A cardiac arrhythmia, also called cardiac dysrhythmia, is a disturbance in the regular rhythm of the heartbeat. Several forms of cardiac arrhythmia are life-threatening and a medical emergency.
Frequency too high/low
A rhythm of the heart at a rate of more than 100 beats/minute is considered a tachycardia. If the ventricles of the heart sustain a tachycardia for a sustained period of time, there can be deleterious effects. Individuals may sense a tachycardia as a pounding sensation of the heart; this is known as "palpitations". However, strictly speaking, palpitations are any sensation of an individual's own heart beat, and can occur at rates less than 100 beats/minute.
The causes of tachycardias are numerous, and include stress, ingested or injected substances (ie: caffeine, alcohol, hyperthyroidism, or various drugs). Individuals who have a tachycardia are often advised to limit or remove exposure to any causative agent.
A slow rhythm, known as bradycardia (less than 60 beats/min), is usually not life threatening, but may cause symptoms.
Either arrhythmia requires medical attention to evaluate the risks associated with the arrhythmia.
A more serious variety of arrhythmia is known as fibrillation. Fibrillation occurs when the heart muscle begins a quivering motion instead of a normal, healthy pumping rhythm. Atrial fibrillation is the quivering, chaotic motion in the upper chambers of the heart, known as the atria. Atrial fibrillation is often due to serious underlying medical conditions, and should be evaluated by a physician. It is not typically a medical emergency.
If fibrillation occurs in the ventricles (lower chambers) of the heart, it is always a medical emergency. If left untreated, ventricular fibrillation can lead to death within minutes. When a heart goes into ventricular fibrillation, effective pumping of the blood stops. The individual goes into cardiac arrest, and will not survive unless cardiopulmonary resuscitation (CPR) and defibrillation are provided immediately.
CPR can prolong the survival of the heart muscle, but defibrillation is the intervention which is most likely to restore a more healthy heart rhythm. It does this by applying an electric shock to the heart, after which sometimes the heart will revert to a rhythm that can once again pump blood.
Almost every person goes into ventricular fibrillation in the last few minutes of life as the heart muscle reacts to diminished oxygen, blood flow or trauma or irritants.
Origin of impulse
When an electrical impulse begins in any part of the heart, it will spread throughout the myocardium and cause a contraction; see Electrical conduction system of the heart. Abnormal impulses can begin by one of two mechanisms: automaticity or reentry.
Automaticity refers to a cardiac muscle cell firing off an impulse on its own. Every cardiac cell has this potential: if it does not receive any impulses from elsewhere, its internal "pacemaker" will fire off an impulse after a certain amount of time. A single specialized location in the atria, the sinoatrial node, has a higher automaticity (a faster pacemaker) than the rest of the heart, and therefore is usually the one to start the heartbeat.
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 arrhythmias; 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 slightly different timing than usual and causes a 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 arrhythmia can result.
Reentrant arrhythmias occur when an electrical impulse travels in a circle within the heart, rather than moving outward 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 the impulse spreads through the heart quickly enough that each cell will only respond once, but if conduction is abnormally slow in some areas, part of the impulse will arrive late and will be treated as a new impulse, which can then spread backward. Depending on the timing, this can produce a sustained abnormal rhythm, such as atrial flutter, a self-limiting burst of supraventricular tachycardia, or the 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.
Cardiac arrhythmias are often first detected by simple but nonspecific means: auscultation of the heartbeat with a stethoscope, or feeling for peripheral cardiac arrest brought on by an arrhythmic episode. Victims of SADS are typically in adolescence or their early 20s, and were previously not diagnosed with any type of cardiac disease.
The most common causes of SADS are long QT syndrome, Brugada syndrome, and hypertrophic cardiomyopathy.
List of common cardiac arrhythmias
- Atrial Arrhythmias
- Ventricular Arrhythmias
- Heart Blocks
There are many classes of antiarrhythmic medications and many individual drugs within these classes. See the article on antiarrhythmic agents.
Arrhythmias are also treated electrically. Cardioversion is the application of electrical current across the chest wall to the heart and it is used for treatment of supraventricular or pulsed ventricular tachycardia. Defibrillation differs in that it is used for ventricular fibrillation and pulseless ventricular tachycardia, and more electricity is delivered with defibrillation than with cardioversion. In cardioversion, the recipient is either sedated or lightly anesthetized for the procedure. In defibrillation, the recipient has lost consciousness so there is no need for sedation.
Electrical treatment of arrhythmia includes cardiac pacing. Pacing is usually done for very slow heartbeats from drug overdose or myocardial infarction.
- SADS Foundation (http://www.sads.org/)
- Cardiac Risk in the Young (http://www.c-r-y.org.uk) (UK)