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Definition of a cardiac arrhythmia: Abnormal heart rhythm (simple!).
There are two types of arrhythmias; either the heart beats too fast or the heart beats too slow.
The ECG looks normal (normal P, QRS, T waves and normal PQ and ST intervals). The heart just beats too slowly.
Therapy: implantation of an artificial pacemaker that will stimulate the heart if the sinus node quits.
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There are two major mechanisms for inducing tachycardia’s:
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Focal activity means that an abnormal site is generating impulses. This site is often called “ectopic”, which really means outside the normal location (i.e., the sinus node).
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The site of ectopic activity can be located anywhere in the heart, in the right or left atria, in the right or left ventricle, in the AV-node etc.
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In re-entry, the impulse turns around in a loop or a circuit. This is also called a circus movement arrhythmia. These circuits can occur in the atria or in the ventricles.
There are several mechanisms that may induce focal arrhythmias:
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Enhanced Diastolic Depolarization:
This occurs in the SA-node, usually by excessive sympathetic activity. Because the depolarization is faster, the potential will reach threshold quicker and produce action potentials at a faster rate. An example of this mechanism is sinus tachycardia.
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Triggered activity: Early After Depolarization:
Sometimes, during the plateau phase of the action potential, a spontaneous depolarization may occur. This is often the case when there is too much calcium in the cell. These depolarisations may reach threshold and induce, too soon, a newaction potential:
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Triggered activity: Delayed After Depolarization:
This is similar to the early after depolarisations but this type of activity occur after full repolarisation has taken place, hence their name, delayed (or late) after depolarisations:
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If you remember your cellular electrophysiology, the refractory period consists of two parts:
the relative refractory period
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If the impulse propagates in a straight line, the absolute refractory period follows immediately after the depolarization. The relative refractory period occurs after the absolute refractory period.
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But in a circle, the depolarization may bite in its own tail, which may consist of relative refractory tissue. In fact, the circle cannot become ‘smaller’ than the (total) refractory period!
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In general, there are two types of re-entrant arrhythmias:
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The anatomical re-entry occurs around an obstacle, such as dead tissue, for example caused by an old infarct.
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But, suppose that one of the two impulses was blocked at an early stage? Then the other impulse can continue to propagate around this hole. In fact, it will continue forever! This is an arrhythmia!
Because the impulse is running around an obstacle, this is called an anatomical re-entry or an anatomical circus movement.
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And if there is no hole at all? Yes, you can still induce re-entry. But there is now no hole around which the impulse propagates. It now actually propagates around its own refractory tail! This is called ‘functional re-entry’.
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In the previous examples, there was only one circuit revolving either around a ‘hole’ (= anatomical re-entry) or around its own refractory tail (= functional re-entry).
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But what happens if there is more than one impulse revolving around in the heart?
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This is shown in the next simulation where 5-6 circus movements are simultaneously taking place.
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Obviously, in this situation, there is no longer any rhythm in the heart. Normal rhythm has now been replaced by chaos. This chaos is called ‘fibrillation’
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There has been a huge increase in therapies for treating cardiac arrhythmias; from all kind of drugs to electronic equipment.
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It is now also possible to treat patients and their arrhythmias by inducing blocks (dead tissue) or ‘destroying’ abnormal pacemaker’s area at specific locations in the heart muscles.
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Especially in the field of electronics, many types of pacemakers have been developed which, on demand, can intervene to stop a specific arrhythmia and restore sinus rhythm.
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But the ultimate device is of course the defibrillator! This device has new become available throughout society, as AED (=automatic external defibrillator) in multiple locations such as shopping centres etc.
