Arrhythmia Recognition: The Art of Interpretation

The SVTs-A together make up the second largest group after VTach, accounting for approximately 10% to 20% of all the WCTs. We covered the basics of how the aberrancy comes about and the mechanisms involved in Chapter 6, Electrocardiography and Arrhythmia Recognition, starting on page 87. The SVTs-A can be broken down into four groups based on the underlying mechanisms that lead to their formation. They are listed here as part of the rhythms that make up WCTs.

The five main groups that comprise the WCTs include (in descending order of frequency):

Additional Information

To honor Dr. Harvey⁶ (see the previous Additional Information box), we have created a five-finger mnemonic to help you recall the various groups that are found under the WCT umbrella term. The five fingers are assigned as shown in Table 34-1.

Table 34-1 Five-Finger Approach to the WCTs

Finger	Group
Thumb: VTach	Ventricular tachycardia
Index: IVCD/BBB	SVT-A due to preexisting IVCD or BBB
Middle: MP3s	SVT-A due to metabolic, physiologic, paced, or pharmacologic causes
Ring: Rate-Related	SVT-A due to rate-related aberrancy
Pinky (or A Pinky): Accessory Pathway	SVT-A due to transmission through an accessory pathway

Note: The order in this table does not follow the order of decreasing frequency used in the main text.

Our five-finger approach to the WCTs (Figure 34-2) starts with the thumb. The thumb is the most important digit and is responsible for opposition. Opposition is a complex combination of abduction, flexion, and internal rotation that allows the thumb to reach across the palm and allows us to grip and hold objects, like calipers! If you ever want to see how important opposition is, try to hammer a nail or cut something with a knife without using your thumb. Since VTach is the most common and deadliest of the WCTs, making up more than 80% of cases,^1-5 we are assigning the thumb the honor of representing VTach.

The illustration of a palm shows the five fingers of WCT. — Figure 34-2 The five-fingers of WCT: Thumb = VTach, Index = IVCD and preexisting BBB, Middle = MP3s, Ring = Rate-Related, and A Pinky = Accessory Pathway.

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Description

The other four fingers represent the SVTs-A. Unfortunately, we could not come up with a mnemonic that used the descending order of frequency in our list. However, it is still a great memory aid.

In Chapter 32, Ventricular Tachycardia, we took an in-depth view of VTach from beginning to end. In Chapter 36, Wide-Complex Tachycardia: Criteria, we will look at the various morphologic criteria and algorithms available to us in order to help identify this rhythm in a bit more depth. This will allow us a deeper level of understanding of the rhythm and why it is so different from the SVTs-A. To avoid redundancy of this material, the reader is referred to those chapters for further reading.

So far, we’ve mostly concentrated on individual complexes. You need to keep in mind, however, that the same principles hold for rhythms as well. In general, the longer refractory periods provide us with an upper threshold rate for normal conduction. In other words, when a rhythm exceeds a specific rate for that patient, the complexes will continue to fall during the refractory period and an aberrantly conducted rhythm is established. This is what we mean when we say “rate-related aberrancy.” The rate that distinguishes normal conduction from aberrant conduction is known as the critical rate.

Keep in mind that the electrical conduction system’s critical rate is different for each person, but for most of us with a normally functioning system, the bar is set pretty high and we typically never reach that number. Internal and external influences, however, will manipulate the critical rate up or down. A partial list of the factors that decrease the rate threshold by increasing refractoriness include the following:

Aberrancy can take the shape of either a right bundle branch block (RBBB) or a left bundle branch block (LBBB) pattern. An RBBB pattern is more common, accounting for approximately 80% of the cases. The LBBB pattern is found in approximately 20% but can be higher when there is structural heart disease present. In addition, the width of the QRS intervals will vary depending on the point where the aberrancy takes off due to the underlying process involved.

You should always try to compare your ECGs or strips to old ones found in the chart or elsewhere. If you are lucky enough to find an old ECG with a morphologically similar rhythm, you may have just won the arrhythmia lottery, because prior workups could easily allow you to identify the present arrhythmia and evaluate the prior treatment strategies that were used. This could save you a lot of time or at least demonstrate the chronicity of the rhythm abnormality.

One of the most important signs to look for when evaluating for a possible aberrancy is the initial deflection (either positive or negative) of the QRS complex (Figure 34-3). If the initial deflection from the baseline is identical in orientation and morphology compared to the native beat, aberrancy is almost guaranteed as the mechanism for QRS widening. Why is this?

The illustration shows the occurrence of supraventricular and ventricular complexes. — Figure 34-3 Septal depolarization is triggered from a small septal branch that breaks off the left bundle branch. Septal depolarization typically occurs from left to right, forming a vector that travels rightward. Note that the initial component (septal depolarization) is identical in all three cases because they all use the same pathway. Note that the ventricular complex on the right has a sharper onset and is negative. The onset of the QRS complex is in the opposite direction of the supraventricular-conducted one.

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In contrast, when a ventricular pacer fires, it creates a depolarization wave that travels through the ventricles by direct cell-to-cell contact (Figure 34-3). The septal depolarization occurs as part of that wave but does not create its own vector. Typically, this creates an initial deflection of the QRS complex that is the opposite of one created using the HPS. Note, however, that the initial deflection can still mimic the supraventricular complexes depending on the site of origin and route of transmission taken by the depolarization wave.

Let’s take a closer look at the HPS. The bundle of His immediately breaks up after leaving the AV nodal area. Here, it branches into the left bundle and right bundle branches. At the start of the left bundle, a small hairlike fiber breaks off, the sole purpose of which is to depolarize the septal wall. Since aberrancy typically occurs distally in the bundle branches, both the native complex and the aberrant one depolarize the septum exactly the same. Likewise, the depolarization waves at that level will be identical, leading to identical morphology.

Ventricular complexes, on the other hand, originate in an ectopic ventricular focus and travel by direct cell-to-cell contact, taking a completely different depolarization route. This causes the appearance of the initial deflection to be broader and grossly different from the native beat.

A final note: The refractory period of the bundle branches of the electrical conduction system are always proportional to the length of the previous cardiac cycle. In other words, if you have a long P-P interval preceding a complex, then the subsequent refractory period is long. Any beat that arrives early will find the system “napping” and the transmission of the impulse cannot proceed normally. The result is that the following complex will conduct aberrantly. This mechanism is responsible for the frequently observed sequence of a long–short P-P intervals leading to an aberrantly conducted complex frequently seen in atrial fibrillation and known as the Ashman’s phenomenon (see the Additional Information box titled Ashman’s Phenomenon in Chapter 20, Atrial Fibrillation, page 285).

Further pacemaker-induced tachyarrhythmias are beyond the scope of this text, and interested students should consult more advanced material as needed.