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A cardiac arrhythmia is a condition where the normal heart rhythm is disrupted and a different rhythm or no rhythm at all is in its place. When this happens, the blood cannot travel around the body adequately and the patient has symptoms consistent with low blood pressure.

The heart rhythm is normally what's called a "normal sinus rhythm". It originates in the sinoatrial node, which is a spontaneous pacemaker and then travels down the septum of the heart to the atrioventricular node. From there, the heart electrical activity passes up along the outside of the heart and the heart beats or pumps. If anything is wrong with one of the heart's pacemakers, it results in a heart arrhythmia.

Doctors use an electrocardiogram or ECG in order to show the heart rhythm. It is a very accurate test for rhythm abnormalities of the heart. A continuous ECG in the form of a Holter monitor can tell if there is an arrhythmia that is not occurring all the time.

There are two types of arrhythmias: the first is a reentrant arrhythmia and the second is a non-reentrant arrhythmia. In reentrant arrhythmias, there is a signal that is repetitively propagated without provocation. The rhythm just circulates around the heart indefinitely. A non-reentrant arrhythmia has nothing to do with a repetitively propagated heart rhythm. Instead, there is blockage of one or more pacemaker. You can divide arrhythmias up into tachyarrhythmias or fast rhythms and bradyarrhythmias or slow rhythms. There can also be an absence of rhythm or asystole, in which there is no electrical activity traveling around the heart. The upper end of a normal rhythm is 100 beats per minute. The lower end of a normal heart rhythm is 60 beats per minute.

The different kinds of arrhythmia are as follows:

• AV heart block. This is a non-reentrant arrhythmia in which some of the sinus beats do not pass through to the AV node and beyond. They are blocked at the AV node. There can be a Wenckebach rhythm which results in an increased length of time between a SA rhythm beat and an AV nodal beat with a dropped beat resulting after two to four longer beats. In a regular AV block, only a certain amount of beats are passed through, such as one in two beats or one in three beats.
• Alternans arrhythmia. The duration of the beat and the shape of the beat on the ECG changes over time and alternates from one type of beat to another. The rate of a T wave alternans rhythm is about 110 beats per minute. There is an increased risk of sudden cardiac death.
• Parasystole. This occurs when the SA node beats at its own rate and a second ectopic pacemaker within the ventricles has its own rhythm. An abnormal ectopic beat occurs every once in a while when the ectopic pacemaker beats at the right time.
• Sinus tachycardia. This occurs when the sinus pacemaker is beating too fast at rest. It is usually due to underlying health conditions that cause the heart to beat too fast. It is not usually dangerous.
• Sinus bradycardia. This occurs when the heart beats to slowly, below 60 beats per minute. Unless it is very slow, it is not a dangerous type of rhythm.

Reentrant arrhythmias can be those that affect one chamber of the heart or all chambers of the heart. The rhythm spirals in a one directional ring around the heart muscle. Here are some reentrant arrhythmias:

Ventricular tachycardia. This is when the rhythm is very rapid and the electrical signal bypasses the pacemakers causing a wide complex rhythm that is generally very fast, up to 200 beats per minute. It can last longer than thirty seconds and be "sustained" or it can be unsustained. It is a dangerous rhythm because it doesn't send blood around to the heart very well and it can degenerate into a ventricular fibrillation, which is often fatal.

Ventricular fibrillation. This is life-threatening because no blood goes through the heart. The ventricles fibrillate with irregular electrical activity.

Atrial flutter. This is a reentrant supraventricular rhythm with rates of atrial flutter that approach 350 beats per minute. The ventricular rate is much slower than that because not all atrial flutter beats get past the AV node. It can degenerate into atrial fibrillation over time.

Atrial fibrillation. This is when there is no specific electrical activity in the SA node, which fibrillates the atria of the heart. The ventricles beat off the AV node in a slower rhythm. Sometimes an atrial fibrillation rhythm can be faster than a normal rhythm and this needs to be treated with heart rate lowering drugs.

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