Pre-excitation syndromes and Conduction Blocks

Conduction System
Abnormalities and
Preexcitation Syndromes
Conduction Abnormalities
An obstruction in normal electrical
conduction system
„ 3 types of conduction blocks
„
– Sinus node block
– AV block (PR interval)
» 1st degree
» 2nd degree – Wenckebach, Mobitz I
» 2nd degree – Mobitz II
» 3rd degree
– Bundle branch block (widened QRS complex)
1
1st Degree AV Block
Prolonged delay in
conduction at the AV
node or His bundle
„ Look in lead II and V5 to
verify
„
– PR interval > 0.20 seconds
– QRS for every P wave
2
PR interval ~ 0.26 sec
2nd Degree AV block – Mobitz I
„
„
„
„
Each atrial impulse encounters a
longer and longer delay in the AV
node until one impulse fails to
make it through
PR interval progressively
lengthens until one QRS complex
is dropped
QRS does not always follow a P
wave
Also called
Weinckebach
3
2nd Degree AV Block – Mobitz II
„
„
„
Caused by a block below the
the AV node in which not all
atrial impulses reach the
ventricles
PR interval is normal length
(0.20 sec)
P wave is not always followed
by a QRS complex
NO QRS follows P wave
Progressively lengthened PR interval
Varying PR interval
AV Block, 2:1
4
3rd degree AV Block
„
„
„
„
Also known as AV
dissociation, there is no
communication occurring
between the atria and the
ventricles.
PR interval varies from cycle
to cycle
P wave is not always followed
by a QRS complex.
Atrial (P wave) rate will be
twice that of the ventricles
(QRS complex).
Notice P waves may be hidden
in QRS or T waves
Atrial Rate ~ 70 bts/min
Ventricular Rate ~ 35 bts/min
5
Bundle Branch Blocks
„ Conduction block in either the right or
left bundle branch
„ Must look at the width of the QRS
complex usually in the precordial leads
„ Right bundle branch block (RBBB) –
may be normal or abnormal occurrence
„ Left bundle branch block (LBBB) –
usually signifies underlying cardiac
disease
Right Bundle Branch Block
„ QRS complex widened to greater than
0.12 seconds
„ RSR’ pattern in V1 and/or V2 with ST
segment depression and T wave inversion
„ Reciprocal changes in V5 and V6
„ May also be “incomplete” meaning QRS
complex between 0.10 seconds and 0.12
sec in length
6
RBBB
RSR’
QRS complexes in most leads are > 0.12 sec
www.cardiology.org
RBBB
7
Left Bundle Branch Block
„ QRS complex widened to greater than
0.12 seconds
„ Broad or notched R wave with
prolonged upstroke in leads V5, V6, I,
and avL with ST segment depression
and T wave inversion
„ Reciprocal changes in V1 and V2
„ LAD may also be present
LBBB
QRS complexes in most leads are > 0.12 sec
Notched QRS
www.cardiology.org
8
LBBB
Hemiblocks
„ Conduction block of just one the the
fascicles in the left ventricular conduction
system
„ Left Anterior Hemiblock –
– Normal or borderline QRS duration
– LAD
„ Left posterior Hemiblock
– Normal or borderline QRS duration
– RAD
9
Left Anterior Hemiblock
www.cardiology.org
LAH
10
Preexcitation Syndromes
„ This means that current is transmitted
faster than it should be from atria to
ventricles
„ Due to accessory pathways
„ Wolff-Parkinson-White (WPW)
syndrome
„ Lown-Ganong-Levine (LGL) syndrome
WPW syndrome
„ PR interval < 0.12 sec
„ Wide QRS complexes
„ Presence of delta wave in some
leads
11
WPW
Delta Wave
www.cardiology.org
LGL syndrome
„ PR interval < 0.12 sec
„ Normal QRS width
„ No delta wave
„ May be presence of PSVT or atrial
fibrillation with LGL
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