Understanding the inner workings of the heart is vital to 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 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 impacts the heart’s effectiveness as a pump. For example, atrial fibrillationOne of the most common dysrhythmias, the atria have a host of sites that are concurrently firing at a combined rate of 350-600/minute; since the junction allows maximum 240 impulses through per minute, the result is a chaotic rhythm most... results in the loss of atrial kickThe contraction of the atria prior to ventricular contraction causes an increased volume and stretch to the ventricles – resulting in increased force of contraction and increased stroke volume (Starling’s Law); this extra stroke volume increases cardiac output by 10-35%.....
This is an abbreviated description of the heart’s electrical system.
The SA (Sinoatrial) Node has the ability to self-initiate an electrical impulse. This ability, called automaticityA cardiac cell’s (usually pacemaker cells such as the SA node, AV node or His-Purkinje network) ability to self-initiate an impulse; note that abundant catecholamines and/or ischemia enhances automaticity – non-pacemaker cells may become pacemaker cells. The SA node, usually..., makes 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... a 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 for the heart. The fact that the SA node normally fires at rates greater than other pacemakers (60-100/minute) makes the SA node most often the dominant pacemaker.
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... also has the ability to initiate impulses, serving as a back-up in the case of SA nodalCommonly refers to the junction; for example, junctional rhythm is also known as nodal rhythm; may also refer to the sinoatrial node. 1. Six Second ECG Guidebook (2012), T Barill, p. 200 failure. The AV node significantly slows down the transmission of the electrical 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..., providing time for atrial kick prior to ventricular contraction.
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... carries the impulse from the AV node in 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 to 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... in the ventricle. The Bundle of His and the AV node, together called 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..., can serve as a pacemaker at 40-60 beats/minute.
The bundle branches and the 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 depolarization 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.. These electrical structures also can self-initiate impulses if necessary with typical rates of 20-40/minute. We refer to the bundle branches/Purkinje fibers as 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. Impulses that originate above the ventricles are associated with a narrow 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....
Lastly, 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.... is the product of three factors: intrinsicCharacteristic or property natural to the heart and its structures. 1. Six Second ECG Guidebook (2012), T Barill, p. 198 impulse formation, sympathetic, and parasympathetic stimulation. Sympathetic stimulationIschemia and sympathetic stimulation can enhance a ventricle’s automaticity, stimulating the ventricle to initiate an impulse before a sinus initiated wave reaches the ventricles. This solitary wave doesn’t ride the Autobahn. Rather, this one wave must traverse both ventricles. The... increases heart 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. and 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; whereas parasympathetic stimulation slows heart rate and reduces contractility. Measuring heart rate is a useful sign when assessing a person’s state of homeostasis.
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. 56