1. Blocks Devin Herbert, R3 Aug 23, 2012
We will review the ECG criteria for a number of different heart blocks.
So, why do we care about heart blocks? Well, the identification of some mandates urgent management in the ED(ie STEMI treatment or transvenous pacemaker insertion) while others are less exciting, but are markers of conduction system disease (secondary to drugs, metabolic aberrations, ischemic or infiltrative diseases) which do influence our understanding of the patient.
Then, we will look at some example ECG’s, which contain one or more of the blocks we’ll discuss.
Ask questions along the way, but, the faster we can get through the slides, the more ECG’s we will get to scrutinize.

2. So, lets re-familiarize ourselves with the cardiac conduction system
So, lets re-familiarize ourselves with the cardiac conduction system. For the purposes of this presentation, this simplified version will suffice.
We will return to this diagram a number of times, when discussing each of the various heart blocks and the anatomic location of their conduction system pathology.
We will talk specifically about AV nodal blocks, right and left BBB’s and the hemiblocks (LAFB, LPFB), so have a quick look for the involved structures for each.

3. AV nodal blocks 1st degree: PR >200msec
2nd degree: Type I (Wenckebach) vs. Type II
3rd degree: complete heart block
1st degree (prolonged physiologic block) and 2nd degree (type I - long refractory period) are benign findings, whereas 2nd degree type II and 3rd degree blocks are signs of significant nodal pathology, with true conduction “block”.
In some cases or 2nd degree AV block, we see 2:1 P:QRS ratio. Assume Type II, to avoid dismissing a serious Dx.
Complete heart block is an example of AV dissociation, of which there are many. To meet complete heart block criteria, there should be a fast atrial rate and a slow escape rate, with no fusion or capture.

4. Next we will discuss the bundle branch blocks, namely right and left

5. Right BBB: QRS >120msec
Bundle Branch Blocks
Right BBB: QRS >120msec
RSRI in V1
Slurred S in V6
Left BBB: QRS >120msec
monomorphic S in V1
monomorphic R in V6
Remember, It is possible to get incomplete LBBB or RBBB, which meet the same morphology criteria, but are btw msec.

8. Another simple way to conceptualize the look of BBB’s, is to think about vectors.
In the precordial leads V1 overlies the right side of the heart and V6 the left
So, if the predominant QRS deflection is towards V1 and away from V6 think RBBB and vice versa.

9. Next we will have a look at the hemiblocks.

10. rS in III (usually II and aVF also)
Hemiblocks
LAFB: LAD (-30 to -90o)
R wave in V1
rS in III (usually II and aVF also)
LPFB: RAD (90 to 180o)
Exclusion of RVH
A point of clarification about terminology: Left anterior fascicular block is commonly referred to as left anterior hemiblock
Left posterior fascicular block is commonly referred to as left posterior hemiblock
Easy way to identify LAFB (pos in I, neg in aVF, neg in II)
LPFB less common, as the fibers are diffuse, no true bundle (therefore larger area of conduction system disease to see the pattern).
The most common cause of RAD is RVH (which has a pos R wave in V1 and/or V2 with R:S >1).

11. Since we’ve been discussing axis,
Start with I and aVF
Then, leads II and aVL are perpendicular
Leads III and AvR are perpendicular

12. Extreme right Left axis Right axis Normal
When thinking about the axes in hemiblocks, think of a simple diagram like this.

13. 1st degree AV block + bifascicular block
RBBB + LAFB or LPFB
Trifascicular block
1st degree AV block + bifascicular block
Finally, the bifascicular and trifascicular blocks, which are essentially combinations of blocks we have already discussed.
They are worth remembering, both because identifying them will make you look cool and they are markers of more extensive disease, particularly in the setting of acute ischemia.

14. Practice So, it’s finally time to practice.
For the upcoming practice cases, I’ll ask for a volunteer to walk us through each of the examples. Please identify the rhythm and block(s). Don’t worry about other details. Ask for help if you need it.

15. Sinus brady
1 degree AV
RBBB

16. Sinus bradycardia
1st degree AV block
RBBB

17. Sinus
2 degree AV with 2:1 conduction (likely type 2)
RBBB

18. 2nd degree AV block, 2:1 conduction (likely Type II)
Sinus
2nd degree AV block, 2:1 conduction (likely Type II)
RBBB

19. Sinus brady
RBBB
LAFB, therefore bifascicular block
LVH

20. Sinus bradycardia
RBBB
LAFB
= Bifascicular block

21. Sinus
2 degree AV type 2
RBBB

22. 2nd degree AV block, Type II
Sinus
2nd degree AV block, Type II
RBBB

23. complete heart block
Ventricular escape
RBBB
LAFB, therefore bifascicular block
LVH

24. Complete heart block
Ventricular escape
RBBB
LAFB
= Bifascicular block

25. AV dissociation
LBBB

26. AV dissociation
LBBB

27. Sinus brady
2 degree AV block with 2:1 conduction

28. 2nd degree AV block, 2:1 conduction
Sinus bradycardia
2nd degree AV block, 2:1 conduction

29. A fib
RBBB
LAFB
Therefore, bifascicular block

30. A Fib
RBBB
LAFB
= Bifascicular block

31. Complete heart block
Junctional escape

32. Junctional escape
Complete heart block

33. Sinus
1 degree AV block
RBBB
LAFB, therefore trifascicular block

34. Sinus
1st degree AV block
RBBB
LAFB
= Trifascicular block

35. Sinus tachycardia
2nd degree AV block, type I
Pt in cardiogenic shock post inferior STEMI, on dopamine infusion

36. 2nd degree AV block, Type I
Sinus tachycardia
2nd degree AV block, Type I

37. The end

38. Sinus
2 degree AV block with 2:1 conduction (type 1 or 2)
RBBB

39. 2nd degree AV block, 2:1 conduction
Sinus
2nd degree AV block, 2:1 conduction
RBBB