| Electrocardiogram “ECG” A recording of the electrical changes that occur in the myocardium during a cardiac cycle is called an electrocardiogram (ECG). These changes result from the depolarization and repolarization associated with the contraction of muscle fibers. Because the body fluids can conduct electrical currents, such changes can be detected on the surface of the body. To record an ECG, metal electrodes are placed in certain locations on the skin. These electrodes are connected by wires to an instrument that responds to very weak electrical changes by causing a pen or stylus to mark on a moving strip of paper. When the instrument is operating, up-and-down movements of the pen correspond to electrical changes occurring as a result of myocardial activity. Because the paper moves past the pen at a known rate, the distance between pen deflections can be used to measure the time elapsing between various phases of the cardiac cycle. The ECG pattern includes several deflections, or waves, during each cardiac cycle. Between cycles, the muscle fibers remain polarized, and no detectable electrical changes occur. Consequently, the pen does not move but creates a baseline as the paper passes through the instrument. When the S-A node triggers a cardiac impulse, however, the atrial fibers are stimulated to depolarize, and an electrical change occurs. As a result, the pen is deflected, and when this electrical change is completed, the pen returns to the base position. This first pen movement produces a P wave that is caused by a depolarization of the atrial fibers just before they contract. When the cardiac impulse reaches the ventricular fibers, they are stimulated to depolarize very rapidly. Because the ventricular walls are much more extensive than those of the atria, the amount of electrical change is greater, and the pen is deflected to a greater degree than before. Once again, when the electrical change is completed, the pen returns to the baseline, leaving a mark called QRS complex, which usually consists of a Q wave, an R wave, and an S wave. This complex appears just prior to the contraction of the ventricular walls. Near the end of the ECG pattern, the pen is deflected for a third time, producing a T wave. This wave is caused by the electrical changes occurring as the ventricular muscle fibers become repolarized relatively slowly. The record of the atrial repolarization is missing from the pattern because the atrial fibers repolarize at the same time that the ventricular fibers depolarize. The recording of the atrial repolarization is thus obscured by the QRS complex. Action Potential This is the complex section of cardiac anatomy and requires some patience to understand. In the heart are specialized tissue collections that have a unique property, they rhythmically emit electrical impulses. The cause of this phenomena is the "leaky membrane" that allows the regular exchange of Sodium, Potassium, and Calcium ions and causes a change in the polarization of the cells. Sodium ions move into the cell and start the depolarization, Calcium ions extend that depolarization. When the Calcium ions stop entering the cell, Potassium ions move in and the repolarization of the cell begins. To simplify this, the Sodium starts the cells stimulation, the Calcium extends that stimulation to allow the entire muscle to contract before Potassium comes along and tells it to relax for a moment and get ready for the next wave. The most important thing to remember about this action is the period where the cells reset for the next wave This refractory period has two stages, the Absolute and Relative refractory periods. Let's use an example to simplify this concept. Think of a toilet, (strange I know, but read on,) when you flush it you have initiated the impulse, (the water floods the bowl and changes the pressure.) It takes a few seconds but the action is complete when the bowl empties, (but in the heart it's a few tenths of a second.) Now if you try to flush it again before the upper chamber has filled with water nothing happens. This is similar to the Absolute refractory phase in that the muscle is drained and needs a moment to recharge, so an impulse sent to it would not cause it to contract (push the handle down and nothing really happens.). |
Cardiovascular Diseases
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