The 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... of 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... 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 depolarization. Normal ventricular depolarization begins with 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 (causing a 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...) followed by a simultaneous depolarization of the remaining ventricular walls via the right and left 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 left bundle branch splits into the septal, anterior and posterior fascicles.
The 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.
Ventricular depolarization moving towards a positive electrode 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 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... 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 branches.
Normal ventricular depolarization begins with the septal fascicleWith regards to the heart, a fascicle is a smaller branch of the left bundle branch – two fascicles serve the left ventricle (one of which divides again); the three fascicles ensures rapid depolarization across the entire left ventricle. 1.... 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 the septal, anterior and posterior fascicles.
As mentioned, 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 waveform) 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 wave 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.
Figure 6.10 outlines the steps needed to determine whether a QRS complex is upright, inverted or neutral. For QRS complexes such as ‘A’ and ‘E’, the answer is straightforward. For the other QRS complexes (B-D, F), the net deflection of the QRS complex must be calculated. Take QRS complex ‘B’. The R wave is 5 mm in height and the S wave is 7 mm in depth leaving a net deflection of +5-7 or -2. The QRS complex ‘B’ has a negative net deflection of -2.
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. 79, 151, 161-162