In re-entry, the impulse turns around in a loop or a circuit. This is also called a circus movement arrhythmia.
Mechanisms of Focal Arrhythmias:
There are several mechanisms that may induce focal arrhythmias: a. Enhanced Diastolic Depolarisation b. Triggered Activity
- early after depolarization
- delayed after depolarization
2. 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.
3. 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 new action potential.
Re-entry literally means ‘to re-enter’ or ‘to come back’. In this case, it means that the action potential in the heart ‘comes back’ or re-excites itself. It can only do that if it turns around and ‘bite in its own tail’, just like the fox in Firefox.
Because the path of the impulse is no longer straight but propagates in a circle, this type of re-entry is often also called ‘circus movement’ arrhythmia.
Note that if the impulse runs in a circle, it can only re-excite the cells in front of the advancing wave if those cells have recovered from the previous excitation. In other words, the refractory period of the cells now becomes very important.
If you remember your cellular electrophysiology, the refractory period consists of two parts:
the absolute refractory period
the relative refractory period
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.
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!
Types of Re-entrant Arrhythmias:
In general, there are two types of re-entrant arrhythmias:
The anatomical re-entry occurs around an obstacle, such as dead tissue, for example caused by an old infarct.
In the following movie, I have simulated the initiation of such an anatomical re-entry. I first start with a normal focal activity:
But, suppose that one of the two impulses was blocked at an earlier 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.
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’.