If the first deflection is not negative, the Q is absent. Q: the first negative deflection after the p-wave.The letters "Q", "R" and "S" are used to describe the QRS complex This wave possibly results from "afterdepolarizations" of the ventricles. One heart beat consists of an atrial depolarization -> atrial contraction -> p-wave, ventricular depolarization -> ventricular contraction -> ORS-complex and the resting phase (including the repolarization during the T-wave) between two heart beats. There is no cardiac muscle activity during the T wave. The T wave represents the repolarization of the ventricles. As the endocardial cardiomyocytes depolarize slightly earlier than the outer layers, a typical QRS pattern occurs (figure). The QRS complex is the average of the depolarization waves of the inner (endocardial) and outer (epicardial) cardiomyocytes. Normal atrial repolarization is not visible on the ECG (but can be visible during atrial infarction and pericarditis). The signal produced by pacemaker cells in the SA node is conducted to the right and left atria. This depolarization starts in the SA (sinoatrial) node. The P wave is the result of the atrial depolarization. Here the ECG electrode is represented as an eye.Įxample of the different QRS configurations On the ECG, an action potential wave coming toward the electrode is shown as a positive (upwards) signal. During repolarization the ion concentration returns to its precontraction state. These calcium ions cause the actual muscular contraction.įinally the potassium ions stream out of the cell. Subsequently, the calcium ions stream into the cell. The final result, which is shown on the ECG, is actually the average of billions of microscopic electrical signals.ĭuring the depolarization, sodium ions stream into the cell. The individual action potentials of the individual cardiomyocytes are averaged. Courtesy of Arie Verkerk and Antoni van Ginneken, AMC, Amsterdam, The Netherlands. The electrical signal is written in blue and shows the action potential. The glass electrode measures the electrical current in the heart cell (with the patch-clamp method). ![]() This movie shows the contraction of a single (rabbit) heart cell. Through the depolarization by surrounding cells they become positively charged and they contract. In rest the heart cells are negatively charged. The ECG represents the sum of the action potentials of millions of cardiomyocytes The ECG is a graph of these electrical cardiac signals. The electrical activity of a single cell can be registered as the action potential.Īs the electrical impulse spreads through the heart, the electrical field changes continually in size and direction. If the cardiac muscle cells are electrically stimulated, they depolarize (the resting potential changes from negative to positive) and contract. The inside of the cell is negatively charged relative to the outside (resting potential). Cardiac muscles are electrically charged at rest. This stimulation is also called activation or excitation. Just like skeletal muscles, heart muscles are electrically stimulated to contract. What does the ECG register? The electrocardiogram An electrocardiogram (ECG or EKG) is a register of the heart's electrical activity. Note that the layout is different for each machine, but most machines will show the information above somewhere. Finally we have the ECG leads themselves.These will be discussed below.If this is not set at 10 mm, there is something wrong with the machine setting. So the height and depth of these signals are a measurement for the voltage. ![]() At the beginning of every lead is a vertical block that shows with what amplitude a 1 mV signal is drawn.
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