Every lead view of an ECGElectrocardiogram; also called an EKG; a representation of electrical voltage measured across the chest over a period of time. 1. Six Second ECG Guidebook (2012), T Barill, p. 196 has a positive electrode. The heart is viewed electrically from the vantage point of the positive electrode. The positive electrode is important for another reason as well. A depolarizing wave travelling towards the positive electrode produces an upright waveformA wave of an ECG. 1. Six Second ECG Guidebook (2012), T Barill, p. 209. This principle is pivotal in the quest to fully understand the ECG.
A depolarizing electrical wave that is directed towards a positive lead produces an upright waveform on an ECG. Conversely, an inverted waveform results when an depolarizing wave moves away from a positive lead.
Take an ECG tracing from the bipolar leadThe standard three lead system that forms Einthoven’s triangle with the white electrode (right shoulder), black electrode (left shoulder) and red electrode (left lower abdomen) forming three bipolar leads (positive lead and negative lead) - Lead I (white to black)... II. The positive red electrode is located near the apex of the heart. As a result, the apex of the heart is best viewed by lead II. Consider as well the depolarizing atrial wave (P waveA wave generated from the depolarization of the atria; the P wave is upright when originating from the SA node. The P wave represents the depolarization of the right and left atria. The P wave begins with the first deviation...) with respect to this positive red electrode. A depolarizing wave travelling from the SA nodeThe SA node, usually the dominant pacemaker, is located in the right atrium at the opening of the superior vena cava. The SA (sinoatrial) node is a clump of hundreds of specialized cardiac cells that have the ability to self-initiate... out to the left atrium and the AV nodeIs located in the inferior aspect of the right atria; functions to slow the conduction speed to allow for atrial conduction prior to ventricular conduction (atrial kick); also serves as a pacemaker if the SA node fails to fire. The... is directed towards the positive electrode in lead II. The P wave produced on lead II, then, would be upright (refer to Figure 4.10).
Alternately, an impulse originating from the AV junctionConducts the impulse through the fibrous plate that separates the atria and the ventricles; consists of the AV node and the bundle of His; functions also to slow the conduction speed to allow for atrial conduction prior to ventricular conduction... depolarizes across the atriaRight and left atria (1/3 volume and muscle mass of the ventricles) pump blood to the ventricles. Chambers and Layers of the Heart 1. Six Second ECG Guidebook (2012), T Barill, p. 190 away from the positive red electrode. A resulting inverted P wave provides compelling evidence that this is a junctional rhythmA cardiac rhythm that occurs as a backup pacemaker when the sinus node fails to initiate an impulse; the junction typically fires at 40-60/minute; the P wave is either absent or inverted. A cardiac rhythm is usually named first by... initiated by the AV junctionConnects the atria to the ventricle and slows the impulse conduction speed sufficiently to allow for atrial kick; the junction consists of the AV node and the bundle of His; the junction is a supraventricular structure. 1. Six Second ECG....
Figure 4.9 provides a graphical example of contributing electrical vectors (small arrows) that average to form the mean direction of depolarizationThe rapid influx of positive ions (sodium and/or calcium) into a cell – depolarization is necessary for contraction to occur. A depolarizing wave moves through the myocardium on average along a trajectory or vector. A vector is a force moving..., known as the electrical axisThe direction of an average (mean) vector is called the electrical axis. The overall direction of ventricular depolarization is called the QRS axis. Many novice practitioners of 12 lead ECG interpretation find identifying the QRS axis challenging and somewhat confusing.... (large arrow). It is common for the electrical axis of the atria to point down and to the left.
Figure 4.10 illustrates how the direction and shape of the P wave helps to locate the site of an originating supraventricularLocated above the ventricle – includes the bundle of His, AV node, atria and the SA node. 1. Six Second ECG Guidebook (2012), T Barill, p. 206 impulse. Lead II has its positive electrode near the heart’s apex.
1. Six Second ECG GuidebookA Practice Guide to Basic and 12 Lead ECG Interpretation, written by Tracy Barill, 2012 Introduction The ability to correctly interpret an electrocardiogram (ECG), be it a simple six second strip or a 12 lead ECG, is a vital skill... (2012), T Barill, p. 77-78