A matrix of fibres located throughout 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... that connects the impulse from 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... to the myocardial tissue. The bundle branches and the Purkinje network facilitate rapid 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... 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 Purkinje network also creates a typical heart rateThe number of QRS complexes per minute; note that HR may not equal perfused pulse rate. See also: - Step 1 Heart Rate - Rate - Rule of 300s (Triplicate Method) - Six Second Count - The Caliper Method 1.... of 20-40 when necessary.
The heart’s 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 composed of five significant components: 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..., 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 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..., the bundle branches, and the Purkinje network. These electrical structures work as a cohesive interdependent team, conducting electrical impulses rapidly throughout the heart. Their location within the heart is depicted in the Figure 3.1.
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, the depolarizing wave travels 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 through the AV node, and the Bundle of His. The bundle branches then carry this wave through to the Purkinje network where the wave enters the ventricular 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..., the myocardium and 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... in that order. The atria and the ventricles depolarize then contract from the inside out.
This 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... (the bundle branches /Purkinje network) shortens both distance and the time taken for depolarization. A rapid depolarization results causing a narrow QRSThe 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... of less than 0.12 seconds (less than 3 mm in width).
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.
The Purkinje network also acts as a back up pacemakerAn electronic pulse generator that stimulates depolarization of the atria and/or the ventricles. 1. Six Second ECG Guidebook (2012), T Barill, p. 201 site, typically firing at a rateHeart Rate; calculated by counting the number of QRS complexes in six seconds and multiplying by 10; rate is also determined by measuring the number of large squares between two R waves; i.e. –1 large square = heart rate of 300/minute. of 20-40 beats per minute.
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 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... (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. 46, 57, 203