Cardiac arrhythmia (also dysrhythmia) is a term for any of a large and heterogeneous group of conditions in which there is abnormal electrical activity in the heart. The heart beat may be too fast or too slow, and may be regular or irregular. Some arrhythmias are life-threatening medical emergencies that can result in cardiac arrest and sudden death. Others cause aggravating symptoms such as an abnormal awareness of heart beat (palpitations), and may be merely annoying. Others may not be associated with any symptoms at all, but pre-dispose toward potentially life threatening stroke or embolus.
Subscribe in a readerThe term cardiac arrhythmia covers a very large number of very different conditions, many of which receive separate articles in depth elsewhere in Wikipedia. The most common symptom of arrhythmia is an abnormal awareness of heartbeat, termed palpitations. These may be infrequent, frequent, or continuous. Some of these arrhythmias are harmless (though annoying) 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 increase in risk of blood clotting within the heart, and also an insufficient amount of blood is transported to the heart beacause of weak heart beat, and thus increase the risk of embolisation and stroke, or increase in the risk of heart failure, or increase in the risk of sudden cardiac death.
Some arrhythmias are very minor and can be regarded as variants of normal. In fact, most people will sometimes feel their heart skip a beat, or give an occasional extra strong beat - neither of which is usually a cause for alarm. The term sinus arrhythmia refers to a normal phenomenon of mild acceleration and slowing of the heart rate that occurs with breathing in and out. It is usually quite pronounced in children, and steadily lessens with age.
If an arrhythmia results in a heart beat 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 fainting. Some types of arrhythmia result in cardiac arrest, or sudden death. Medical assessment of the abnormality using an electrocardiogram is the best way to diagnose and assess the risk of any given arrhythmia. Mechanisms and aetiology 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 of the atria to contract, then activates the atrioventricular (or AV) node which is normally the only electrical connection between the atria and the ventricles or main pumping chambers. The impulse then spreads through both ventricles via the His 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 100 beats per minute. The resting heart rate in children is much faster. Bradycardias A slow rhythm, (less than 60 beats/min), is labelled bradycardia. This may be caused by a slowed signal from the sinus node (termed sinus bradycardia), a pause in the normal activity of the sinus node (termed sinus arrest), or by blocking of the electrical impulse on its way from the atria to the ventricles (termed 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. Tachycardias Any 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 is called sinus tachycardia. Other things 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). 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, reentry or triggered activity. A specialised form of re-entry problem is termed fibrillation. 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 which 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 to 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 can result. Re-entry Re-entry dysrhythmias 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. Every cardiac cell is able to transmit impulses in every direction, but will only do so once within a short period of time. Normally an action potential impulse will spread through the heart quickly enough that each cell will only respond once. However, if conduction is abnormally slow in some areas, 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. Re-entry circuits are responsible for atrial flutter, most paroxysmal supraventricular tachycardia, and dangerous ventricular tachycardia. By analogy, imagine a room full of people all given these instructions: "If you see anyone starting to stand up, then stand up for three seconds and sit back down." If the people are quick enough to respond, the first person to stand will trigger a single wave which will then die out; but if there are stragglers on one side of the room, people who have already sat down will see them and start a second wave, and so on. 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, and 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 which 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, but can result from the action of anti-arrhythmic drugs.
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