Depolarization

The 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 myocardiumThe muscle layer of the heart; the middle layer that is responsible for contraction of the heart. The muscular myocardium is the thickest layer and the workhorse of the heart. It is composed of specialized muscle and electrical cells that... on average along a trajectory or vector. A vector is a force moving in a direction symbolized by an arrow. The larger the force, the larger the arrow.

For example, an impulse initiated by 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... moves towards 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... and the left atrium. On average the depolarizing wave travels down and to the left. Atrial depolarization, then, has a vector that points down and towards the left. This average vector is 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.... of atrial depolarization (refer to Figure 4.9).

The depolarization of the myocardium is represented on 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 by a series of waveforms, one for atrial depolarization and soon after a larger waveformA wave of an ECG. 1. Six Second ECG Guidebook (2012), T Barill, p. 209 for ventricular depolarizationThe depolarization of the myocardium is represented on an ECG by a series of waveforms, one for atrial depolarization and soon after a larger waveform for ventricular depolarization. Normal ventricular depolarization begins with the septal fascicle of the left bundle.... While an in depth discussion of each of these waveforms is forthcoming, an important characteristic shared by all ECG waveforms is the direction of a wave. Is the waveform upright or is it pointing downwards? A simple observation but one packed with significance.

Every lead view of an ECG has a positive electrode. As mentioned earlier, 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 waveform. 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 node out to the left atrium and the AV node 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....

Ventricular depolarization moving towards a positive electrode also produces an upright waveform. The resulting waveform, though, is often more complexA collection of waveforms (i.e. QRS complex and the ECG complex). 1. Six Second ECG Guidebook (2012), T Barill, p. 194 than the P wave produced by atrial depolarization. Ventricular depolarization (QRS complexThe electrical representation of ventricular depolarization; the atrial repolarization is also a part of the QRS. ECG interpretation relies heavily on the QRS complex. The QRS complex represents the depolarization of the ventricles. The repolarization of the atria is also...) normally traverses three or four areas of the ventriclesThe larger chambers of the heart (3 times the volume and muscle thickness than the atria), responsible for the pumping of blood to the lungs and the rest of the body. simultaneously thanks to the bundle branchesThe bundle of His terminates in the right and left bundle branches, insulated rapidly conducting electrical pathways that connect with the Purkinje network and thus begin depolarizing waves across the ventricles; the left bundle branch splits into three smaller branches....

The direction of the QRS complex is usually only considered with 12 lead ECG interpretation particularly when determining the electrical axis of ventricular depolarization. For single and dual lead viewsLocating the positive electrode is crucial to determining which area of the heart is viewed electrically. Metaphorically, the positive electrode serves as a mini-video camera aimed at the heart in the direction of the negative electrode. These leads and their..., only the width of the QRS complex is useful for rapid ECG interpretation.

The direction of the P wave, though, is an important consideration for rapid ECG interpretation. Based on a principle of vector theory, the P wave can provide insight into the location of an impulse that originates above the ventricles. Equipped as well with the knowledge that a narrow QRS complex is produced by a 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, the identification of supraventricular rhythms is definitely within reach.

Figure 4.9 provides a graphical example of contributing electrical vectors (small arrows) that average to form the mean direction of depolarization, known as the electrical axis (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 supraventricular 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. 77-79, 82, 108, 110, 195