PR interval

Good Samaritan CSG

Main Points

• PR interval– Derivation– Preexcitation– AV blocks

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 .12-.20 (3-5 small boxes)

• Allows atrial-assisted filling of ventricles (“timing belt of the heart”)

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

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

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

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

AV blocks

• Type 1 – PR longer than .20 sec– Every beat is conducted– PR interval is constant

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.

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.

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.

Advanced 2nd Degree Block

• Complete heart block with occaisional “capture beats

• Capture beat has a shorter RR than preceding beats

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

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

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”

AV dissociation type 2

• Subsidiary pacemaker accelerates to point where it overdrive supresses SA node

• Known as “usurpation”

AV dissociation type 3

• Complete heart block with new pacemaker arising below block

• Classic AV dissociation/3rd degree heart block we think of

Questions