When we use the term AV dissociation, remember that we are talking about a situation in which the atria and the ventricles are each beating to their own drummer, but there is still some occasional or limited communication between them. In other words, it is not a complete block of all of the transmission of information.
Since there is some limited communication between the atria and the ventricles, occasionally, the timing of a supraventricular impulse will fall in such a way as to trigger conduction through the normal conduction system in the ventricles. The normal P wave in these cases will stimulate the ventricles in such a way as to cause a QRS complex that appears normal on the strip or ECG. This normal-looking complex is known as a capture beat (Figure 31-8). They are called capture beats because the normal supraventricular impulse “captures” the ventricles and forces them to behave normally.
Figure 31-8 Accelerated idioventricular rhythm with AV dissociation. Note that the fourth P wave in the strip falls in such a way as to “capture” the ventricles normally. This capture gives rise to a normal, narrow QRS complex.
© Jones & Bartlett Learning.
Capture beats, along with fusion complexes, are the keystone features in the diagnosis of accelerated idioventricular rhythms and ventricular tachycardia. If you ever see a wide complex rhythm with a normal-appearing QRS complex smack in the middle of it, it is either accelerated idioventricular or ventricular tachycardia, depending on the underlying ventricular rate.
So, let’s recap. Accelerated idioventricular rhythm is a wide complex rhythm that has complexes of ventricular morphology, is very regular, and may or may not be associated with AV dissociation, fusion complexes, and capture beats.
Here are some helpful diagnostic hints for spotting this rhythm (Figure 31-9). Use your calipers to map out any inconsistencies or “bumps” in the morphology of the complexes. Many times, these bumps turn out to be P waves and mapping them brings out the presence of AV dissociation. Look for morphologic changes in the QRS complexes, and try to see if they are due to different pacemakers or to fusion. Regularity, similar coupling intervals, and similarities to both parents will point in the direction of fusion. The presence of capture beats is classic for AV dissociation, which is a major determinant of accelerated idioventricular and ventricular tachycardia. As a side point, if the sinus rate and the ventricular rates are close to each other, the complexes immediately surrounding the capture beats will be fusion complexes, and you will see this pattern repeating itself on a long strip.
Figure 31-9 Go ahead and analyze this strip. Hint: It has everything we just talked about. The ventricular rate is 90 BPM. Don’t forget to use your calipers!
© Jones & Bartlett Learning.
Now that we have given you some pointers, go ahead and analyze the complicated strip in Figure 31-9. The answer is in the Additional Information box if you would like to review it.
ARRHYTHMIA RECOGNITION
Idioventricular Rhythm
| Rate: | 20 to 50 BPM |
| Regularity: | Regular (ventricular) |
| P wave:
Morphology: Upright in II, III, and aVF: |
Retrograde or AV dissociation
Variable Sometimes |
| P:QRS ratio: | None or retrograde |
| PR interval: | None |
| QRS width: | Wide |
| Grouping: | None |
| Dropped beats: | None |
DIFFERENTIAL DIAGNOSIS
Idioventricular Rhythm
Idioventricular rhythm may be due to increased automaticity or it may be due to an escape mechanism, reflecting the failure of the supraventricular pacemakers. This list is not inclusive but reflects the most common causes of the rhythm disturbance.
Additional Information
Accelerated Idioventricular Rhythm
The first thing you should have done is gotten an overall feel for the rhythm. It is a wide-complex rhythm with one single event in the middle. The event is not a wide beat among narrow ones, but it is a narrow one among a bunch of wide ones. That should immediately make you think of a capture beat. The next thing is to use your calipers and map out the tips of the QRS complexes. The two areas on either side of the capture beat are perfectly regular and are composed of wide complexes. This is consistent with an idioventricular rhythm or an accelerated idioventricular. The ventricular rate is 90 BPM, so it is accelerated idioventricular.
Whenever you see a capture beat, you have AV dissociation. So, let’s find the P waves. Take an overall look at the strip, starting with the P wave on the capture beat. Are there any “bumps” that are on some complexes but not others? Yes, on complex V1 there is a bump at the bottom of the S wave (see P5 on Figure 31-10). Take your calipers and put one pin on the P wave on the capture beat and the other on the bump at the bottom of V1. Now, march the calipers in both directions, placing a little mark where the pin falls. Are there other “bumps” where the pins fall? Yes! Those are your P waves marching through the strip (look at the P1–P8 markers below the strip on Figure 31-10).
Figure 31-10 Analysis of accelerated idioventricular rhythm strip.
© Jones & Bartlett Learning.
DescriptionNow, since we know we have a capture beat and AV dissociation, let’s see if we have any fusion complexes. Do any of the ventricular complexes look different from the others? Yes, the complexes labeled F1 through F3 are definitely different from the ones on the other end of the capture beat. These complexes are a little smaller and a little narrower than the ones at the end. Notice that these complexes have P waves in front of them to varying degrees, while the ones at the end have the P waves after the QRS complexes. They have a little of the appearance of the capture beat and a little of the appearance of the full ventricular beats. F1–3 are, therefore, fusion complexes due to a partial capture of the supraventricular impulse from the P waves. F1 has the P wave the farthest from the QRS and is the shortest and narrowest of the three.
Let’s recap. We have a wide-complex rhythm that is regular, has a ventricular rate of 90 BPM, AV dissociation, a capture beat, and some fusion beats. There is only one possibility for this combination: accelerated idioventricular!
This strip is not an easy one. It is, however, typical of what you will see in clinical practice. When you approach a strip, take a few seconds to look at it before you panic. You will see that there is a pattern that you can tease out, if you have the knowledge and the patience to see it. We hope that this analysis has helped you and that you will be able to carry away some valuable learning points as we progress through the rest of this chapter and the next. As you will see, many lessons from this chapter will carry over to your analysis of ventricular tachycardias.
ARRHYTHMIA RECOGNITION
Accelerated Idioventricular Rhythm
| Rate: | 50 to 100 BPM |
| Regularity: | Regular (ventricular) |
| P wave:
Morphology: Upright in II, III, and aVF: |
Retrograde or AV dissociation
Variable Sometimes |
| P:QRS ratio: | None or retrograde |
| PR interval: | None |
| QRS width: | Wide |
| Grouping: | None |
| Dropped beats: | None |
DIFFERENTIAL DIAGNOSIS
Accelerated Idioventricular Rhythm
This list is not inclusive but reflects the most common causes of the rhythm disturbance.