Today, I want to tell you about a couple of intrinsic cardiac pacemakers. We'll start with the atrioventricular or AV node. Like our discussion of the SA node, the discussion is pretty technical. Look at it as a way to impress people around the watercooler!
The AV node is a "back up" for the SA node. Should the SA node fail to fire within a certain time frame, the AV node will stimulate conduction at a heart rate of 40 to 60 bpm. The AV node ensures that the brain, the heart and the rest of the body get enough blood flow.
The AV Node as a Cardiac Pacemaker
How do we know when a heartbeat is coming from the AV node? By looking at the P wave.
In a complex generated by the AV node, the QRS complex is not preceded by a normal P wave and PR interval (0.16 to 0.20 msec). The PR interval, if ahead of the QRS (antegrade) is shorter than 0.12msec. The P wave—which will look different from the normal P wave—can also follow the QRS (representing that the atria were electrically stimulated from below), or be buried (and thus unseen) in the QRS.
Sometimes the AV node is referred to as the AV junction, and electrical stimulation that exceeds its intrinsic rate may be called a junctional tachycardia (or "junctional tach") or AV tachycardia (or AV tach). The term tachycardia is used whenever the heart rate exceeds the usual rate for the source of the electrical stimulation. So, a sinus tachycardia exceeds 100 bpm, and a junctional tach is faster than 60 bpm.
The AV node is actually quite important. It slows the electrical impulse 1/20th of a second. While that may not sound like much, it’s enough for the valves to close after the atria fill the ventricles with blood, so that it does not slosh back into the atria when the ventricles contract.
The Purkinge Fibers Act as Cardiac Pacemakers
Lastly, the cardiac pacemakers known as the Purkinge fibers that line the two ventricles may also "kick in" should the heart rate fall below 40 bpm. This can happen for several reasons:
- Ischemia (lack of blood flow) to the SA and/or the AV node as in during an acute heart attack;
- As the result of disease/aging of the conduction system;
- Toxic drug levels; and
- Several other possible reasons.
The Purkinge fibers will—at the very least—act as intrinsic cardiac pacemakers to keep the heart going at 30 to 40 bpm, which is barely enough to sustain vital functions, and can be enough to buy precious time for us in a hospital setting.