CASE: A 70 yo male presents to the ED with dizziness. He has normal vital signs and his ECG appears as below:
ANSWER:
AV Dissociation without complete heart block
DISCUSSION ON COMPLETE AND INCOMPLETE HEART BLOCK
Remember that AV dissociation can occur WITHOUT complete heart block. Let’s discuss blocks. The definition is the every QRS is related to a P and every P is related to a QRS. What we are talking about here is does every P wave produce a QRS complex on the ECG. Now remember even though we are speaking mostly about a test (the ECG) we have to remember that the test does not live in isolation of the physical exam. It’s like looking at an abg where the pH is 7.0 and not asking what the clinical situation is. If its DKA that’s bad, if it’s a seizure, well, that abg will get better on its own.
I think the biggest problem in understanding ECG’s is the terminology. So that will be a big part of this discussion. So just to point out how confusing the terminology is we won’t be discussing bundle branch block. So really when we refer to “heart block” what we are really speaking about is a delay or obstruction in conduction either at the AV node (nodal) or just below the AV node (infranodal) in the bundle of his. AV blocks are usually thought of in “degree’s” of block from first to second. It is a sort of continuum from first degree AV block which is essentially forgettable to complete heart block which can be lethal. But here again the terminology is confusing. First degree heart block isn’t really block. It’s just conduction at the AV node that is slowed. So, a p wave always produces a QRS but it just might take longer than normal (>200 ms). Also, it essentially has no clinical context and so we won’t give it another thought.
TERMINOLOGY
A note on terminology. When we say conduction in the context of conduction ratios in AVBs, we are refering to the ratio of P waves that conduct to the ventricles. Thus, a 3:2 conduction means that, there are 3 P waves, 2 of them conduct normally to create QRS complexes. This is different than block when we are comparing the number of P present to the number of P waves that have been blocked. For example, a 3:2 block means that for every three P waves, two get blocked and only one gets through to create a single QRS complex 1.
SECOND DEGREE HEART BLOCK
It’s a Second because only Some of the P waves are conducted through. This degree comes in 2 flavors. The first type has a lengthening PR interval (You’d think a PQ interval but often there aren’t Q waves and there are more often R waves) until one finally drops. The PR interval is progressively lengthening as though it were giving you warning until it eventually drops the QRS. This give us time to figure out what to do… as in time to follow up with cardiology it the patient is not symptomatic. This is in contrast to the type II where the QRS suddenly drops off without warning and therefore needs something done immediately1.
PEARL: The difference between Mobitz Type I and Type II: A QRS that gets progressively longer and then drops (Mobitz Type I aka Wenckebach) is not as bad as one that just drops suddenly (Type II)
MOBITZ TYPE I (Wenckebach)
Let’s start with Mobitz Type I (or the Wenckebach phenomenon). Here the P-P interval is constant but R-R intervals shorten between each complex in a group. Note that these rhythms exhibit a grouped appearance. The groupings can occur at any conduction ratios. However, there will always be one less QRS complex than there are P waves. The first PR interval is the shortest. The longest PR interval is the last one. The key to this diagnosis is the lengthening PR 1.
PEARL: The key to diagnosing Type I Block is a changing PR interval
MOBITZ TYPE II
Simply stated, Mobitz II second-degree AVB is diagnosed when you have non-conducted P waves with constant PR intervals. In Mobitz II, the QRS complexes can either be narrow or wide, depending on the location of the conduction block. However, since Mobitz II rarely occurs in the AV node the QRS complexes in Mobitz II are usually wide.
PEARL: Mobitz II has a CONSTANT PR Interval and will often have a wide complex QRS
BLOCKED PAC vs MOBITZ II
Sometime PAC’s are present and then are blocked from depolarizing the ventricles for whatever reason. The net result is that the P wave will be visible, but the QRS complex will not be. This may resemble a Mobitz II block if not thought out. Remember, the P-P interval in Mobitz II is very, very regular. If you see a prematurely occurring P wave that is blocked and the P-P interval is not the same, think blocked PAC 1.
PEARL: A blocked PAC can be differentiated by measuring the PR interval
HEART BLOCK IN AMI
This is an important distinction. In a patient with an acute MI, type I AV block usually accompanies inferior infarction (especially if a right ventricular infarct occurs). This however is transient and does not require temporary pacing. Conversely when a type II AV block occurs in the setting of acute anterior MI, this will require temporary or permanent pacing, and is associated with high mortality2.
PEARL: TYPE II BLOCK IS ALWAYS WORSE THAN TYPE I ESPECIALLY IN AMI
COMPLETE HEART BLOCK
This lack of conduction is typically due to a defect in the (infra-nodal) ventricular electrical conduction system. Therefore, there must be TWO separate pacemakers. This is why it is said that the P’s and QRS’s march out but aren’t related. One pacemaker is supraventricular (sinus, or atrial). The other is ventricular and is mandatory in order to maintain cardiac output1. On the ECG you will see, constant P-P intervals (supraventricular pacemaker) and constant R-R intervals (ventricular pacemaker). The difference is that the PR interval is completely changing and never exactly repeats throughout the entire strip. The morphology of the final rhythm will depend on the two individual rhythms that make it up. The rate of the ventricular rhythm will depend on location of the pacemaker. A junctional pacemaker will typically have narrow QRS complexes that are between 40 and 60 BPM. A ventricular pacemaker will typically have wide QRS complexes and a rate between 30 and 45 BPM. However, any pacemaker can have accelerated automaticity or may trigger a reentry circuit. In those cases, the ventricular rhythm may be tachycardia 1.
Complete Heart Block (CHB) vs AV Dissociation (AVD)
The difference between CHB and AVD is important and not just semantical. You CAN have AVD without CHB, but not vice versa.
In CHB none of the atrial beats are conducted to the ventricles. Furthermore, because the problem in CHB is usually infra-nodal you should see wide complexes and the pacemaker rate should only be idioventricular or junctional. Therefore, the atrial rate should be much faster than the ventricular rate. CHB: because of two separate pacemakers one in the atria and one in the ventricle there should be constant P-P intervals and R-R intervals but a varying P-R interval.
Conversely, in AVD you will have the occasional conducted beat even though the atria and ventricles frequently still beat independent of each other. In AVD you will likely have narrow QRS complexes with changing morphologies. Additionally, since conduction across the AV node is still possible, you may have the ventricular rate the same or faster than the atrial rate. Lastly in AVD the P-P and R-R intervals may be not exactly constant because of conducted escape beats that skew the timing.
PEARL: AVD can be distinguished on the ECG from CHB by finding conducted P waves, a ventricular rate at least as fast as the atrial rate, and possibly varying QRS morphologies.
FINAL PEARL: CHB IS VERY SYMPTOMATIC AND ALMOST ALWAYS HAS LOW BLOOD PRESSURE AND BRADYCARDIA
BACK TO OUR ECG EXPLANATION:
In our ECG we see P wave (BLUE ARROW) that appears to be conducted. We see an R-R interval (YELLOW) and P-P interval (RED) that is slightly different after the conducted P waves (YELLOW VS RED). We also see a ventricular rate that is similar or faster than atrial rate. Finally, when looking at the monitor (not shown here) the patient had converted back to sinus.
SUMMARY TABLE
| Complete Heart Block
(a condition like influenza) |
AV dissociation
(a symptom like fever) |
|
| Definition | All atrial impulses blocked | Ventricle is “separated” from atria by a different focus arising from AV junction or ventricle |
| Clinical Findings | Bradycardia
JVP with cannon a waves No Response to atropine or exertion |
Occurs at any HR
Response to atropine or exertion. Converts to sinus |
| Atrial vs Ventricular Rate | High/Low | Equal or Low/High |
REFERENCES
- Garcia, T. B. Introduction To 12-Lead ECG: The Art Of Interpretation (Garcia, Introduction to 12-Lead ECG) (Jones & Bartlett Learning, 2002).
- MD, D. P. Z., PhD, P. L. M. D., MS, R. O. B. M. D., MD, D. L. M. & MD, G. F. T. Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine, 2-Volume Set, 11e (Elsevier, 2018).
Reviews in EM (REM) 