An electrocardiogram (ECG) is a non-invasive diagnostic tool that records the heart’s electrical activity using electrodes placed on the body’s surface. A standard 12-lead ECG provides a multi-dimensional view of cardiac function using two distinct sets of leads. Limb leads, derived from electrodes on the arms and legs, primarily provide information about the heart’s electrical forces in the frontal plane. Precordial leads (V1 through V6) are positioned directly on the chest wall and offer a view of the heart’s electrical activity in the horizontal plane. These six chest leads are valuable because their proximity to the heart allows them to capture localized electrical events indicative of underlying cardiac abnormalities.
Anatomical Placement of V1 Through V6
The correct placement of the six precordial electrodes is a precise procedure that ensures the resulting tracing accurately reflects the heart’s electrical function. V1 is placed in the fourth intercostal space (ICS) immediately to the right of the sternal border, and V2 is positioned symmetrically to the left of the sternal border. V4 is placed next, positioned in the fifth ICS along the midclavicular line. V3 is then placed midway between the established positions of V2 and V4. The final two electrodes are aligned horizontally with V4 along the fifth ICS. V5 is positioned along the anterior axillary line, and V6 is placed on the midaxillary line. This precise arrangement provides a sweeping electrical view of the heart from the right side of the sternum to the left side of the chest.
Diagnostic Viewpoints of the Heart
The precordial leads are functionally grouped to monitor specific regions of the heart muscle, offering a detailed electrical map of the ventricles.
Leads V1 and V2 are considered the septal leads because they primarily view the electrical activity of the interventricular septum. Changes in these leads can also reflect issues originating in the right ventricle due to their proximity. An abnormality, such as ST-segment elevation, often suggests a potential problem in the septal area, which is typically supplied by the left anterior descending coronary artery.
Moving laterally, leads V3 and V4 are known as the anterior leads, positioned to view the anterior wall of the left ventricle. This region is a large territory of the heart. These leads are important for identifying issues like anterior wall myocardial infarction. Changes in V3 and V4, such as specific patterns of ST-segment deviation, are indicative of injury to this muscle mass, which is predominantly fed by the left anterior descending artery.
The final pair, V5 and V6, are the lateral leads, providing an electrical view of the lower-lateral wall of the left ventricle. Because the left ventricle has the largest muscle mass, these leads capture significant electrical forces. Abnormalities in V5 and V6, often paired with changes in limb leads I and aVL, signify electrical changes in the lateral cardiac territory, typically supplied by the circumflex artery. The progression of the QRS complex across V1 to V6, from predominantly negative to positive, reflects the heart’s normal electrical vector moving toward the left lateral wall.
Supplementary Lead Configurations
While the standard six precordial leads are sufficient for routine assessment, certain clinical situations require specialized electrode placements to fully assess all cardiac territories.
Right-Sided Leads
Right-sided precordial leads, most commonly V3R and V4R, are employed when a right ventricular infarction is suspected. Since the standard 12-lead ECG offers limited direct information about the right ventricle, these leads are placed in a mirror-image fashion on the right side of the chest. V4R is positioned in the fifth right intercostal space at the midclavicular line. ST-segment elevation in V4R is highly diagnostic for right ventricular involvement.
Posterior Leads
Posterior precordial leads (V7, V8, and V9) are used to detect infarction on the posterior wall of the left ventricle, a region not directly viewed by the standard leads. These electrodes are placed on the patient’s back at the same horizontal level as V4, V5, and V6. V7 is positioned on the posterior axillary line, V8 at the mid-scapular line, and V9 near the paraspinal border. The primary indication for these leads is when the standard ECG shows ST-segment depression in leads V1–V3, which can be a reciprocal sign of a true posterior wall infarction. Obtaining these supplementary views is important for accurate diagnosis and guiding time-sensitive treatment decisions.