1. PR interval
Good Samaritan CSG

2. Main Points
PR interval
Derivation
Preexcitation
AV blocks

3. PR interval derivation
Measured from beginning of P to beginning of QRS – more properly “PQ”
From exiting SA node to leaving terminal perkinjie system
Normal (3-5 small boxes)
Allows atrial-assisted filling of ventricles (“timing belt of the heart”)

4. Preexcitation 3 variants, often simply referred to as a group as “WPW”
All involve accessory paths that allow direct activation of ventricles without usual av-his-perkinjie delay
2 effects – short PR from bypassing normal delaying mechanism, and slurred initial R/S deflection from direct and dyssynchronous activation of ventricle rather than more simultaneous activation from conducting system
Dangerous as re-entrant rhythms are much more stable at high rates than normal
AV nodal blocking agents should be avoided, as an anti-dromic tachycardia can be induced
Instead of going down the “regular”path and back up the “accessory path”, slow av conduction reverses the flow, so a narrow tachy becomes a wide tachy

5. Pre-excitation – WPW Type 1 – WPW
Pathway from atria myocardium to ventricle myocardium
Short PR from bypassing av node
Delta wave from direct activation of myocardium

6. Preexcitation – James variant
Type 2 – James variant
Pathway from atria myocardium to post AV node his bundle
Short PR from bypassing AV node
No delta wave, as inserts into normal conducting system

7. Preexcitation – Mahaim variant
Type 3 – Mahaim variant
Pathway from his to myocardium
Normal PR as impulse passed through AV
Delta wave as inserts into myocardium

8. AV blocks Type 1 – PR longer than .20 sec Every beat is conducted
PR interval is constant

9. AV blocks
2nd degree – involve variable PR intervals, with conduction of at least some beats
Types 2 and Advanced AV block are likely to progress, pacemaker evaluatiion is warranted
Three kinds –
Type 1 – progressive PR lengthening (wenkebach)
Type 2 – Fixed ratio of p’s make a lesser number of QRS. Conducted p’s have a constant PR
Advanced AV block – Complete AV block with occasional “capture beats” that make it through the AV node.

10. 2nd Degree Type 1
The RR interval of the pause is less than the two preceding RR intervals, and the RR interval after the pause is greater than the RR interval before the pause.

11. 2nd Degree Type 2
PR intervals are constant until a nonconducted P wave occurs. The RR interval of the pause is equal to the two preceding RR intervals.

12. Advanced 2nd Degree Block
Complete heart block with occaisional “capture beats
Capture beat has a shorter RR than preceding beats

13. 3rd Degree AV Block No conducted beats from atria to ventricles
P waves with “march through”
Width of QRS suggests place of new pacemaker – Wide = ventricular, Narrow = junctional

14. AV dissociation Atria and ventricles march to entirely different drums
Not synonymous with complete heart block, although that is one of the causes
Generally can call when v rate is faster than a rate

15. AV dissociation type 1
Type 1 occurs when primary pacemaker (SA node) slows to point of normally suppressed pacemaker taking over
i.e. sa node slows so junction loses overdrive suppression and takes over
Known as “default”

16. AV dissociation type 2
Subsidiary pacemaker accelerates to point where it overdrive supresses SA node
Known as “usurpation”

17. AV dissociation type 3
Complete heart block with new pacemaker arising below block
Classic AV dissociation/3rd degree heart block we think of

18. Questions