Wave of atrial repolarization is invisible because of low amplitude. The first deflection is the P wave associated with right and left atrial depolarization. Waves and Intervals on the ECG Atrial and ventricular depolarization and repolarization are represented on the ECG as a series of waves: the P wave followed by the QRS complex and the T wave. Irrespective of whether it is a limb lead or chest lead, a current surging directly in the direction of the recording electrode will cause a positive deflection on the ECG a current flowing in the direction but not directly toward the recording electrode will be registered as a positive deflection of lower amplitude a current running at right angle to the direction of the recording electrode will cause no deflection or a biphasic deflection a current flowing away in a direction opposite to that of the recording electrode will be registered as a negative deflection and a current flowing away but not directly will cause a negative deflection of smaller amplitude. Leads V5 and V6 record the current flow generated by the left ventricle directly. Leads V3 and V4 record directly the electrical activities of the ventricular septum and the anterior wall of the left ventricle. Leads V1 and V2 record the current flux over the right ventricle directly. There are also six chest leads with sensing electrodes positioned horizontally around the left anterior hemi-thorax between the 4 th and 5 th interspaces: Lead aVF recordsdirectly from the feet below at a coordinate of 90 o. Lead aVL records from the left shoulder at a coordinate of -30 o. 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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