The specialized 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... and Purkinje networkA matrix of fibres located throughout the myocardium that connects the impulse from the bundle branches to the myocardial tissue. The bundle branches and the Purkinje network facilitate rapid depolarization throughout the ventricles. The Purkinje network also creates a typical... facilitate rapid conductivityThe ability of a cell to receive and transmit an electrical impulse. 1. Six Second ECG Guidebook (2012), T Barill, p. 194. The bundle branches and Purkinje network are composed of Purkinje fibers, specialized cardiac cells that are tailored for fast conductivity. These rapidly conducting cells carry the impulses through connective tissueConnective tissue in the heart provides structure, strength and protection to ensure efficient and effective electrical conduction and mechanical function. Connective tissue in the heart provides structure, strength and protection to ensure efficient and effective function. Connective tissue protects necessary..., reaching contractile cardiac tissue at the distal ends of the Purkinje network. This encapsulated electrical network is extremely efficient, rapidly carrying a depolarizing wave throughout 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 ventricles’ electrical systemUnderstanding the inner workings of the heart is vital to ECG interpretation and to responding effectively to acute cardiac events. The mechanical aspects of the heart are intimately connected to the heart’s electrical system. A dysfunctional electrical system often negatively... is exceptionally efficient. To produce a forceful, coordinated contraction, the electrical wave must travel quickly through the large ventricles.
With 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, only one wave is propagated. Via the bundle branches, this impulse is split into at least three simultaneous waves, thus reducing the distance each wave must travel. Less distance equates to less time. As a result, the time taken to depolarize the ventricles is reduced considerably.
Note that the speed of contraction translates directly into the force of contraction. The faster that the ventricles can depolarize and subsequently contract, the greater the force of contraction. A greater force of contraction increases both stroke volumeThe amount of blood ejected by either the right or left ventricle with one beat (contraction). While heart rate is an undisputed contributor to cardiac output, stroke volume is the other major player. As heart rates vary to changes in... and cardiac outputWhat is it? Why is it Vital? The amount of blood pumped out of the ventricle in a minute (most often refers to the blood pumped by the left ventricle) What is cardiac output? Simply, cardiac output is the amount.... Force of contraction is referred to as contractilityA muscle cell’s ability to shorten or contract through the action of actin and myosin - mediated by the calcium ion; the faster the influx of calcium, the more forceful the contraction. 1. Six Second ECG Guidebook (2012), T Barill, p. 194.
We established that an electrical wave envelops the ventricles very quickly IF the bundle branches and the Purkinje network are utilized. This is comparable to getting off the back country roads and racing down the freeway. The rapidly conducting bundle branches could be called the AutobahnAn electrical wave envelops the ventricles very quickly IF the Bundle Branches and the Purkinje Network are utilized. This is comparable to getting off the back country roads and racing down the freeway. The rapidly conducting bundle branches could be... of the heart.
Where is the only location to ramp onto the Autobahn of the heart? Remember that the bundle branches are largely encapsulated in connective tissue. The impulse must have travelled through the bundle of HisPart of the AV junction, the bundle of His conducts the impulse through the fibrous plate that separates the atria and the ventricle; the bundle of His is also a pacemaker, firing at 40-60/minute. The bundle of His serves as... to arrive at the bundle branches (the Autobahn). This is the only entry point to the Autobahn.
Where are we going with this? If the impulse travels through the bundle of His, then it originated in either the bundle of His or above the bundle of His (i.e. 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..., the atria or 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...). Simply stated, for a rapid wave 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... to envelop the ventricles, the impulse must originate above the ventricles.
A 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... represents 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.... 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 is the graphical representation of the electrical activity of the heart, with the horizontal axis (width) of the ECG being a measurement of time. A narrow QRS then equates to rapid ventricular depolarization, taking very little time. A narrow QRS, then, occurs when the impulse originates above the ventricles.
Consider for a moment an ectopicA depolarizing wave that originates anywhere outside of the SA node. 1. Six Second ECG Guidebook (2012), T Barill, p. 196 impulse originating in 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.... Instead of three simultaneous depolarizing waves, one wave depolarizes the right and then the left ventricleThe left ventricle ejects blood into the aortic arch to the body. Within the arch, the coronary arteries branch off first followed by three main arteries that branch to the brain (carotids) and the upper thorax (subclavian artery). The chambers.... Of course, more distance is covered by the one wave, taking more time. Instead of the Autobahn, the wave travels the back country roads. With more time taken for ventricular depolarization, a wide QRS results.
When interpreting an ECG, the location of the originating impulse for each beat is quite important. In fact, the naming of most cardiac rhythms begin with the site that the impulse originated. For example, a rhythmOften refers to a discernible pattern in time or distance between QRS complexes and/or P waves. that consistently originates from the sinoatrial nodeLocated in the upper right atrium near the inlet for the superior vena cava, serves as the predominant pacemaker for the heart with rates usually of 60-100/minute. The SA node, usually the dominant pacemaker, is located in the right atrium... is called a sinus rhythmAlso known as regular sinus rhythm or sinus rhythm, this cardiac rhythm is not a dysrhythmia; sinus rhythm originates in the sinoatrial (SA) node with a rate of 60-100/minute; P waves are upright in most leads and the QRS is.... If a beat originates 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..., it is called a junctional beat.
Therefore, based on what we know about the QRS and the speed of depolarization, a narrow QRS occurs with 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 rhythms. A wide QRS complexA collection of waveforms (i.e. QRS complex and the ECG complex). 1. Six Second ECG Guidebook (2012), T Barill, p. 194 is commonly associated with ventricular rhythms. This is perhaps the most important step in identifying cardiac rhythms. Is the QRS wide or narrow?
Asking the question, “Is the QRS wide or narrow” is an important step in ECG interpretation. A narrow QRS occurs when an impulse that originates from above the ventricles travels down the rapidly conducting bundle branches to depolarize the ventricles.
A wave of depolarization normally begins with the SA node. This electrical wave from the SA node passes quickly across the atria, through 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... (the AV node and the bundle of His) then across the ventricles via the bundle branches and the Purkinje network.
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. 50-53