- The recording produced by this noninvasive procedure is termed an electrocardiogram.
The EKG device detects and amplifies the tiny electrical changes on the skin that are caused when the heart muscle depolarizes during each heartbeat.
- At rest, each heart muscle cell has a negative charge, called the membrane potential, across its cell membrane. Decreasing this negative charge towards zero, via the influx of the positive cations is called depolarization, which activates the mechanisms in the cell that cause it to contract.
- During each heartbeat, a healthy heart will have an orderly progression of a wave of depolarization that is triggered by the cells in the sinoatrial node, spreads out through the atrium, passes through the atrioventricular node and then spreads all over the ventricles.
- This is detected as tiny rises and falls in the voltage between two electrodes placed either side of the heart which is displayed as a wavy line either on a screen or on paper. This display indicates the overall rhythm of the heart and weaknesses in different parts of the heart muscle.
- Sinoatrial Node-A small body of specialized muscle tissue in the wall of the right atrium of the heart that acts as a pacemaker by producing a contractile signal at regular intervals
- Atrioventricular Node-a node of specialized heart muscle located in the septal wall of the right atrium; receives impulses from the sinoatrial node and transmits them to atrioventricular bundle
- Usually, more than two electrodes are used, and they can be combined into a number of pairs
- For example: left arm (LA), right arm (RA) and left leg (LL) electrodes form the three pairs LA+RA, LA+LL, and RA+LL
- The output from each pair is known as a lead. Each lead looks at the heart from a different angle. Different types of ECGs can be referred to by the number of leads that are recorded
Placement of Electrodes:Ten electrodes are used for a 12-lead ECG. The electrodes usually consist of a conducting gel, embedded in the middle of a self-adhesive pad onto which cables clip. Sometimes the gel also forms the adhesive. They are labeled and placed on the patient's body as follows
Electrode label (in the USA) Electrode placement RA On the right arm, avoiding thick muscle. LA In the same location where RA was placed, but on the left arm. RL On the right leg, lateral calf muscle. LL In the same location where RL was placed, but on the left leg. V1 In the fourth intercostal space (between ribs 4 and 5) just to the right of the sternum (breastbone). V2 In the fourth intercostal space (between ribs 4 and 5) just to the left of the sternum. V3 Between leads V2 and V4. V4 In the fifth intercostal space (between ribs 5 and 6) in the mid-clavicular line. V5 Horizontally even with V4, in the left anterior axillary line. V6 Horizontally even with V4 and V5 in the midaxillary line.