Cardiac dysrhythmia#Re-entry
A circulating excitation represents a basal form of the disorder of impulse conduction in the heart dar. During a normal cardiac action comes every excitation by activation of ventricular void, because it is then surrounded only by refractory, so difficult, if not excitable tissue. Even within the Purkinje fibers, the excitation can propagate in both directions and goes off immediately when hit two pulses. If now, for example in a bundle to a aufzweigenden Purkinje fiber an anatomical or functional obstacle in front, can, before the operation at this point is completely blocked, a dangerous limbo occur: a unidirectional block.
The damaged area thereby slowing the passage of an excitation until it against the following refractory, so difficult, if not excitable, sometime. This excitation, the damaged area but now traversed in the opposite direction, as they reached it at a point where it no longer may be refractory. Is now the ( running in the opposite direction ) behind this retrograde or antidromic excitation remaining Refraktärstrecke shorter than the orbit, the excitation is not cleared within the circle and can permanently through it. One speaks of a circular excitation.
The danger lies in a re-entry ( " reentry ") of the excitation wavelength in the surrounding tissue, if this is no longer refractory. It can form a thrill, so to speak, becomes self-sustaining. This represents the cause of severe, concomitant with the risk of fibrillation tachycardic arrhythmias
Prerequisite
As a prerequisite applies according to the equation (see below) that the circular path must be longer than the corresponding Refraktärstrecke the potentially circular excitation. This is favored at reduced refractory period and decreased conduction velocity.
- Sr is the Refraktärstrecke which is proportional to the wavelength.
- Tr is the refractory period, which is proportional to the potential length.
- V is the line speed, which is dependent on the number of sodium channels.
Common sites for reentry phenomena
- Particularly the area around the AV range
- Accessory AV pathway (Wolff- Parkinson -White syndrome )
- Ventricular acute ischemic zones
- Marginal zones of all infarcts
Effect of antiarrhythmic drugs on circulating excitation
Class Ia antiarrhythmic drugs:
- Prolongation of the absolute refractory
- Greater reduction in conduction velocity
By increasing the likelihood of a reduced wavelength lambda Prophylactic long-term administration it comes to the increased probability less Reentrykreise ( to a minimum of 4 myocardial circumference) - proarrhythmic.
Class Ib antiarrhythmic drugs:
- Hardly change to decrease the absolute refractory
- Weak reduction of the conduction velocity
Preferred reducing the conduction velocity in partially depolarized tissue compared to healthy tissue (prototype lidocaine). A circulating excitation can be broken at the site of unidirectional block by its conversion into a bidirectional block without the conduction appreciably retard the healthy tissue - low proarrhytmisch.
Class Ic antiarrhythmic drugs:
- No extension of the absolute refractory
- Strong reduction of the conduction velocity
By greatly increasing the likelihood of a reduced wavelength lambda Prophylactic long-term administration, the probability of small Reentrykreise is increased most - strong proarrhytmisch (see CAST study - Cardiac Arrhythmia Suppression Trial).
Class III antiarrhythmic agents:
- Marked prolongation of the absolute refractory period
- Insignificant change in conduction velocity
Through increased refractory period, it will inevitably lead to longer wavelengths. Lambda can be larger, even larger than the final diameter of the heart - low proarrythmisch.