4 12-Lead ECG Interpretation

A systematic approach

A 12-lead ECG records the heart’s electrical activity from different directions; it does not provide 12 separate views of the heart. Interpret the tracing systematically, check that the recording and lead placement are plausible, and compare with prior ECGs when available.

A practical sequence is: rate and rhythm, axis, intervals and QRS morphology, chamber patterns, then ST-T changes and localization. Interpret ECG findings alongside symptoms, examination, and other tests.

Estimating the frontal-plane axis

The QRS axis estimates the average direction of ventricular depolarization in the frontal plane. In adults, a commonly used normal range is approximately $-30^\circ$ to $+90^\circ$.

A quick quadrant estimate uses the QRS polarity in leads I and aVF:

• Positive in I and positive in aVF: normal axis quadrant.

• Positive in I and negative in aVF: leftward quadrant; check lead II to distinguish a borderline axis from left-axis deviation.

• Negative in I and positive in aVF: right-axis deviation.

• Negative in both I and aVF: extreme or indeterminate axis.

For example, a positive QRS in I and a negative QRS in aVF suggests a leftward axis. If lead II is also negative, the axis is likely below $-30^\circ$, meeting the usual definition of left-axis deviation. For a more precise estimate, find the most isoelectric limb lead; the axis lies roughly perpendicular to it.

Axis deviation is a clue, not a diagnosis. Conduction disease, ventricular hypertrophy, infarction, body position, and lead reversal can affect the axis.

Recognizing chamber patterns

Look at P waves for atrial abnormalities and QRS voltage and morphology for ventricular hypertrophy.

• Right atrial abnormality: tall, peaked P waves, especially in the inferior leads.

• Left atrial abnormality: a broad or notched P wave in lead II, or a prominent terminal negative portion of the P wave in V1.

• Left ventricular hypertrophy (LVH): increased QRS voltage, sometimes with left-axis deviation and lateral ST depression or T-wave inversion, described as a strain pattern. One familiar voltage rule is $S_{V1}+R_{V5\text{ or }V6}\ge 35\text{ mm}$; this is only one of several criteria.

• Right ventricular hypertrophy (RVH): right-axis deviation with a relatively prominent R wave in V1 and deep S waves in lateral chest leads; interpret this pattern in clinical context.

Voltage criteria are imperfect. A normal ECG does not exclude structural enlargement, and ECG criteria alone do not establish it. Body habitus, age, lead placement, and coexisting conduction abnormalities can alter the tracing. Echocardiography or other imaging is used to assess chamber structure.

Identifying bundle branch blocks

A complete bundle branch block generally has a QRS duration of at least $120\text{ ms}$ and a characteristic morphology. Identify the pattern across several leads rather than relying on one finding.

• Right bundle branch block (RBBB): an rsR′ or rSR′-type pattern in V1–V2, often described as “rabbit ears,” with a broad terminal S wave in leads I and V6.

• Left bundle branch block (LBBB): a broad, often notched R wave in leads I, aVL, V5, and V6; usually no normal septal q waves in those lateral leads; and a predominantly negative QRS in V1.

Bundle branch block changes ventricular activation and often causes secondary ST-T changes, commonly opposite in direction to the main QRS deflection. These changes can make ischemia more difficult to assess, particularly with LBBB. Use symptoms, previous ECGs, serial assessment, and appropriate clinical criteria. A new or unexplained block warrants clinical evaluation, but the tracing alone does not determine its cause or treatment.

Localizing ST-T abnormalities

Each lead views electrical activity from a different direction. ST-segment or T-wave abnormalities in contiguous leads are more meaningful than an isolated change.

• Septal: V1–V2.

• Anterior: V3–V4.

• Lateral: I, aVL, V5–V6.

• Inferior: II, III, aVF.

• Posterior: additional leads V7–V9.

• Right ventricle: additional leads V3R–V4R, especially when right-ventricular involvement is suspected.

ST elevation in contiguous inferior leads (II, III, aVF) points to an inferior injury pattern; reciprocal ST depression in I or aVL may support it. ST depression in V1–V3, especially with prominent R waves, can be a clue to posterior injury and may prompt posterior leads when appropriate.

Lead patterns suggest a region, not a single guaranteed culprit artery. Coronary anatomy varies, and ECG localization is not definitive by itself.

Clinical interpretation

Interpret abnormalities in context and do not treat an ECG label in isolation. In a person with possible acute coronary syndrome, the ECG is part of urgent clinical assessment; an initially nondiagnostic tracing does not rule out an evolving event.

Persistent or recurrent symptoms may require repeat ECGs and further evaluation under local emergency protocols. Suspected acute cardiac emergencies should be escalated promptly.