Electrical Axis

The direction of an average (mean) vector is called the electrical axis. The overall direction of 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... is called the QRS axis.

Many novice practitioners of 12 lead 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 interpretation find identifying the QRS axis challenging and somewhat confusing. Fortunately, this skill only rarely impacts the acute management of a patient in a cardiac crisis. Hence, the topic is considered optional skill sets in the systematic analysis of a 12 lead ECG.

Background

If the identification of the QRS axis is often considered a non-essential skill, is there any merit to establishing a QRS axis? The 12 lead ECG becomes a more robust assessment tool when QRS axis is utilized in a systematic analysis. A QRS axis can help identify ventricular and atrial hypertrophy, correlate QRS deviation with the extent of a myocardial infarctionThe necrosis or death of myocardial tissue due to insufficient supply of oxygen to the infarcted region. The ability to identify cardiac ischemia, injury and infarction is vital in the management of the majority of cardiac emergencies. Most sudden cardiac... and even help differentiate between 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 tachycardiaA cardiac rhythm with a rate above 100/minute; for example, if the impulse originates from the atria with a rate of 160/minute, the rhythm is called an atrial tachycardia. A cardiac rhythm is usually named first by the location of... with aberrant conductionFor about 15% of rhythms with wide QRS complexes, impaired bundle branch conductivity prolongs ventricular depolarization. Ischemia, infarction, and antiarrythmics can slow or block transmission of an impulse along these bundle branches. The resulting widening of the QRS complex is... and ventricular tachycardiaIs a cardiac rhythm characterized by rapidly occurring wide QRS complexes with absent or infrequent P waves evident; the absence of atrial kick, the lack of filling time, and the tendency for ventricular tachycardia to change to ventricular fibrillation makes.... A QRS axis has merit.

Let’s begin the discussion of QRS axis with a brief review of the 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....

Revisiting the QRS ComplexA collection of waveforms (i.e. QRS complex and the ECG complex). 1. Six Second ECG Guidebook (2012), T Barill, p. 194

The QRS complex provides a surprising amount of information about ventricular 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...: the speed of depolarization, the electrical force or voltage involved and the direction of the depolarizing wave across 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..

The QRS complex is produced by ventricular depolarization. The width of the QRS is a function of the time taken for the ventricles to depolarize. The height or amplitudeThe height or depth of waves and complexes of an ECG in millimetres; represents millivolts where 10 mm is 1 millivolt with a properly calibrated monitor. 1. Six Second ECG Guidebook (2012), T Barill, p. 190 of the QRS is a function of the electrical force or voltage of the monitored region of the heart. The orientation of the QRS complex (i.e. upright, downward or diphasic waveformA wave of an ECG. 1. Six Second ECG Guidebook (2012), T Barill, p. 209) is a function of whether the depolarizing wave is directed towards or away from the positive electrode of each lead.

With lead II, the positive electrode resides close to the apex of the heart. Since the depolarizing wave moves towards this positive red electrode, the resulting QRS complex will also be positive (upright). Conversely, with a positive electrode located on the right shoulder as in lead aVR, the QRS complex would be inverted since the ventricles typically depolarize away from the right shoulder.

While often a quick look is sufficient to determine whether the QRS complex is upright or inverted, occasionally a QRS complex requires a simple calculation to arrive at a net deflection. Figure 6.10 outlines the steps required to arrive at a net deflection. The orientation (up or down) of the QRS complex ‘D’ is not easily established. The net deflection would equal the sum of the separate three deflections: Q waveA Q wave is the first negative deflection of the QRS complex that is not preceded by a R wave. A normal Q wave is narrow and small in amplitude. Note that a wide and/or deep Q wave may signify... of -3 mm, an R waveFollowing the depolarization of the interventricular septum, ventricular depolarization then progresses from the endocardium through to the epicardium across both ventricles producing an R wave and an S wave. An R wave is the first positive deflection of the QRS... of +4 mm and an S waveAn S wave is the first wave after the R wave that dips below the baseline (isoelectric line). The end of the S wave occurs where the S wave begins to flatten out. This is called the J point. Figure... of -7 mm = net deflection of -6 mm. Calculating net QRS deflectionWhile often a quick look is sufficient to determine whether the QRS complex is upright or inverted, occasionally a QRS complex requires a simple calculation to arrive at a net deflection. Figure 6.10 outlines the steps required to arrive at... is a well-utilized exercise in identifying the QRS axis.

The Mean VectorThe direction of an average (mean) vector is called the electrical axis. A vector is an arrow that represents the size and direction of a force. The larger the force the larger the arrow. Now take the small vectors from... and the QRS Axis

It is time to introduce the term vector. A vector is an arrow that represents the size and direction of a force. The larger the force the larger the arrow. Now take the small vectors from Figure 6.11 that represent the direction and size of the electrical force of ventricular depolarization. When adding these small vectors, the resulting size and direction of the overall ventricular depolarization could be represented by the large arrow. The direction of an average (mean) vector is called the electrical axis. The overall direction of ventricular depolarization is called the QRS axis.

Figure 6.11 illustrates the direction of normal depolarization of the ventricles from 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... out to the apex of the heart. Strictly speaking, each of the four depolarizing waves move in different directions. The septal fascicleNormal ventricular depolarization begins with the septal fascicle of the left bundle branch (causing a Q wave) followed by a simultaneous depolarization of the remaining ventricular walls via the right and left bundle branches. The left bundle branch splits into... of the left bundle branch depolarizes the septal region from the left septum towards the right ventricleThe right ventricle ejects blood through the main branches of the left and right pulmonary arteries to the lungs. The chambers of the heart are the main drivers within an intricate pathway, delivering blood to the lungs for gas exchange... (#1), away from the positive electrode in lead II. This early depolarizing wave away from the positive electrode produces a small Q wave. Both ventricles then simultaneously depolarize (#2). While the right ventricle depolarizes away from the positive electrode (in lead II), the electrical current of the much thicker left ventricular 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... dominates the overall direction of ventricular depolarization thus producing an upward QRS complex.

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, 161, 163