ECG of a myocardial infarction is dynamic. In the hyperacute phase (minutes) the T waves become tall and peaked. Within the first hours ST elevation develops (the acute injury phase). Over the next hours to days the ST segments begin to normalize, the T waves invert, and pathologic Q waves appear as muscle is lost. Weeks later the ST and T changes may resolve, leaving a Q wave as the lasting fingerprint of the old infarct. This progression is why serial ECGs are ordered: a single tracing is a snapshot, and comparing tracings over time reveals whether an event is active, evolving, or old.

Measuring the ST segment

The ST segment is the portion between the end of the QRS (the J point) and the beginning of the T wave. It is measured at the J point and compared with the isoelectric baseline (conventionally the TP or PR segment). Deviation is reported in millimeters, where one small box equals 1 mm.

• ST depression: the J point sits below baseline; 0.5 mm or more of horizontal or down-sloping depression is significant and suggests subendocardial ischemia.
• ST elevation: the J point sits above baseline and is the key injury marker.

STEMI criteria — the threshold concept

Two rules define a STEMI: the elevation must reach a threshold height, and it must appear in two or more anatomically contiguous leads (leads that view the same wall).

The general rule is 1 mm or more of ST elevation in two contiguous limb or precordial leads, with a higher allowance in the anteroseptal leads V2-V3, where the normal ST takeoff is naturally higher. "Contiguous" means leads that look at neighboring regions of the same wall — for example II, III, and aVF (inferior wall) or V1 through V4 (anterior wall). A single lead with borderline elevation does not meet STEMI criteria; the two-contiguous-lead requirement is what separates real injury from normal variation or artifact.

STEMI versus NSTEMI

Acute coronary syndromes are separated largely by the ECG. A STEMI shows the threshold ST elevation described above and reflects a fully occluded artery needing emergency reperfusion. An NSTEMI (non-ST-elevation MI) shows ischemic changes — ST depression or T-wave inversion — without meeting ST-elevation criteria, yet cardiac troponin is still raised because muscle is dying; the artery is partially or intermittently occluded. Unstable angina looks similar on the ECG but troponin stays normal. The rhythm technician cannot measure troponin, so the practical rule is simple: any new ischemic ST or T change deserves escalation, and ST elevation meeting threshold in contiguous leads deserves the fastest possible escalation. Serial tracings matter here too, because an NSTEMI can evolve, and a repeat 12-lead may reveal new elevation that the first tracing missed.

Distinguishing STEMI from mimics — briefly

Not every ST elevation is a STEMI. Benign early repolarization, pericarditis (diffuse, saddle-shaped elevation with PR depression), left bundle branch block, and paced rhythms all distort the ST segment. The rhythm technician does not diagnose the cause, but recognizing that new, localized, threshold-meeting elevation in contiguous leads is a red flag — and that a paced or LBBB tracing makes ST interpretation unreliable — is within scope.

Escalation: the technician's job

A CRAT-level technician monitors rhythm, but ST changes are time-critical because "time is muscle." When you see new ST elevation meeting threshold in two or more contiguous leads, new deep symmetric T-wave inversions, or evolving ST depression — especially with chest pain — the correct action is to notify the nurse or physician immediately, ensure a full 12-lead ECG is captured, and document the time. Do not wait for the change to "declare itself"; the diagnostic and treatment clock (door-to-balloon time for primary PCI) starts at recognition. A rhythm strip of one or two leads can raise suspicion, but STEMI criteria are applied on the 12-lead, so escalating to obtain that 12-lead is the single most valuable step the technician takes.