In 1901, Willem Einthoven invented the EKG machine. The EKG, or electrocardiogram, is a diagnostic tool widely used to measure the electrical activity of the heart. In addition to providing health care professionals with a record of cardiac electrical activity, the EKG presents valuable information about the heart's structure and function. The heart beats in reaction to an electrical impulse or wave that moves through it. This impulse makes the cardiac muscle contract and pump blood throughout the body. Information provided from the EKG can be used to determine if this impulse is normal or irregular. The electrical impulses of the heart are obtained by the EKG with the use of "electrodes," which are electrical sensors placed on the skin in specific locations.
Two electrodes make up a lead, and each lead measures the heart's activity from a different angle in space. It helps to think of the various leads as cameras placed in different positions around the heart that record the same cardiac activity from different viewpoints. The standard EKG is composed of 12 separate leads. This includes six limb leads recorded by using arm and leg electrodes, and six chest leads obtained by placing a suction cup electrode at six different positions on the chest.
Bipolar Limb Leads
The electrodes for the limb leads are placed on the right arm, the left arm and the left leg. Each limb lead consists of a pair of electrodes, one positive and one negative. Therefore, these leads are called "bipolar" limb leads. The placement and charges of these electrodes effectively create three leads named lead I, lead II and lead III.
Lead I measures horizontally in the frontal plane. The positive electrode is placed on the left arm, and the negative electrode is placed on the right arm.
Lead II provides superior and inferior measurement in the frontal plane. The positive electrode is placed on the left leg, and the negative electrode is placed on the right arm.
Lead III provides superior and inferior measurement in the frontal plane. The positive electrode is placed on the left leg, and the negative electrode is placed on the left arm.
These three limb leads are often called "Einthoven's triangle." Each lead shows a different axis of the heart. The direction from the negative to the positive electrode is the lead's axis. Lead I has an axis of 0 degrees. Lead II forms the right side of the triangle and has an axis of 60 degrees, and lead III forms the left side of the triangle with an axis of 120 degrees.
Augmented or Unipolar Leads
These leads provide additional views of the heart, utilizing the same electrodes as the bipolar limb leads. The EKG electronically combines the negative electrodes of two of the bipolar leads in order to obtain a different axis. This is effectively a combination, or a cross of two of the bipolar leads. The unipolar leads are as follows: AVF, AVR and AVL. The letter "A" indicates that the lead is augmented. The R, L and F identify the extremity on which the lead is placed (R=right arm, L=left arm and F=left foot).
Chest Leads
The six chest leads are unipolar leads that measure cardiac electrical activity in the horizontal plane. These leads are recorded using a positive electrode, which is placed at six anatomical positions on the chest, encircling the heart in the horizontal plane. They are arranged in progressive order from V1 through V6, and are positioned in successive steps from the patient's right to the left side of the chest. The chest leads cover the heart in its normal anatomic position. They are as follows:
• V1: right 4th intercostal space
• V2: left 4th intercostal space
These two chest leads are oriented over the right side of the heart.
• V3: halfway between V2 and V4
• V4: left 5th intercostal space, mid-clavicular line
V3 and V4 are the middle leads located in the midline of the heart.
• V5: horizontal to V4, anterior axillary line
• V6: horizontal to V5, mid-axillary line
Leads V5 and V6 are oriented over the left side of the heart (Rapid Interpretation EKGs).
References
- American Heart Association
- Rapid Interpretation of EKG's; Dale Dubin, M.D.; 2000
- The University of Utah
