Figure 25-1 Laminar versus turbulent flow.
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Step 1: General Concepts in AV Nodal Reentry
Normally, the AV node has only one approach or tract receiving the impulse from the atrial myocardium. As mentioned in Chapter 21, Introduction to Junctional Rhythms, about 10% of patients have two tracts instead of one. The two tracts have their own distinct conduction properties, one conducting the current quickly, but having a much longer refractory period, and the other conducting the current much more slowly but having a shorter refractory period. Because of the differing conduction properties between the two tracts, one atrial depolarization wave could theoretically trigger two separate ventricular complexes. Luckily, this is not what happens in real life. Instead, the two tracts conduct the depolarization wave to the AV node in a very unique way. To help us understand how this process works, we will once again turn to our water model and use a simple analogy.
Suppose water were traveling down two separate channels (Figure 25-1). One of the channels is smooth and the water travels by laminar flow. The other channel is traversed with rocks and boulders, leading to the formation of turbulence. In which channel will the water travel faster?
Figure 25-1 Laminar versus turbulent flow.
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The answer is that smooth, laminar flow is much faster than turbulent flow. Laminar flow is faster because the water does not have anything holding it back or obstructing the flow (Figure 25-2). All the water is heading in the same direction and friction only occurs along the sides, where the water touches the channel walls. In turbulent flow, the water is constantly crashing into and around obstructions, which essentially slows the flow (Figure 25-3).
Figure 25-2 In laminar flow, all of the water is traveling in the same direction and the speed it accumulates is undisturbed.
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Figure 25-3 In turbulent flow, the water has to flow around various obstacles and is constantly crashing. Flow is much slower because the turbulence causes deceleration.
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Now let’s apply that concept to a real-life model. Suppose that you have a system of dry river channels like the one in Figure 25-4. The main channel splits into two branches that are exactly the same with the exception that one of the channels has some obstructions, which would cause turbulent flow if water were flowing through it. Can you predict the sequence of events if there were suddenly a flash flood?
Figure 25-4 Dry riverbed.
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First, water would travel by laminar flow until the branching point of the two channels (Figure 25-5). Then the water would proceed down the two branches. The smooth channel would continue to transport the water by direct laminar flow, and the water would travel very quickly. Let’s call this one the fast channel. The obstructed channel would begin to develop turbulent flow as water winds its way slowly around the rocks (Figure 25-6). Let’s call this channel the slow channel.
Figure 25-5 A flash flood appears.
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Figure 25-6 Water flows more quickly down the smooth “fast” channel than the obstructed “slow” channel.
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Now, what would happen when the water that traveled down the fast channel reaches the shallow, dry pond bed? The pond would begin to fill very quickly. Note that the pond is filling while water is still winding its way down the slow channel. What happens when the pond is completely filled?
When the pond is completely filled, the slow channel would begin to backfill with the water from the pond, which is quickly continuing to rise (Figure 25-7). The two wavefronts would travel along the slow channel until they crashed into each other (Figure 25-8). At that point, the water would stabilize and the water level would equilibrate according to the amount of water entering the system.
Figure 25-7 Once the pond is filled, the slow channel would begin to backfill.
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Figure 25-8 The two wavefronts traveling along the slow channel crash into each other, essentially canceling each other out.
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To recap, water entering the dry riverbed would rush downstream to the fork. There the water would spread evenly down the two channels. The flow down the smooth, fast channel would transport the water by direct laminar flow and would fill the pond first. Once the pond was filled, the water would begin to travel backward into the slow channel until the two wavefronts traveling along the slow channel met and canceled each other out.
This sequence of events is fairly intuitive and easy to understand. As we shall see, it is also identical to what happens during a normal cycle in many patients with dual approaches to the AV node. The depolarization wave travels down the fast channel, depolarizes the AV node, and begins to return retrogradely up the slow channel. There the two currents meet, essentially canceling each other out. Note that only the depolarization wave traveling down the fast pathway depolarizes the AV node and causes only one ventricular depolarization.
The sequence outlined here occurs 99.99999% of the time. Now, let’s move on to the other 0.00001% of the time. This very rare, but very real, scenario leads to the formation of a reentry circuit within the AV node and to the formation of AVNRT. Once again, we will turn to our water model to help simplify the process.
Suppose a tree were washed down by the rapid flow of the flash flood, and the tree propped itself right across the fast channel. What would happen to the flow of water then? Looking at Figure 25-9, we see that the flow of water would be forced down the slow channel. Eventually, the flow of water would reach the pond, fill it, and the water would begin to flow retrogradely up the fast channel until it reached the tree from the other side.
Figure 25-9 If the smooth channel is obstructed, the flow of water would be forced down the slower channel. The water would then fill the pond and flow retrogradely up the fast channel until it reached the bifurcation again.
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Let’s get back to the AV node. If an impulse hit the fast tract at a point when it was refractory (the tree blocking the way), the depolarization wave would have to travel down the slow pathway to depolarize the AV node. From here, the electric impulse would travel retrogradely back up the fast tract. However, this time, the wave will not be canceled out by one coming simultaneously down the tract. When the retrograde impulse reaches the bifurcation area (the area where the tree would be), it would find the slow tract ready to receive the impulse again (remember the slow tract has a very fast recovery time), and a reentry circuit is born. We will get back to this circuit in much more detail later in the chapter.
Keep the concepts introduced by these two analogies in mind as we continue to discuss the formation of the reentry circuit in the next section. These concepts are key to understanding the mechanisms involved in the formation of AV nodal reentry tachycardia.
Reentry is a very foreign concept and very, very difficult to understand. This analogy of water flowing down two channels can serve as a foundation for understanding the concept of AV nodal reentry. This model presented three critical factors needed in order to establish a reentry circuit. Namely:
Now let’s move on and take a look at what really happens in the heart.