Retrograde conduction

O Paroxysmal supraventricular tachycardia is caused by reentry that includes the AV node as a part of the reentrant circuit. Typically, electrical impulses travel forward (antegrade) down the AV node and then travel back up the AV node (retrograde) in a repetitive circuit. In some patients, the retrograde conduction pathway of the reentrant circuit may exist in extra-AV nodal tissue adjacent to the AV node. One of these pathways usually conducts impulses rapidly, while the other usually conducts impulses slowly. Most commonly, during PSVT the impulse conducts antegrade through the slow... 

In the undamaged myocardium, cardiac impulses travel rapidly antegrade through the Purkinje hbers to deliver the excitatory electrical impulse to the ventricular myocardium. During the normal activation sequence, retrograde conduction from ventricular myocardium to the conducting hbers is prevented by the longer duration of the membrane action potential and thus the refractory period in the Purkinje hbers. 

PSVT due to AV nodal reentry may occur by the following sequence of events The occurrence of an appropriately timed premature impulse penetrates the AV node but is blocked in the fast pathway that is still refractory from the previous beat. However, the slow pathway, which has a shorter refractory period, permits antegrade conduction of the premature impulse. By the time the impulse has reached the distal common pathway, the fast pathway has recovered its excitability and now will permit retrograde conduction. The impulse reaches the common proximal pathway, preceded by an excitable gap of tis-... 

Pacemaker-mediated tachycardia arises from retrograde conduction via the normal conduction system of the heart of a paced or intrinsic ventricniar event to the atrium. The tracking of the retrogradely conducted atrial complex must occur for PMT to occur. The successive ventricular-paced event that tracks this atrial complex also results in retrograde conduction to the atrinm. 

Another form of a PMT algorithm is the PVC PVARP extension. PMT most commonly is initiated by a PVC because atrial activity has not had the chance to enter the AV conduction system, making retrograde conduction much more likely. Therefore, a pacemaker may define a PVC as a ventricular-sensed event not preceded by an atrial event. When such a PVC occurs, the pacemaker automatically extends the PVARP in order to decrease the likelihood of sensing of a retrograde P wave. 

Concealed extrasystoles causing pseudo-AV block. (Look for associated unexpected sudden PR prolongation, combination of what appears to be type I and type II and isolated retrograde P-waves from retrograde conduction of the concealed extrasystole)... 

Fig. 19.19 Pacemaker output settings for lowest current drain and maximum longevity induction of an endless loop tachycardia during an atrial capture threshold test in a patient with a Pacesetter Affinity DR pacing system. The automatic PMT detection and termination algorithm recognized the tachycardia and promptly terminated it. Meanwhile, the recording provided details as to the tachycardia interval and the retrograde conduction interval providing guidance as to programming the PVARP.

Marco DD, Gallagher D, Noninvasive measurement of retrograde conduction times in pacemaker patients, PACE 1988 11 1673-1678. 

After the addition of the AEGM, it becomes clear that the tachycardia has a 1 1 association between atrium and ventricles (one P wave for every R wave). This still could be either an SVT or VT with 1 1 retrograde conduction. 

The time interval between an R and P wave during typical AVNRT is than that with retrograde conduction. 

May appear after QRS complex with retrograde conduction to atria... 

Originate from the His bundle or the AV junction. The impulse may activate the ventricles via the normal pathway. Alternatively the atria may also be activated by retrograde conduction. This is where the impulse travels back up towards the atria instead of down towards the ventricles activating the atria. Junctional pranature beats can be seen on the ECG as they occur before the next normal sinus beat. Often there is no P wave present if the atria and ventricles are depolarised simultaneously, as this is masked by the QRS complex (Fig. 6.3). Alternatively the P wave may be inverted (Fig. 6.4) and can occur either before, or after the QRS complex (Fig. 6.5). 

This is caused by the presence of an extra pathway of connection between the atria and the ventricles. This means that impulses generated in the SAN have the potential to travel back up through the extra pathway and depolarise the SAN. This is known as retrograde conduction. This can create a loop, sustaining the tachycardia. 

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