Ischemia Recognition — Differentiate ischemia, injury, and infarction

Historical Context & Motivation

The ability to recognize myocardial ischemia on an electrocardiogram stands as one of the most consequential diagnostic achievements in modern cardiology. Before the advent of electrocardiography, physicians relied almost entirely on clinical symptoms—chest pain, diaphoresis, and dyspnea—to identify patients suffering from coronary artery disease. The development of the electrocardiogram (EKG) transformed cardiovascular medicine by providing a non-invasive, real-time window into myocardial electrical activity that could reveal not only the presence but also the progression and anatomical location of coronary events.

This historical progression underscores a fundamental clinical question that every EKG technician must be able to answer: when you observe an abnormal tracing, does it represent reversible ischemia, ongoing injury, or irreversible infarction? The answer determines the urgency of intervention and profoundly influences patient outcomes.

Core Principles & Definitions

Understanding the electrocardiographic signatures of ischemia, injury, and infarction requires a solid grasp of the underlying pathophysiology. These three conditions represent a continuum of myocardial oxygen deprivation, each producing distinct changes in the electrical behavior of cardiac cells. When coronary blood flow is reduced, the affected myocardium progresses through predictable stages that are reflected on the EKG in specific waveform components: the T wave for ischemia, the ST segment for injury, and the Q wave for infarction.

Visual Explanation — The Ischemia-Injury-Infarction Triad on the EKG

The diagram above illustrates the four key EKG patterns you must be able to differentiate. In the normal tracing, the T wave is upright and the ST segment sits along the isoelectric baseline. In ischemia, the most prominent change occurs in the T wave, which becomes symmetrically inverted due to delayed repolarization of the oxygen-deprived subepicardial tissue. The ST segment remains at baseline because the resting membrane potential has not yet been significantly affected. As the process worsens to injury, the ST segment elevates above baseline, reflecting an active current of injury flowing between damaged and healthy myocardium during the period between ventricular depolarization and repolarization. Finally, in infarction, the dead tissue creates an electrical void, and the initial QRS vector shifts away from the necrotic zone, producing deep, wide Q waves that persist indefinitely as markers of permanent tissue loss.

Electrophysiological Mechanisms

The EKG changes associated with ischemia, injury, and infarction are rooted in the electrophysiology of individual cardiac myocytes and the way their collective electrical behavior is sensed by surface electrodes. Understanding the mechanism behind each pattern—rather than merely memorizing waveform shapes—enables you to predict what you will see on the EKG based on clinical context and to recognize atypical presentations.

Ischemia: Altered Repolarization

Under normal conditions, repolarization proceeds from the epicardium (outer surface) inward toward the endocardium. When subepicardial ischemia occurs—as in transmural ischemia—the epicardial cells experience delayed repolarization, reversing the normal repolarization sequence. Since the repolarization wave now travels from endocardium to epicardium (the same direction as depolarization), the T wave inverts in the leads facing the ischemic territory. Conversely, subendocardial ischemia delays repolarization in the inner wall, exaggerating the normal repolarization direction and producing tall, peaked, symmetric T waves.

Injury: The Current of Injury

In injured myocardium, the resting membrane potential of damaged cells becomes less negative (partially depolarized), typically shifting from the normal −90 mV toward −60 to −70 mV. This creates a voltage gradient between injured and healthy tissue during electrical diastole (the TP segment), producing a diastolic current of injury that effectively depresses the true baseline. During the ST segment, when all ventricular cells should be uniformly depolarized, the injured cells actually repolarize earlier because their action potential duration is shortened, creating a systolic current of injury. The net effect of both mechanisms is apparent ST-segment elevation in leads facing the injured region and reciprocal ST depression in opposing leads.

Infarction: The Electrical Window

Necrotic tissue is electrically inert—it neither depolarizes nor repolarizes. When a recording electrode faces a region of full-thickness infarction, it "looks through" the dead tissue and records the depolarization of the opposite ventricular wall traveling away from the electrode. This retreating vector inscribes a negative deflection at the very beginning of the QRS complex, producing a pathological Q wave. To be classified as pathological, a Q wave must be ≥ 0.04 seconds (40 ms or one small box) wide and ≥ one-third the height of the R wave in that lead. Normal septal Q waves—small, narrow deflections in leads I, aVL, V5, and V6—do not meet these criteria and should not be mistaken for infarction.

Detailed Classification — Localizing Ischemic Changes by Lead

Beyond recognizing whether an EKG pattern represents ischemia, injury, or infarction, the certified EKG technician must be able to localize the affected myocardial territory by identifying which leads show changes. Each coronary artery supplies a specific region of the heart, and the 12-lead EKG provides spatial information through contiguous lead groups. The table below maps lead groups to anatomical territories and their most commonly associated coronary arteries.

The concept of reciprocal changes is clinically important and frequently tested. When ST elevation appears in one lead group, the leads on the opposite side of the heart typically show ST depression. For example, an acute inferior MI with ST elevation in leads II, III, and aVF will often show reciprocal ST depression in the lateral leads I and aVL. Recognizing these reciprocal changes increases diagnostic confidence and helps differentiate true injury patterns from pericarditis, which produces diffuse ST elevation without significant reciprocal changes.

Worked Example — Interpreting an Acute MI on a 12-Lead EKG

A 62-year-old male presents to the emergency department with crushing substernal chest pain radiating to the left arm, onset 45 minutes ago. A 12-lead EKG is obtained and reveals the following findings: ST elevation in leads V1 through V4, T-wave inversion in leads V5 and V6, small Q waves beginning to form in V2 and V3, and ST depression in leads II, III, and aVF. Systematically interpret this EKG.

Comparative Analysis — Ischemia vs. Injury vs. Infarction

Connection to Advanced Theory — STEMI vs. NSTEMI and Evolving MI Patterns

The foundational distinction between ischemia, injury, and infarction serves as the basis for more advanced diagnostic classifications that you will encounter in clinical practice and on the CET examination. The most important of these is the classification of acute coronary syndromes (ACS) into STEMI (ST-elevation myocardial infarction) and NSTEMI (non-ST-elevation myocardial infarction), which have fundamentally different management pathways.

Another advanced concept is the evolution of MI on serial EKGs. In the hyperacute phase (minutes after onset), you may see tall, peaked T waves before ST elevation develops. Over hours, ST elevation becomes prominent and T waves begin to invert. Over days to weeks, the ST segment returns to baseline, T-wave inversion may deepen and then gradually resolve, but pathological Q waves typically persist as permanent markers of the infarction. Understanding this temporal evolution is essential for dating an MI and determining whether findings represent an acute event versus a chronic scar from a prior infarction.

Practice Problems

Lesson Summary — Ischemia, Injury, and Infarction