![]() ![]() Lead aVR records from the right shoulder at a coordinate of -150 o. Lead III records from the foot at a coordinate of 120 o. Lead II records from the foot at a coordinate of 60 o. Lead I records from the left at a coordinate of 0 o. The heart occupies a position in the center of the thorax anda 12-lead ECG is simply a recordingof the current flux of cardiac depolarization and repolarization obtained from 12 different sites on the body surface. Despite the latter, ECG waves are commonly described by their height in mm rather than by their strength in mV. ![]() Two 5-mm-divisions on the vertical axis are calibrated to represent 1 mV. Each1-mm-division on the horizontal axis is 40 ms each 5-mm-division is 200 ms. The ECG Graph Paper Horizontal axis of theECG graph paper represents time in milliseconds (ms) while the vertical axis represents amplitude or voltage in millivolts (mV). The resultant waveform traced on graph paper is called the electrocardiogram (ECG). The wave of depolarization and repolarization described above can be mapped on the body surface by sensing electrodes placed on the extremities and the chest wall. Under abnormal conditions, ectopic foci in the atria, the AV junction, and the ventricles can usurp pacing dominance from this node and generate ectopic beats. Depolarization is followed by repolarization and the sequence of depolarization [activation-and-contraction [repolarization repeats itself to generate rhythmical heart beats. Under physiological conditions, the sinoatrial (SA) node generates pacemaker impulses that spread to the right and left atria, converge on the atrioventricular (AV) node, and continue down the His bundle and bundle branches (right bundle branch or RBB and left bundle branch or LBB) to activate the ventricles. Imagination is more important than knowledge. It is hoped the liberal use of diagrams and pictures can help to improve understanding. This Primer is only meant to introduce the subject to medical students, interns, novice residents, and general physicians in community practice. So why bother to learn how to read an electrocardiogram? The answer is simple: A robotic machine can follow algorithms but it takes a human mind to read beyond and between the waveforms to make interpretation and collate it with clinical findings.Įlectrocardiography can be the topic of a lifelong study. Modern-day ECG machines can make accurate measurements and analysis. Origin of the Heart Beat and Electrocardiogram ![]()
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