Types of Arrhythmias Characteristics Prevalence Demographics and Symptoms

Arrhythmias of the heart are any disorder of the orderly rhythmic beating initiated by the sinoatrial (SA) node (the normal pacemakers for the heart) such that the rate and/or rhythm is disturbed. Such disturbances of rate and rhythm occur as a result of electrophysiological perturbations of cardiac tissue that result in inappropriate changes in one, or more, of the inherent cardiac electrophysiological properties of automaticity, excitability, conduction, and refractoriness.

Arrhythmias are diagnosed primarily on the basis of electrocardiogram (ECG) findings although an initial diagnosis can be made on the basis of symptoms and heart rate. The generally accepted methods for classifying arrhythmias are based upon the anatomical site responsible for arrhythmia; the heart rate; the shape and nature of the ECG; as well as on other evidence.1

Simplistically, arrhythmias always have an inappropriate heart rate, and/or disorders of conduction. Increased heart rates are known as tachycardias while decreased heart rates are bradycardias. Conduction disturbances are either partial, or complete, and are often confined to particular cardiac anatomical sites. The morbidity and mortality associated with arrhythmias vary from the benign to the highly malignant, depending upon the type of arrhythmia. Some arrhythmias are essentially normal such as sinus arrhythmia where the heart rate increases and decreases with breathing. It is commonly seen in fit, healthy people. Others are abnormal, but not of any consequence, as with infrequent premature beats in atria or ventricles in mature adults. On the other hand, ventricular fibrillation is fatal (unless reverted) while atrial fibrillation carries a significant morbidity and mortality risk, especially in the elderly. The most common arrhythmias, based on their anatomical occurrence, are listed below.

6.33.1.1.1 Nodal arrhythmias

There are two anatomical and functional pacemaker nodes that control normal heart beating. These pacemakers can function independently of nerves, but their rates are modified by the autonomic nervous system in that they are accelerated by sympathetic nerve activity and slowed by parasympathetic nerves. The two nodes comprise the SA node, which normally dominates, and the atrioventricular (AV) node, which regulates impulse flow between atria and ventricles. This node assumes the role of the SA node if the latter fails. Disturbances in the nodes are responsible for the following conditions.

6.33.1.1.1.1 Sick sinus syndrome

Sick sinus syndrome is an arrhythmia in which the rate in the SA node is erratic, or nonexistent, and cannot be relied upon for regular beating.

6.33.1.1.1.2 Sinoatrial nodal tachycardia and bradycardia

In SA nodal tachycardia and bradycardia there is an inappropriately fast (tachycardia) or slow (bradycardia) rate originating in the SA node. These rate disturbances are usually due to inappropriate activity in cardiac nerves (sympathetic for tachycardia, and parasympathetic for bradycardia). Tachycardia can also be due to circulating hormones such as epinephrine and thyroid hormone.

6.33.1.1.1.3 Atrioventricular nodal disturbances

Failure of AV node conduction can result in partial or complete AV block. Such conduction failure results in a ventricular rate that is slower than the atrial rate.

6.33.1.1.1.4 Atrioventricular nodal or paroxysmal supraventricular nodal tachycardias

These are tachycardias that occur with certain AV nodal dysfunctions. They give rise to paroxysmal attacks that may be maintained (for hours) or, in many cases, last only for a few minutes. The term paroxysmal is used to indicate an abrupt onset and abrupt spontaneous termination.

6.33.1.1.2 Atrial arrhythmias

The atria are particularly subject to arrhythmias when age and disease have caused structural damage. Developments in atrial defibrillation have been recently reviewed.2

6.33.1.1.2.1 Premature atrial contractions

Premature atrial contractions (PACs) are extra atrial beats that appear occasionally between normal SA nodal beats. They can consist of only an occasional extra beat or many such extra beats. PACs are normally not very troublesome to the patient, but they can indicate the presence of some degree of atrial pathology, and be a harbinger of the more serious atrial tachycardia or atrial fibrillation.

6.33.1.1.2.2 Atrial tachycardia

Atrial tachycardia (AT), also sometimes called atrial flutter, is an arrhythmia in which the atria beat very rapidly at 300 beats min _ 1 or more. Both the atrial and ventricular rate in atrial tachycardia is regular, and the atrial component of the ECG often has a characteristic sawtooth pattern in the intervals between QRS complexes (Figure 1c). This arrhythmia can be chronic or transient. The high beating rate of the atria cannot be conducted on a 1:1 basis through the AV node, and so the ventricles fail to follow each atrial beat. As a result, there is always some degree of AV block with the ratio of atrial to ventricular beats of 2 up to 4:1. AT is compatible with life and may produce few symptoms, although there is an associated increase in morbidity and mortality.2

6.33.1.1.2.3 Atrial fibrillation

Atrial fibrillation can be initiated when the recurring wave of excitation that constitutes atrial tachycardia breaks into multiple wavelets and gives rise to highly disorganized atrial fibrillation. The irregular and incoherent waves of contractions in atrial fibrillation that pass over the surface of the atria have been described as having the appearance of a 'can of worms.' The limited ability of the AV node to successfully transmit all atrial impulses through to the ventricle means that only the occasional wavelet from the fibrillating atria successfully passes through the AV node to activate the ventricles. The result is a fast ventricular rate that has no discernable regularity. The maximum rate at which ventricles can beat is limited to less than 200 beats min_ 1. As a result, the ventricles can still fill, eject blood, and maintain cardiac output, but only to a limited extent. However, atrial fibrillation sometimes results in an inadequate cardiac output and this produces symptoms. The characteristics of atrial fibrillation on an ECG include a noisy, spiky baseline and the absence of atrial P waves (Figure 1d).

6.33.1.1.3 Ventricular arrhythmias

In an analogous manner to atria, ventricular arrhythmias include premature contractions, ventricular tachycardia (VT), and ventricular fibrillation (VF).

6.33.1.1.3.1 Ventricular tachycardia

Ventricular tachycardia occurs when the ventricles are not under the control of the SA node or the atria, but are beating rhythmically at a fast and inappropriate rate. If the AV node is removed by surgery or pathology, the ventricles will generally beat, but only at a much slower and dangerous rate (for example 30 beats min_ 1) since it is not always sufficient to maintain an adequate cardiac output. However, in most VT the ventricular rate is much faster than the SA node rate. Providing the ventricular rate is not too high (an upper limit of around 170 beats min _ 1), the ventricles will be able to fill and maintain an adequate cardiac output. At higher rates cardiac output, and hence blood pressure, can fall to catastrophically low levels. Furthermore, ventricular tachycardia can degenerate into ventricular fibrillation. This latter arrhythmia is fatal unless reverted within a few minutes. The ECG in ventricular tachycardia shows uniform repetitive cycles often at a constant wavelength (Figure 1e) and the usually narrow QRS complex of the ECG generally becomes much broader. This is because the QRS complex depends on the origin and direction of the wave of ventricular muscle depolarization sweeping repetitively across the ventricles. Usually the blood pressure during ventricular tachycardia is low, reflecting a low cardiac output.

6.33.1.1.3.2 Torsades de pointes

Torsades depointes is a special form ofVT. It most often occurs in the presence of a prolonged QT interval on the ECG, a slow heart rate, and/or disturbances in blood electrolyte balance. This can be an iatrogenic arrhythmia and due to drugs, or may be genetic in origin. It is a special form of VT in that it shows a cyclic pattern in which the wave heights of the QRS wave in the ECG vary in a progressive and repeating manner to produce an undulating pattern (Figure 1f), hence the French name, torsades de pointes. The best explanation for this pattern is a wave of depolarization (a rotor) moving around the heart. The arrhythmia is often self-terminating, but during its occurrence, syncope (fainting) can occur. Torsades can degenerate into ventricular fibrillation, and can be induced by drug therapy.

Diabetes Sustenance

Diabetes Sustenance

Get All The Support And Guidance You Need To Be A Success At Dealing With Diabetes The Healthy Way. This Book Is One Of The Most Valuable Resources In The World When It Comes To Learning How Nutritional Supplements Can Control Sugar Levels.

Get My Free Ebook


Post a comment