ECG Interpretation Made Easy; How to read 12 Lead EKG (Basics), USMLE/NCLEX

An ECG records the heart’s electrical activity by detecting signals produced by the SA node, which triggers atrial contraction. These electrical impulses travel from the SA node to the AV node, then through the right and left bundle branches and Purkinje fibers to coordinate ventricular contraction. Abnormalities in this conduction system, such as myocardial infarction, hypertrophy, ventricular tachycardia, and ventricular fibrillation, can be identified through distinct ECG patterns. A 12-lead ECG captures activity from 12 angles by employing hex axial leads and precordial leads for a comprehensive assessment of the heart.

Hexaxial Lead Placement for Multi-Angle Cardiac Imaging Electrodes are strategically placed on the body to capture the heart's electrical signals from multiple directions. Lead I uses a positive electrode on the left arm and a negative one on the right arm, while lead II positions the positive electrode on the left foot with the negative on the right arm. Lead III’s setup involves a positive electrode on the left foot and a negative electrode on the left arm, with each positive electrode acting as a camera to reveal distinct views of the heart.

Augmented Leads Enhance Cardiac Perspective Without Extra Electrodes Mathematical calculations derive three augmented leads—AVR, AVL, and AVF—from the initial leads, offering additional angles to examine the heart. Although these computed leads capture lower voltage signals, the ECG machine amplifies them for clear visualization. This methodology expands the diagnostic framework by providing six comprehensive perspectives of the heart's electrical activity.

Precordial Perspectives: Strategic Chest Lead Positioning Precordial leads capture the heart’s electrical activity directly from the anterior chest, offering a view distinct from the side perspectives of hex axial leads. Electrodes V1 and V2 are accurately positioned in the fourth intercostal space adjacent to the sternum, identified by landmarks such as the sternal notch and Angle of Louis. V4 is placed in the fifth intercostal space, with V3 precisely located between V2 and V4, while V5 and V6 extend laterally—V6 aligning with the mid-axillary line.

Regional Insights: Linking ECG Leads with Cardiac Territories Specific ECG leads correspond to distinct areas of the heart, allowing for precise localization of myocardial events. Leads II, III, and aVF monitor the inferior surface, V1 and V2 focus on the septal region, V2 through V4 observe the anterior wall, and leads I, aVL, V5, and V6 assess the lateral aspect. This integrated view of hex axial and precordial leads provides a comprehensive framework for identifying cardiac abnormalities.

The heart’s electrical activity follows a vector from the SA node, through the AV node and bundle branches to the pericanal fibers, moving in a left lateral direction. An electrode in front of this flow, like lead II, captures prominent positive deflections, while one behind it, such as aVR, shows negative deflections because the current is moving away. A positive deflection in a normally negative aVR indicates an abnormal shift in the heart’s axis, signaling an alteration in the regular current flow.

An ECG starts with an isoelectric baseline, signaling no electrical activity, and then shows deflections where positive waves occur with current flowing in the expected direction and negative deflections when the current is reversed. The P wave represents atrial depolarization as the current moves from the SA node toward the AV node, while a brief delay in conduction from the left to thicker right bundle branch creates a small, negative Q wave. This sequence culminates in a pronounced QRS complex indicating robust ventricular depolarization, followed by a T wave of ventricular repolarization that mirrors the direction of the initial current flow.

ECG Grid Measurements and Interval Standards ECG paper is structured with small boxes measuring 1 mm and corresponding to 0.04 seconds, while each large box consists of five small boxes amounting to 5 mm and 0.20 seconds. These defined measurements enable precise calculations of key intervals such as the PR interval, which normally extends from the P wave to the R wave for 0.12 to 0.20 seconds. The P wave is expected to be smooth, round, and upright with a duration not exceeding 0.10 seconds, making box counting essential for identifying conduction anomalies.

Waveform Analysis and Conduction Interval Interpretations The QRS complex should be narrow and sharp, with variations like QR or RS forms, where a wider complex indicates abnormal conduction. The ST segment is maintained near the isoelectric line, and the T wave should present an asymmetrical shape characterized by a slow rise and fast fall; symmetry can suggest ischemia or electrolyte disturbances. The QT interval, reflecting complete ventricular activity, normally spans 0.33 to 0.44 seconds and adjusts with heart rate, while an occasional U wave typically carries no significant clinical meaning.

The standard 12-lead ECG outlines the heart’s electrical activity using hexagonal leads, particularly those derived from AVR, AVL, and AVF, which capture distinct aspects of conduction. The precordial leads add critical spatial information that enhances understanding of cardiac function. Key waveform components including the P wave, PR interval, QRS complex, ST segment, QT interval, and U wave are interpreted with reference to time measurements marked by big and small boxes. A forthcoming discussion will address rhythm abnormalities and rate determination, building on these foundational insights.