ecg, ekg, electrocardiogram, health, medicine
A Khorovets. What Is An Electrocardiogram (ECG)?. The Internet Journal of Health. 1999 Volume 1 Number 2.
An electrocardiogram (ECG) is the representation of the electrical activity of the heart (cardiac) muscle as it is recorded from the body surface. This article describes in understandable terms the basics of an ECG.
An electrocardiogram (ECG) is the representation of the electrical activity of the heart (cardiac) muscle as it is recorded from the body surface. The muscle cells of the heart are linked so closely to one another that electrical impulses can easily spread from one cell to the next. Certain groups of cardiac cells are designed to rapidly transmit electrical activity through the heart. These specialized cells include the atrial conduction tracks, the atrioventricular (AV) node, the bundle of His, the bundle branches, and the distal ventricular conduction system.
In the resting state, cardiac muscle cells are “polarized”, with the inside of cells being negatively charged with respect to the outside. This charge is created by having a greater concentration of certain charged particles (ions) on one side of the cell membrane as compared with the other side. For example, the concentration of potassium ions is much higher inside the cells while the concentration of sodium ions is much higher outside. In response to stimuli, movement of these ions occurs, particularly a rapid inward movement of sodium. This causes a rapid loss of internal negative potential and thus generates electricity. This process is known as “depolarization”.
The heart has some very specialized cells. The so-called automatic cells of the heart are capable of spontaneous depolarization. They are important in the generation of heart rhythm, and because of this they are also known as pacemaking cells. Under normal conditions, the pacemaking cells in a special area in the upper part of the heart called the sinoatrial (SA or sinus) node depolarize most rapidly and set the heart rate. Other pacemaking cells are situated in the atria (upper chambers of the heart), between the atria and the ventricles (called the AV node), and in the ventricles (lower chambers of the heart).
Reading a basic ECG
The first crude ECG was described in 1903 by Willem Einthoven. His procedure has been expanded, and now the probes used to measure electricity (called electrodes) are placed on the right and left arms, left leg, and across the chest wall. Currently there are 12 standardized “leads” for the standard ECG: bipolar leads I, II and III, described by Einthoven, augmented unipolar leads aVR, aVL and aVF and unipolar precordial leads V1-V6. How do these “leads” work and what do they mean?
When current passes to the positive end of the bipolar (2-sided) electrode, it causes a positive deflection, which corresponds to an upward movement of the pen on the ECG paper. Passage of current away from the positive pole of the bipolar electrode causes a negative deflection and a downward movement of the pen on the ECG paper. Current flowing at an oblique angle to the electrode causes smaller deflection, and current flowing perpendicular to the electrode does not cause any deflection in the recorder. Thus each lead “sees” the heart in a different way. This information is recorded on paper as a series of deflections and waves.
The distances between the deflections on an ECG are called segments and the distance between waves are called intervals.
Commonly encountered ECG abnormalities include:
Lake C. Chapters 1 and 2. Clinical Monitoring. WB Saunders Company, 1990. ISBN 0-7216-2961-X
Schamroth L. The Disorders of Cardiac Rhythm. Blackwell Scientific Publication. ISBN 0-632-08300-X