The electrical representation 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...; the atrial repolarizationFollows depolarization, involving the return to a pre-depolarization state; the myocardial cell’s electrical potential returns from +30 mV to its polarized state of –90 mV; the ions potassium, calcium and sodium are largely involved; note that contraction of the myocardial... is also a part of the QRS.
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 relies heavily on 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 represents 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 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 repolarization of 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 is also buried in the QRS complex.
Three distinct waveforms are often present in a normal QRS complex. These waveforms follow the pathways of ventricular depolarization. Depolarization of the ventricular septum proceeds first from left to right away from the positive electrode in lead II. This early depolarization causes a small downward deflection called 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....
A Q wave is the first negative deflection of the QRS complex that is not preceded by 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.... A normal Q wave is narrow and small in 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. Note that a wide and/or deep Q wave may signify a previous 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... (MI).
Following the depolarization of the interventricular septum, ventricular depolarization then progresses from the endocardiumThe smooth innermost layer of the heart covers the inner chambers and the cardiac valves. The mechanical structures of the heart include the heart’s layers, chambers, septum, valves, and the major vessels (including the coronary arteries). Each of these structures... through to the epicardiumThe external layer that covers the heart – also called the visceral layer of the heart; between the visceral layer and the outer parietal layer is the pericardial sac; note that the coronary arteries travel along the epicardium before burrowing... across both ventricles producing an R wave 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.... An R wave is the first positive deflection of the QRS complex. An S wave is the first wave after the R wave that dips below the baselineThe baseline or isoelectric line is a reference point for the waves, intervals and segments. While the PR segment is often used as the baseline, the TP segment - between the end of the T wave and the beginning of... (isoelectric lineAlso called the baseline, the straight line that is present when no electrical activity is present. The baseline or isoelectric line is a reference point for the waves, intervals and segments.While the PR segment is often used as the baseline,...). The end of the S wave occurs where the S wave begins to flatten out. This is called the J pointA small notch in the QRS where the ST segment begins. The point where the S Wave ends and begins to flatten out. Figure 4.13 The QRS Complex, ST Segment and the T Wave Figure 4.13 depicts the component parts....
Why is the QRS complex so much larger 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...? The ventricles are about 3 times the size of the atria. The larger ventricle will produce a larger waveformA wave of an ECG. 1. Six Second ECG Guidebook (2012), T Barill, p. 209.
Abnormal ventricular depolarization produces a QRS complex that often has additional waveforms. For example, a second positive deflection of a QRS complex after an R wave is labelled R’ (R prime). Similarly, a second S wave that dips below the baseline after the R wave is labelled S’ (S prime). Refer to Figure 4.14 for an illustrated example. A downward notch in the R wave that does not dip below the baseline is simply called – yes – a notch in the R wave.
A narrow QRS complex occurs quickly over a period of less than 0.12 seconds (less than 3 mm in width). A narrow QRS occurs with normal ventricular depolarization that originates above the ventricles. Figure 4.14 provides a variety of different QRS complexes, produced by normal and abnormal ventricular depolarization. A normal, narrow QRS complex may be predominantly upright, predominantly inverted, completely inverted (called a QS complex) or biphasic (part upright, part inverted).
While the direction of the QRS complex is generally not important with basic ECG interpretation, the width of the QRS complex is key. The width of the QRS complex often indicates the location of the originating electrical impulse. This is a rather important point since the first and foremost word of an ECG interpretation is the location of impulse initiation.
For example, rhythms that come 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... are sinus rhythms, 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... are junctional rhythms, and that originate from the ventricle are ventricular rhythms. Simple. If the QRS is narrow – taking very little time to occur – the cardiac rhythmOften refers to a discernible pattern in time or distance between QRS complexes and/or P waves. originates from 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 site. Quickly determining whether the QRS is narrow or wide is a vital step in rapid ECG interpretation.
Figure 4.13 depicts the component parts of the QRS complex. The QRS complex consists of a series of waves, the ‘Q’, ‘R’, and ‘S’ waves. The ‘Q’ wave is the first negative deflection from baseline. The ‘R’ wave is the first positive deflection above baseline. The ‘S’ wave follows the ‘R’ wave with a negative deflection. A QRS complex may or may not have all three waveforms. The ST segmentSegment (line) between the end of the QRS complex and the beginning of the T wave; the end of the QRS is marked by the J point. Between the QRS complex and the T wave, lies the ST segment. The... begins at the J point and continues to the beginning of the T waveThe wave that arrives after the QRS; is a graphical presentation of ventricular repolarization. Expect a T wave to follow every QRS complex. The T wave is a graphic representation of the repolarization of the ventricle. The T wave is....
Figure 4.14 depicts several QRS complex shapes or morphologies. QRS complex #1 demonstrates the labelling convention for subsequent positive deflections above the baseline after the R wave. This second deflection is labelled R’. Note that a third upright deflection would be labelled R’’ (R double prime). QRS complex #2-4 are all normal QRS complexes of different shapes. QRS complex #3 is a biphasic QRS complex would be labelled RS. The QRS complex #5 is a QS 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. 83-85