The Ventricle as a Pacemaker

The ventricular myocardium offers our last defense against asystole or the lack of any rhythm. If you remember from Chapter 1, Anatomy and Basic Physiology, the first and principal pacemaker is the sinus node. In cases where the sinus node fails, the atrial myocardium becomes our first line of defense. This continues down the line until you reach the ventricular pacemaker. Keep in mind, ventricular rhythms are dangerous for many reasons. One of the principal reasons is this: What is your fail-safe if a ventricular pacemaker fails to fire? Answer: There is none! That is it . . . end of the line . . . the buck stops here . . . or whatever other cliché you choose to say that there is no other fail-safe pacemaker after the ventricular myocardium.

The intrinsic rates of the various portions of the ventricle are as follows: The bundle branches fire at a rate of 40 to 45 beats per minute (BPM). The Purkinje system fires at an intrinsic rate of 35 to 40 BPM. Finally, the ventricular myocardium usually fires at an intrinsic rate of 30 to 35 BPM, but can be as low as the 20s.

Here is a little clinical pearl. Whenever you have a patient with a slow ventricular rate that is wide and bizarre, you’d better have pacemaking capabilities at hand or, better yet, on your patient. If you have an external transthoracic pacemaker available, the pads should be on the patient’s chest and the unit should be accessible and ready to go at a moment’s notice. In addition, if you have the capability or the personnel, you should have an internal transvenous pacemaker and wire ready, just in case the transthoracic pacer fails. Remember, there is no other intrinsic pacemaker that can save you if the ventricles fail. Your only response at that point is CPR, electricity via a pacemaker, and prayer.

Additional Information

Where Is the Ectopic Focus?

Now, let’s look at an interesting aspect of morphology. Since the site of origin of a ventricular depolarization is the most important factor in determining morphology, we can use the inverse of that statement to help us identify where the complex originated. In other words, we can use certain morphologic types to help guide us to the most likely location of the ectopic pacemaker.

In general, ventricular complexes that originate in the left ventricle will take on a right bundle branch block (RBBB) pattern. Ventricular complexes that originate in the right ventricle take on a left bundle branch block (LBBB) pattern. These statements are generally true, but why does this occur? (For a complete review of the LBBB and RBBB patterns, please see Chapter 5, Introduction to 12-Lead ECGs.)

Take a look at Figure 29-5. In this figure, the ectopic focus is in the left ventricular myocardium. Due to its anatomic location, most of the ventricular myocardium will lie to the right of the focus. The depolarization wave has to head primarily from left to right. So, in what direction would you expect the vector formed by the depolarization wave to head? Left to right. If you think about it, this is very similar to the vector that is formed when the right bundle branch is blocked (Figure 29-6). Since the main vectors of these two complexes point in the same direction, their respective morphologies should be similar. The similar vectors are the reason why a ventricular ectopic focus on the left ventricle will give rise to a complex that is morphologically similar to one caused by an RBBB. Likewise, a right ventricular ectopic focus will give rise to a main ventricular vector that heads to the left and this will resemble an LBBB pattern.

Figure 29-5 An irritable focus, represented by the purple star, acts as a pacemaker and causes an impulse to occur. This gives rise to the purple depolarization wave and the yellow vector. Electrocardiographically, this is represented by the wide slurring of the terminal portion of the QRS.

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Figure 29-6 A right bundle branch block causes the depolarization wave to travel rightward to the right ventricle to depolarize the ventricle by direct cell-to-cell contact. This gives rise to the blue depolarization wave and the yellow vector and the typical right bundle branch block pattern.

© Jones & Bartlett Learning.