pr interval good samaritan csg. main points pr interval –derivation –preexcitation –av blocks
TRANSCRIPT
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PR interval
Good Samaritan CSG
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Main Points
• PR interval– Derivation– Preexcitation– AV blocks
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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”)
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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
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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
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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
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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
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AV blocks
• Type 1 – PR longer than .20 sec– Every beat is conducted– PR interval is constant
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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.
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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.
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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.
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Advanced 2nd Degree Block
• Complete heart block with occaisional “capture beats
• Capture beat has a shorter RR than preceding beats
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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
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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
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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”
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AV dissociation type 2
• Subsidiary pacemaker accelerates to point where it overdrive supresses SA node
• Known as “usurpation”
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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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Questions