Electrocardiograph: Precordial Leads Explained
The electrocardiograph, invented in 1903, assists in the diagnosis of specific cardiac abnormalities by detecting transmission of electrical impulses through the heart’s conductive tissues. This is accomplished by means of leads (or electrodes), attached to a patient’s limbs and chest, which transmit electrical impulses to the recording device. Of the 12 leads, the six limb leads measure electrical activity in the vertical plane. Electrocardiographic assessment is not complete without input from the precordial leads, which help determine the extent and location of myocardial damage.
The precordial leads, designated as leads V1, V2, V3, V4, V5, and V6, are placed at different angles in specific regions of the chest to register cardiac electrical activity in the horizontal plane and to reveal valuable information about specific regions of the heart. As shown above, the precordial leads are placed on the anterior chest in the following manner:
- V1 - is placed on the fourth intercostal space at the right sternal border
- V2 - is placed on the fourth intercostal space at the left sternal border
- V3 - is placed halfway between V2 and V4
- V4 - is placed on the fifth intercostal space at the left midclavicular line
- V5 - is placed on the same level as V4 at the left anterior axillary line
- V6 - is placed on the same level as V4 at the left midaxillary line
The precordial leads are also classified based on the region of the heart they are monitoring:
- V1 and V2 are called the septal leads. Electrical activity of the inter-ventricular septum is best measured in these leads.
- V3 and V4 are called the anterior leads. Electrical activity of the anterior (front) wall of the left ventricle is best measured in these leads.
- V5 and V6 are called the left precordial or lateral precordial leads. Electrical activity of the lateral (left) wall of the left ventricle is best measured in these leads (together with leads I and aVL).
The virtual point of intersection of the precordial leads is the atrioventricular (AV) node. The positive electrode of each precordial lead is placed on the chest while the virtual negative electrode of each precordial lead is located on the back.
It should be noted that each precordial lead has only one physical positive electrode. There are no corresponding negative electrodes placed on the back and are only derived mathematically.
Each of the different electrodes (the six limb leads and the six precordial leads) monitors heart activity from its own specific angle and position. As a result specific areas of the heart are best analyzed through a particular set of leads. For example:
- Electrical activity of the inferior region of the heart is best analyzed through the inferior leads (leads II, III, and aVF).
- Electrical activity of the lateral (left) wall of the left ventricle is best measured through the lateral leads (leads I, aVL, V5, and V6).
Lead II is situated between the lateral leads and the inferior leads; therefore, it is also referred to as an inferolateral lead.
In summary, the precordial leads contribute significantly to the diagnostic utility of the electrocardiograph. Among healthcare professionals, the EKG serves as a map to guide them through the anatomical and physiological terrains of the heart, allowing in-depth assessment of various cardiovascular conditions. For a more detailed discussion and review of EKG principles and other topics related to critical care, please visit ExpertCollege.com.