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Cardiac Arrhythmias - AV Nodal Reentry Tachycardia

AVNRT

          AV Nodal Reentrant Tachycardia is one of the most common forms of tachycardias that originates in the upper chambers of the heart. AVNRT is a reentry tachycardia that is contained within the AV node. In many people, the AV node has two pathways, one faster pathway along with a secondary slower pathway. These pathways must connect with each other at the top and bottom of the AV node. If two pathways are present, it is referred to as dual nodal physiology.

          The pathway that has a faster conduction speed takes longer to recover to the point where it can accept a new signal. The slower pathway recovers more quickly and can accept a new signal ealier than the pathway that conducts faster. If an early signal happens in the time period when the fast pathway is not ready to recieve a new signal but the slow pathway is, then the early signal will travel down the slow pathway only. When the early signal reaches the lower connection, it is possible that the faster pathway is now ready to conduct a signal. If this happens and the fast pathway has the ability to conduct a signal back against the normal direction of conduction, the early signal will split with part of it travelling down into the ventricles along the normal conduction paths. The other part of the signal will return backwards, or retrograde, up the fast pathway. When this signal reaches the top of the node where the upper connection between the two pathways is located, it will travel back down the slow pathway while part of the signal also conducts back up into the atrium. The circuit is now in a loop that will continue until something changes and the circuit is interrupted.

Understanding one of the most common Arryhthmias

          Atrial-Ventricular Nodal Reentry Tachycardia (AVNRT) is one of the most frequent abnormal rhythms encountered in the EP lab. This is also a great one to sharpen your electrogram interpretation skills on. When starting out on the process of learning to read the language of EGM's, pattern recognition is often the first step taken. AVNRT provides us with a great opportunity to utilize pattern recognition skills.

          The description above explains how a premature atrial depolarization may initiate AVNRT when dual nodal physiology is present. Because the reentry circuit is contained within the direct region of the AV node, most of the electrograms fall into a single line as if all the electrodes were picking up electrical activity at the same time. The recordings below will show the various steps in identifying AVNRT.

AH pre jump

          One of the key indicators of AV Nodal Reentry Tachycardia is the AH jump. An AH jump is considered to be present if the AH interval increases by 50ms or more with a reduction of the extra-stimulus interval of 10-20ms.

          In this case, we have programmed stimulation of the RA at 600ms followed by an extra-stimuli at 300ms. The AH interval measures at 146ms.

AH post jump

          On the next pacing train, the extra stimulus is entered at an interval of 290ms, 10ms less than the previous train.

          The AH for this run of programmed stimulation measures out to be 211ms, 61ms more than the previous pacing train. This jump is considered to be positive proof of dual nodal physiology.

         The AH jump is one of the key pieces of evidence for patients who have AVNRT. While the jump is not always over 50ms (as shown in the case below), the presence of a jump does confirm dual nodal physiology. A good analogy for the AH jump is a freeway with an overpass that crosses above another road. Under normal conditions, traffic moves faster passing over the bridge that it would if the cars exited the freeway by way of the off ramp, crossed the road and then reentered the freeway using the on ramp.

         Now let's assume that some careless driver tries to cut in front of another driver. This unexpected event results in an accident that blocks the lanes over the bridge. For traffic to get through, the vehicles must use the longer route comprised of the offramp, crossing the road below the bridge and returning to the freeway by way of the onramp. The cars travel a similar distance, but delays such as waiting for the light to cross and the decreased speed limit on the ramps causes the vehicles to arrive at the far side of the bridge later than they normally would.

        The following images were all recorded from a single procedure. They show a jump that is close to the 50ms interval with a decrease of only 10ms and then tachycardia occurs.

          In the image shown here, an extra stimulus has been added at 320ms post the primary pacing train. The AH is measured at 159ms.

          The next extra stimulus is delivered at 310ms. This results in a jump in the AH interval of 45ms. This is a good indication that dual nodal physiology may be present.

          Two stim trains later the extra stimulus is being delivered at 290ms and tachycardia is induced.

          Note how the A and V signals on both the His and CS catheters appears to be fused into one common signal. All the signals from these catheters appear to "line up" at the same point in time.

 

          Because the circuit of the reentry loop occurs within the immediate proximity of the AV node, atrial and ventricular activation appear to happen simultaneously.

          Only the High Right Atrial electrograms appear to be a bit delayed. Everything else appears to line up with the ventricular depolarization. The P wave which indicates the smaller atrial depolarization is hidden within the QRS. It is possible that a small retrograde P wave may be visualized in the trailing aspect of the QRS.

          Here is the 12 lead surface electrocardiogram from the patient shown above. Note how the P wave is buried within the QRS and is only visible in some leads. (aVF)
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