We have mentioned on previous occasions that the AV node functions as the gatekeeper for the ventricles. This function is very obvious in atrial flutter. Normally, the AV node senses the atrial depolarization, holds the impulse for a few milliseconds, and then allows the depolarization wave to proceed forward to stimulate the ventricles. In atrial flutter, and in many supraventricular tachycardias, this gatekeeping function is extended to include a selective transmission of impulses in order to not overwhelm the ventricles with a very fast, very dangerous tachycardia.
Under normal, nontachycardic atrial rates, the AV node transmits the impulses at a 1:1 (1 to 1) conduction rate. The first number before the colon sign is the number of atrial complexes that are occurring. The number after the colon sign refers to the number of impulses conducted to the ventricles to allow ventricular depolarization to occur. Putting it all together, this type of notation refers to the number of atrial depolarizations that have to occur for the AV node to allow conduction through its tissues to allow one ventricular contraction. A number of 2:1 refers to 2 atrial contractions to 1 ventricular contraction; 3:1 means 3 atrial contractions to 1 ventricular contraction, and so on. You can also have other rates of conduction through the AV node. For example, a rate of 3:2 conduction refers to 3 atrial contractions for 2 ventricular contractions. As you can see, there are many possible combinations.
Now, we need to clarify a very important point in electrocardiographic terminology. The AV node can cause a block to the supraventricular impulses as either a protective mechanism or due to a pathologic process. This dual meaning, one good and one bad, leads to some serious confusion since they both use the word block. In addition, a block and a conduction ratio are not the same thing. For example a 3:2 block means that for every three P waves created, two are blocked. Only one is conducted. However, a 3:2 conduction ratio means that for every three P waves, two are conducted to the ventricles. To avoid confusion, and to adhere to the more clinically useful method, we will strictly be using the conduction ratio.
Another reason to use the term conduction rather than block is to remind us of those times when the block is protective, as in atrial flutter. In other words, blocks are pathologic and conduction is not. Thinking about the issues will quickly remind us of the treatment options available to us clinically. When the conduction is protective, we need to stop or slow down the supraventricular reentry circuits or automatic foci. When the block itself is the problem, we need to address it by increasing conduction through the node or completely bypassing it with an artificial pacemaker. Remember, you can’t get it if you don’t think about it. Using the term “conduction” makes you think about it.
Atrial flutter can actually have both types of “block” present at the same time, a physiologic one to prevent the tachycardia from getting out of control and a pathologic one secondary to some other reason. The pathologic block can be present because of ischemia, drug effect, increased parasympathetic activity, or just underlying AV nodal disease. We will revisit this issue after we have studied the pathologic AV blocks in detail in Chapter 28, Atrioventricular Blocks.