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===Triggered beats===
===Triggered beats===
Triggered beats occur when problems at the level of the ion channels in individual heart cells result in abnormal propagation of electrical activity and can lead to sustained abnormal rhythm. They are relatively rare and can result from the action of anti-arrhythmic drugs. See early and delayed [[Afterdepolarization]]s.
Triggered beats occur when problems at the level of the ion channels in individual heart cells result in abnormal propagation of electrical activity and can lead to sustained abnormal rhythm. They are relatively rare and can result from the action of anti-arrhythmic drugs. See early and delayed [[Afterdepolarization]]s.

===Histamine response===
Histamine modifies the heart rhythm and is responsible for many cardiac arrhythmias. The major arrhythmogenic actions of histamine are an H1-receptor-mediated slowing of atrioventricular conduction and H2-receptor-mediated changes in sinus rate and ventricular automaticity.<ref>{{cite journal |author=Andrew A. Wolff and Roberto Levi |title=Histamine and Cardiac Arrhythmias |journal=Circulation Research |volume=58 |issue=1 |date=January 1986 |url=http://circres.ahajournals.org/content/58/1/1.full.pdf }}</ref> Also, some antihistamines appear to increase the risk of dysrhythmias [http://circ.ahajournals.org/content/122/14/1426.long].


==Diagnostic approach==
==Diagnostic approach==

Revision as of 06:58, 12 January 2015

"Arrhythmia" redirects here. For the novel by Alice Zorn, see Arrhythmia (novel).
Not to be confused with Erythema.
Arrhythmia
SpecialtyCardiology, rhythmology Edit this on Wikidata

Cardiac dysrhythmia (also known as arrhythmia or irregular heartbeat) is any of a group of conditions in which the electrical activity of the heart is irregular, faster, or slower than normal.[1] The heartbeat may in fact be too fast or too slow, or may follow an irregular rhythm. A heart beat that is too fast - by convention above 100 beats per minute in human adults - is called tachycardia, and a heart beat that is too slow - conventionally below 60 beats per minute - is called bradycardia. Although many arrhythmias are not life-threatening, some can cause serious impairment of the cardiac function, up to the extreme of a cardiac arrest.[2]

Arrhythmias can occur in the upper chambers of the heart (atria), in the lower chambers of the heart (ventricles) or at the level of the atrioventricular junction. Arrhythmias may occur at any age.

Cardiac arrhythmias are one of the most common causes of death in western countries, specifically, of sudden death [3] in particular for patients brought in by ambulance to a hospital. Other arrhythmias cause symptoms such as awareness of an abnormal heart beat (palpitations) and may be merely uncomfortable. Still others, such as atrial fibrillation, may not be associated with any symptoms at all, or to vague symptoms only, but may predispose the patient to potentially life-threatening stroke or embolism. Complex arrhythmias have also been observed in patients carrying cardiac pacemakers/defibrillators due to wire faults, and other technical or mechanical issues affecting the device itself.

The term sinus arrhythmia [4] refers to a normal phenomenon of alternating mild acceleration and slowing of the heart rate that occurs with breathing in and out. It is usually quite pronounced in children and steadily decreases with age. This can also be present during meditation breathing exercises that involve deep inhaling and breath holding patterns.[5]

Proarrhythmia is a term used to indicate a new arrhythmia or the more frequent occurrence of pre-existing arrhythmias, paradoxically precipitated by antiarrhythmic therapy, which means it is a side effect associated with the administration of some existing antiarrhythmic drugs, as well as drugs for other indications. In other words, it is a tendency of antiarrhythmic drugs to facilitate emergence of new arrhythmias. Some arrhythmias are minor and can be regarded as normal variants. In fact, most people will on occasion feel their heart skip a beat or give an occasional extra strong beat; neither of these is usually a cause for alarm.[6]

Classification

Arrhythmia may be classified by rate (tachycardia, bradycardia), mechanism (automaticity, reentry, triggered) or duration (isolated premature beats; couplets; runs, that is 3 or more beats; non-sustained= less then 30 seconds or sustained= over 30 seconds).

It is also appropriate to classify by site of origin:

Atrial

Junctional arrhythmias

Ventricular

Heart blocks

These are also known as AV blocks, because the vast majority of them arise from pathology at the atrioventricular node. They are the most common causes of bradycardia:

First, second and third degree block also can occur at the level of the sinoatrial junction. This is referred to as sinoatrial block typically manifesting with various degrees and patterns of sinus bradycardia

Sudden arrhythmic death syndrome

Sudden arrhythmic death syndrome (SADS), is a term used as part of sudden unexpected death syndrome to describe sudden death due to cardiac arrest brought on by an arrhythmia in the presence or absence of any structural heart disease on autopsy. The most common cause of sudden death in the US is coronary artery disease specifically because of poor oxygenation of the heart muscle, that is myocardial ischemia or a heart attack [7] Approximately 180,000 to 250,000 people die suddenly of this cause every year in the US. SADS may occur from other causes. There are many inherited conditions and heart diseases that can affect young people which can subsequently cause sudden death without advance symptoms.[8]

Causes of SADS in young people include viral myocarditis, long QT syndrome, Brugada syndrome, Catecholaminergic polymorphic ventricular tachycardia, hypertrophic cardiomyopathy and arrhythmogenic right ventricular dysplasia.[9][10]

Signs and symptoms

The term cardiac arrhythmia covers a very large number of very different conditions.

The most common symptom of arrhythmia is an abnormal awareness of heartbeat, called palpitations. These may be infrequent, frequent, or continuous. Some of these arrhythmias are harmless (though distracting for patients) but many of them predispose to adverse outcomes.

Some arrhythmias do not cause symptoms, and are not associated with increased mortality. However, some asymptomatic arrhythmias are associated with adverse events. Examples include a higher risk of blood clotting within the heart and a higher risk of insufficient blood being transported to the heart because of weak heartbeat. Other increased risks are of embolisation and stroke, heart failure and sudden cardiac death.

If an arrhythmia results in a heartbeat that is too fast, too slow or too weak to supply the body's needs, this manifests as a lower blood pressure and may cause lightheadedness or dizziness, or syncope (fainting).

Some types of arrhythmia result in cardiac arrest, or sudden death.

Medical assessment of the abnormality using an electrocardiogram is one way to diagnose and assess the risk of any given arrhythmia.

Differential diagnosis

Normal electrical activity

Main article: Electrical conduction system of the heart

Each heart beat originates as an electrical impulse from a small area of tissue in the right atrium of the heart called the sinus node or Sino-atrial node or SA node. The impulse initially causes both atria to contract, then activates the atrioventricular (or AV) node, which is normally the only electrical connection between the atria and the ventricles (main pumping chambers). The impulse then spreads through both ventricles via the Bundle of His and the Purkinje fibres causing a synchronised contraction of the heart muscle and, thus, the pulse.

In adults the normal resting heart rate ranges from 60 to 80 beats per minute. The resting heart rate in children is much faster. In athletes, however, the resting heart rate can be as slow as 40 beats per minute, and be considered as normal.

Bradycardias

Normal sinus rhythm, with solid black arrows pointing to normal P waves representative of normal sinus node function, followed by a pause in sinus node activity (resulting in a transient loss of heart beats). Note that the P wave that disrupts the pause (indicated by the dashed arrow) does not look like the previous (normal) P waves — this last P wave is arising from a different part of the atrium, representing an escape rhythm.

A slow rhythm (less than 60 beats/min), is labelled bradycardia. This may be caused by a slowed signal from the sinus node (sinus bradycardia), a pause in the normal activity of the sinus node (sinus arrest), or by blocking of the electrical impulse on its way from the atria to the ventricles (AV block or heart block). Heart block comes in varying degrees and severity. It may be caused by reversible poisoning of the AV node (with drugs that impair conduction) or by irreversible damage to the node. Bradycardias may also be present in the normally functioning heart of endurance athletes or other well-conditioned persons.

Tachycardias

In adults and children over 15, resting heart rate faster than 100 beats/minute is labelled tachycardia. Tachycardia may result in palpitation; however, tachycardia is not necessarily an arrhythmia. Increased heart rate is a normal response to physical exercise or emotional stress. This is mediated by the sympathetic nervous system on the sinus node and called sinus tachycardia. Other conditions that increase sympathetic nervous system activity in the heart include ingested or injected substances, such as caffeine or amphetamines, and an overactive thyroid gland (hyperthyroidism) or anemia.

Tachycardia that is not sinus tachycardia usually results from the addition of abnormal impulses to the normal cardiac cycle. Abnormal impulses can begin by one of three mechanisms: automaticity, re-entry or triggered activity. A specialised form of re-entry which is both common and problematic is termed fibrillation.

Although the term "tachycardia" is known over one hundred years, basis for the classification of arrhythmias are still being discussed.[11]

Heart defects

Congenital heart defects are structural or electrical pathway problems in the heart that are present at birth. Anyone can be affected with this because overall health does not play a role in the problem. Problems with the electrical pathway of the heart can cause very fast or even deadly arrhythmias. Wolff-Parkinson-White syndrome is due to an extra pathway in the heart that is made up of electrical muscle tissue. This tissue allows the electrical impulse, which stimulates the heartbeat, to happen very rapidly. Right Ventricular outflow tract Tachycardia is the most common type of ventricular tachycardia in otherwise healthy individuals. This defect is due to an electrical node in the right ventricle just before the pulmonary artery. When the node is stimulated, the patient will go into ventricular tachycardia, which does not allow the heart to fill with blood before beating again. Long QT Syndrome is another complex problem in the heart and has been labeled as an independent factor in mortality. There are multiple methods of treatment for these including cardiac ablations, medication treatment, or altering your lifestyle to have less stress and exercise. It is possible to live a full and happy life with these conditions.

Automaticity

Automaticity refers to a cardiac muscle cell firing off an impulse on its own. All of the cells in the heart have the ability to initiate an action potential; however, only some of these cells are designed to routinely trigger heart beats. These cells are found in the conduction system of the heart and include the SA node, AV node, Bundle of His and Purkinje fibers. The sinoatrial node is a single specialized location in the atrium that has a higher automaticity (a faster pacemaker) than the rest of the heart and, therefore, is usually responsible for setting the heart rate and initiating each heart beat.

Any part of the heart that initiates an impulse without waiting for the sinoatrial node is called an ectopic focus and is, by definition, a pathological phenomenon. This may cause a single premature beat now and then, or, if the ectopic focus fires more often than the sinoatrial node, it can produce a sustained abnormal rhythm. Rhythms produced by an ectopic focus in the atria, or by the atrioventricular node, are the least dangerous dysrhythmias; but they can still produce a decrease in the heart's pumping efficiency, because the signal reaches the various parts of the heart muscle with different timing than usual and can be responsible for poorly coordinated contraction.

Conditions that increase automaticity include sympathetic nervous system stimulation and hypoxia. The resulting heart rhythm depends on where the first signal begins: If it is the sinoatrial node, the rhythm remains normal but rapid; if it is an ectopic focus, many types of dysrhythmia may ensue.

Re-entry

Re-entrant arrhythmias occur when an electrical impulse recurrently travels in a tight circle within the heart, rather than moving from one end of the heart to the other and then stopping.[12][13]

Every cardiac cell is able to transmit impulses of excitation in every direction but will do so only once within a short time. Normally, the action potential impulse will spread through the heart quickly enough that each cell will respond only once. However, if there is some essential heterogeneity of refractory period or if conduction is abnormally slow in some areas (for example in heart damage) so the myocardial cells are unable to activate the fast sodium channel, part of the impulse will arrive late and potentially be treated as a new impulse. Depending on the timing, this can produce a sustained abnormal circuit rhythm.

As a sort of re-entry, the vortices of excitation in the myocardium (autowave vortices) is considered to be the main mechanism of life-threatening cardiac arrhythmias.[2] In particular, the autowave reverberator is a typical in thin walls of the atria, with the atrial flutter producing. Re-entry are also responsible for most paroxysmal supraventricular tachycardia, and dangerous ventricular tachycardia. These types of re-entry circuits are different from WPW syndromes in which the real pathways existed.

Although omega-3 fatty acids from fish oil can be protective against arrhythmias, in the case of re-entrant arrhythmias, fish oil can worsen the arrhythmia.[14]

Fibrillation

When an entire chamber of the heart is involved in a multiple micro-reentry circuits and, therefore, quivering with chaotic electrical impulses, it is said to be in fibrillation.

Fibrillation can affect the atrium (atrial fibrillation) or the ventricle (ventricular fibrillation); ventricular fibrillation is imminently life-threatening.

  • Atrial fibrillation affects the upper chambers of the heart, known as the atria. Atrial fibrillation may be due to serious underlying medical conditions and should be evaluated by a physician. It is not typically a medical emergency.
  • Ventricular fibrillation occurs in the ventricles (lower chambers) of the heart; it is always a medical emergency. If left untreated, ventricular fibrillation (VF, or V-fib) can lead to death within minutes. When a heart goes into V-fib, effective pumping of the blood stops. V-fib is considered a form of cardiac arrest. An individual suffering from it will not survive unless cardiopulmonary resuscitation (CPR) and defibrillation are provided immediately.

CPR can prolong the survival of the brain in the lack of a normal pulse, but defibrillation is the only intervention that can restore a healthy heart rhythm. Defibrillation is performed by applying an electric shock to the heart, which resets the cells, permitting a normal beat to re-establish itself.

Triggered beats

Triggered beats occur when problems at the level of the ion channels in individual heart cells result in abnormal propagation of electrical activity and can lead to sustained abnormal rhythm. They are relatively rare and can result from the action of anti-arrhythmic drugs. See early and delayed Afterdepolarizations.

Histamine response

Histamine modifies the heart rhythm and is responsible for many cardiac arrhythmias. The major arrhythmogenic actions of histamine are an H1-receptor-mediated slowing of atrioventricular conduction and H2-receptor-mediated changes in sinus rate and ventricular automaticity.[15] Also, some antihistamines appear to increase the risk of dysrhythmias [1].

Diagnostic approach

Cardiac dysrhythmias are often first detected by simple but nonspecific means: auscultation of the heartbeat with a stethoscope, or feeling for peripheral pulses. These cannot usually diagnose specific dysrhythmias but can give a general indication of the heart rate and whether it is regular or irregular. Not all the electrical impulses of the heart produce audible or palpable beats; in many cardiac arrhythmias, the premature or abnormal beats do not produce an effective pumping action and are experienced as "skipped" beats.

The simplest specific diagnostic test for assessment of heart rhythm is the electrocardiogram (abbreviated ECG or EKG). A Holter monitor is an EKG recorded over a 24-hour period, to detect dysrhythmias that may happen briefly and unpredictably throughout the day.

A more advanced study of the heart's electrical activity can be performed to assess the source of the aberrant heart beats. This can be accomplished in an Electrophysiology study. A minimally invasive procedure that uses a catheter to "listen" to the electrical activity from within the heart, additionally if the source of the arrhythmias is found, often the abnormal cells can be ablated and the arrhythmia can be permanently corrected.

Management

The method of cardiac rhythm management depends firstly on whether or not the affected person is stable or unstable. Treatments may include physical maneuvers, medications, electricity conversion, or electro- or cryo-cautery.

Physical maneuvers

A number of physical acts can increase parasympathetic nervous supply to the heart, resulting in blocking of electrical conduction through the AV node. This can slow down or stop a number of arrhythmias that originate above or at the AV node (see main article: supraventricular tachycardias). Parasympathetic nervous supply to the heart is via the vagus nerve, and these maneuvers are collectively known as vagal maneuvers.

Antiarrhythmic drugs

Main article: Antiarrhythmic agents

There are many classes of antiarrhythmic medications, with different mechanisms of action and many different individual drugs within these classes. Although the goal of drug therapy is to prevent arrhythmia, nearly every antiarrhythmic drug has the potential to act as a pro-arrhythmic, and so must be carefully selected and used under medical supervision.

Other drugs

A number of other drugs can be useful in cardiac arrhythmias.

Several groups of drugs slow conduction through the heart, without actually preventing an arrhythmia. These drugs can be used to "rate control" a fast rhythm and make it physically tolerable for the patient.

Some arrhythmias promote blood clotting within the heart, and increase risk of embolus and stroke. Anticoagulant medications such as warfarin and heparins, and anti-platelet drugs such as aspirin can reduce the risk of clotting.

Electricity

Dysrhythmias may also be treated electrically, by applying a shock across the heart — either externally to the chest wall, or internally to the heart via implanted electrodes.

Cardioversion is either achieved pharmacologically or via the application of a shock synchronised to the underlying heartbeat. It is used for treatment of supraventricular tachycardias. In elective cardioversion, the recipient is usually sedated or lightly anesthetized for the procedure.

Defibrillation differs in that the shock is not synchronised. It is needed for the chaotic rhythm of ventricular fibrillation and is also used for pulseless ventricular tachycardia. Often, more electricity is required for defibrillation than for cardioversion. In most defibrillation, the recipient has lost consciousness so there is no need for sedation.

Defibrillation or cardioversion may be accomplished by an implantable cardioverter-defibrillator (ICD).

Electrical treatment of dysrhythmia also includes cardiac pacing. Temporary pacing may be necessary for reversible causes of very slow heartbeats, or bradycardia, (for example, from drug overdose or myocardial infarction). A permanent pacemaker may be placed in situations where the bradycardia is not expected to recover.

Electrical cautery

Some cardiologists further sub-specialise into electrophysiology. In specialised catheter laboratories, they use fine probes inserted through the blood vessels to map electrical activity from within the heart. This allows abnormal areas of conduction to be located very accurately, and subsequently destroyed with heat, cold, electrical, or laser probes.

This pulmonary vein isolation may be completely curative for AV nodal reentrant tachycardia and sometimes for atrial fibrillation, but for other forms of arrhythmia the success rate remains disappointing.

Research

Dysrhythmias due to medications have been reported since the 1920s with the use of quinine.[16] In the 1960s and 1970s problems with antihistamines and antipsychotics were discovered.[16] It was not until the 1980s that the underlying issue, QTc prolongation was determined.[16]

References

  1. ^ Cardiac Arrhythmia - All About It
  2. ^ a b Mandel, William J., ed. (1995). Cardiac Arrhythmias: Their Mechanisms, Diagnosis, and Management (3 ed.). Lippincott Williams & Wilkins. ISBN 978-0-397-51185-3.
  3. ^ Mehra R. Global public health problem of sudden cardiac death. Journal of Electrocardiology. Nov-Dec 2007;40(6 Suppl):S118-22.
  4. ^ F1, Hayano J. Respiratory sinus arrhythmia: why does the heartbeat synchronize with respiratory rhythm? Chest. 2004 Feb;125(2):683-90.
  5. ^ http://dx.doi.org/10.1016/j.ijcard.2009.06.058
  6. ^ familydoctor.org[full citation needed]
  7. ^ Douglas P. Zipes, MD; Hein J. J. Wellens, MD Sudden Cardiac Death. Circulation. 1998; 98: 2334-2351 doi: 10.1161/01.CIR.98.21.2334
  8. ^ Deo R, Albert CM. Epidemiology and genetics of sudden cardiac death. Circulation 2012;125:620–37.
  9. ^ Chugh, Sumeet S.; Reinier, Kyndaron; Teodorescu, Carmen; Evanado, Audrey; Kehr, Elizabeth; Al Samara, Mershed; Mariani, Ronald; Gunson, Karen; Jui, Jonathan (2008). "Epidemiology of Sudden Cardiac Death: Clinical and Research Implications". Progress in Cardiovascular Diseases. 51 (3): 213–28. doi:10.1016/j.pcad.2008.06.003. PMC 2621010. PMID 19026856.
  10. ^ Winkel BG, Holst AG, Theilade J, et al. Nationwide study of sudden cardiac death in persons aged 1–35 years. Eur Heart J 2011;32:983–90.
  11. ^ Moskalenko, A. (2012). "Tachycardia as "Shadow Play"". In Yamada, Takumi (ed.). Tachycardia. Croatia: InTech. pp. 97–122. ISBN 978-953-51-0413-1. {{cite book}}: Cite has empty unknown parameter: |trans_chapter= (help); External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
  12. ^ Wiener, Norbert; Rosenblueth, Arturo (1946). "The mathematical formulation of the problem of conduction of impulses in a network of connected excitable elements, specifically in cardiac muscle". Archivos del Instituto de Cardiología de México. 16 (3): 205–65. PMID 20245817.
  13. ^ Allessie, M. A.; Bonke, F. I.; Schopman, F. J. (1976). "Circus movement in rabbit atrial muscle as a mechanism of tachycardia. II. The role of nonuniform recovery of excitability in the occurrence of unidirectional block, as studied with multiple microelectrodes". Circulation Research. 39 (2): 168–77. doi:10.1161/01.RES.39.2.168. PMID 939001.
  14. ^ Denruijter, H; Berecki, G; Opthof, T; Verkerk, A; Zock, P; Coronel, R (2007). "Pro- and antiarrhythmic properties of a diet rich in fish oil". Cardiovascular Research. 73 (2): 316–25. doi:10.1016/j.cardiores.2006.06.014. PMID 16859661.
  15. ^ Andrew A. Wolff and Roberto Levi (January 1986). "Histamine and Cardiac Arrhythmias" (PDF). Circulation Research. 58 (1).
  16. ^ a b c Heist, EK; Ruskin, JN (5 October 2010). "Drug-induced arrhythmia". Circulation. 122 (14): 1426–35. PMID 20921449.
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