Download PR interval

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
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