In this chapter, we will introduce you to your first in-depth discussion of a “block.” If you reviewed the rhythms in Chapter 7, How to Interpret a Rhythm Strip, you likely noted that there are a lot of “blocks” thrown around. At first, you are not confused because, frankly, you haven’t thought about it. But, sooner or later, you will scratch your head and ask yourself: Are the terms interchangeable since they are used in different contexts and settings, or have I missed the boat completely? The answer is that there is a reason for all of these terms, and they apply to completely different processes. I hope this discussion helps guide you out of the maze.
Presently, this chapter focuses on sinus blocks and pauses. The word sinus isolates this discussion to the sinoatrial (SA) node and the nearby surrounding area. To get sinus blocks or pauses, the area surrounding the sinus node itself has to be diseased or ischemic, or at least partially refractory. The term sinus block refers to those events that cause a complete block in transmission at the level of the sinus node. Blocks of normal pacemaking function can cause a secondary pacemaker to fire, or the pacemaker can skip a beat, as occurs in these cases. There is no resultant depolarization of the atria or distal parts. That means there is no P wave, QRS complex, ST segment, or T wave at all. The next time the sinus pacemaker fires, it will either capture or again be completely blocked, and so forth. Note that in these cases the sinus node is not reset and the missing complex creates a stoppage of all cardiac activity occurring in a multiple of the normal P-P interval.
Sinus pauses refer to a slowing of the transmission of the sinus pacemaker from reaching the rest of the atrial myocardium. Once it reaches there, however, the complex fires as expected. This occurrence produces an interval that is not quite a multiple of the normal P-P interval. That is why we call these pauses and not blocks. Sinus arrest can have properties of both; so typically, it is not an exact multiple of the normal P-P interval. The differentiation comes in the fact that these silent periods are long. Lack of cardiac activity can cause serious hemodynamic problems and some very stressful moments for the clinicians involved.
Another form of block occurs at the level of the atrioventricular (AV) node. These are known as the AV blocks. There are multiple types, as we will discuss in great depths later in this text, but the common cause in them all is a diseased or stressed AV node. The AV blocks cause slowing or blockage of the depolarization wave as it moves through the AV node. In that sense, they are similar to the sinus blocks and pauses discussed earlier.
Now, let’s change our focus a bit and discuss the bundle branch blocks. As discussed in Chapter 1, Anatomy and Basic Physiology, there are two main bundle branch blocks (the left and the right), and the left bundle is further split into a left anterior fascicle and a left posterior fascicle. In Chapter 6, Electrocardiography and Arrhythmia Recognition, we discuss how any physiologic or anatomic obstructions to the impulse moving through these specialized bundles of the electrical conduction pathways are referred to as “blocks” or “hemiblocks,” respectively. These “blocks” stop transmission through the specialized conduction system and cause the depolarization waves to veer off track and conduct by the slower cell-to-cell conduction routes. As such, they really don’t stop conduction, they just reroute the depolarization wave, causing some predictable but wide and bizarre aberrancies to develop on the surface tracing. These patterns are known as the left bundle branch block pattern, right bundle branch block pattern, left anterior hemiblock pattern, and left posterior hemiblock pattern. In addition, combinations of the right bundle branch block and one of the hemiblocks can occur if two pathways are blocked simultaneously.
So, when someone says the word “block” to you, always stop and ask yourself exactly which one they are referring to. With time and a little more experience, these terms will rattle off your tongue, impressing all around you. For now, just try to make some sense of it all.
—Daniel J. Garcia