Cardiac arrhythmia is a group of conditions in which the muscle contraction of the heart is irregular or is faster or slower than normal. Cardiac dysrhythmia is technically more correct, as arrhythmia would imply that there is "no rhythm," but this term is not used frequently.
Some arrhythmias are life-threatening medical emergencies that can cause cardiac arrest and sudden death. Others cause aggravating symptoms, such as an awareness of a different heart beat, or palpitation, which can be annoying. Some are quite benign and normal. Sinus arrhythmia is the mild acceleration followed by slowing of the normal rhythm that occurs with breathing. In adults the normal heart rate ranges from 60 beats per minute to 100 beats per minute. The normal heart beat is controlled by a small area in the upper chamber of the heart called the sinoatrial node or sinus node. The sinus node contains specialized cells that have spontaneous electrical activity that starts each normal heart beat.
Frequency too high or too low
A heart rate faster than 100 beats/minute is considered a tachycardia. With exercise the sinus node increases its rate of electrical activity to accelerate the heart rate. The normal fast rate that develops is called sinus tachycardia. Arrhythmias that are due to fast, abnormal electrical activity can cause tachycardias that are dangerous. If the ventricles of the heart experiences one of these tachycardias for a long period of time, there can be deleterious effects. Individuals may sense a tachycardia as a pounding sensation of the heart, known as palpitations. If a tachycardia lowers blood pressure it may cause lightheadedness or dizziness, or even fainting (syncope). If the tachycardia is so fast that the heart can not function, it leads to death, which may occur suddenly.
Most tachycardias are not dangerous. Anything that increases adrenaline or its effects on the heart will increase the heart rate and potentially cause palpitations or tachycardias. Causes include stress, ingested or injected substances (ie: caffeine, alcohol (see Holiday heart syndrome), and an overactive thyroid gland (hyperthyroidism). 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. When it causes symptoms implantation of a permanent pacemaker may be needed.
Either dysrhythmia requires medical attention to evaluate the risks associated with the arrhythmia.
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 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 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 dysrhythmia can result.
Reentrant dysrhythmias 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 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 ECG recorded over a 24-hour period, to detect dysrhythmias that may happen briefly and unpredictably throughout the day.