Conduction Blocks Flashcards
Definition of conduction block
any obstruction or delay of the flow of electricity along the normal pathway
3 types of blocks:
sinus node block
AV block
bundle branch block
What does the EKG look like in a sinus node block
looks like a pause in the normal cardiac cycle
AV block occurs between
sinus node and purkinje fibers (includes AV node and His bundle)
what is a fasicular block/hemiblock?
only part of one of the bundle branches are blocked
First degree AV Block is what
prolonged delay in conduction at AV node or His bundle
-prolonged PR interval
Diagnosis of first degree AV block is
PR interval longer than .2s (1 big square)
Second degree AV block
not every atrial impulse can pass through the AV node -more P waves than QRS 2 types: Mobitz type I Mobitz type II
EKG of Mobitz type 1 block looks like
progressive lengthening of PR interval and then a dropped beat
starts again
Grouped beating seen in
2nd degree AV blocks
-Mobitz 1 & 2
Mobitz 2 usually due to block
below the AV node in the His bundle
EKG of Mobitz 2 looks like
two or more normal beats with normal PR interval and then a dropped beat
- no progressive lengthening
- ratio of p waves to QRS is constantly varying
- QRS may be wide due to LV conduction delay
What ratio makes it impossible to tell Mobitz 1 from Mobitz 2?
2:1
Pwave:QRS
General treatment for Mobitz1
Generally no treatment needed
General treatment for Mobitz 2
could progress to third degree heart block
may require pacemaker
Third degree AV block also known as
complete heart block
Response of heart for third degree AV block
idioventricular escape beat
AV dissociation occurs
EKG of Third degree AV block looks like
regular intervals of P waves and QRS but they do not correlate
QRS wide= ventricular origin
QRS narrow = being paced from AV junction
Atrial rate is always faster then the ventricular rate
PR waves of third degree AV block are
completely variable
What are stokes-adams attacks?
Treatment?
no ventricular escape rhythms with 3rd degree AV block
4 or more seconds without ventricular activity
-syncope or near syncope
Require pacemaker
A ventricular escape beat occurs after_____ and is _____ premature
after a long pause
never premature
Causes of third degree AV blocks:
Degeneration of conduction system (#1 cause)
Acute MI
-emergent pacemaker almost always required
Definition of bundle branch block
conduction clock in either the left or right bundle branches
RBBB has what in leads V1 and V2
R’ in QRS
QRS= RSR’ (like rabbit ears)
QRS>.12 seconds
Dx of bundle branch block is done by looking at
width and configuration of QRS complex
What causes the wide negative S wave deflection in the left chest leads during RBBB
rightward spread of the delayed slowed RV depolarization
LBBB shows QRS changes in leads
over LV:
I, aVL, V5, V6
-already have tall r waves
-now see prolongation and sometimes a notch
During LBBB broad deep s waves are seen in
over RV: V1 and V2
Wide QS in lead V1 and wide R wave in lead V6 with slight notching at peak is diagnostic of
LBBB
Lead V1 has wider RSR’ and lead V6 has QRS is diagnostic of
RBBB
Inverted T waves with slight ST depression in right precordial leads with wide QRS is diagnostic of
RBBB
Inverted T waves with ST depression in left lateral leads with wide QRS is diagnostic of
LBBB
RBBB can be seen in diseases of:
diseases of conduction system
otherwise normal hearts
LBBB can be seen in
Rarely in healthy hearts
Reflects heart disease
What is the critical rate?
Bundle branch blocks may occur at a particular heart rate
-slow rates the ventricles conduct normally, but above a certain rate the BBB develops
The critical rate is directly related to_______
If the rate is very rapid_____
the time it takes a bundle branch to repolarize
If HR is so rapid the branch can’t repolarize, there will be a temporary block to conduction
RBBB precludes the dx of
Right ventricular hypertrophy
-R waves are distorted
LBBB precludes the dx of
Left ventricular hypertrophy
-R waves are distorted
Hemiblocks refers to
block of just one of the fascicles of the left bundle branch
- septal
- left anterior
- posterior
Major effect that Hemiblocks have on EKG is
axis deviation
Left anterior hemiblock leads to __________ deviation
Left axis deviation
between -30 and -90
-Normal QRS duration, no ST segment or T wave changes
-No other cause of deviation present
Left posterior hemiblock leads to ________ deviation
Right axis deviation
between +90 and 180
-Normal QRS duration, no ST segment or T wave changes
-No other cause of deviation present
left posterior hemiblock occurs in _____
diseased hearts
A bifascicular block is
RBBB + hemiblock (left anterior or left posterior)
-only one fascicle supplying current to both ventricles
RBBB + left anterior hemiblock EKG would show
QRS wider than .12sec , RSR’ in V1 & V2
+ Left axis deviation
RBBB +left posterior hemiblock EKG would show
QRS wider than .12sec , RSR’ in V1 & V2
+ Right axis deviation
RSR’ in V1 but QRS duration is between 0.10 and 0.12 seconds is called
Incomplete bundle branch block
QRS widening >.10 seconds without the usual criteria for either bundle branch blocks or bifascicular block are called
nonspecific intraventricular conduction delay
Popular type of pacemaker that fires when patients own intrinsic heart rate falls below threshold:
Demand pacemaker
Dual chamber pacemakers are also called
AV sequential pacemakers
3rd degree heart block with origin near the AV node, what kind of pacemaker would be used?
ventricular or AV sequential pacemaker
With ventricular pacemaker EKG will show
following spike will be wide and bizarre like a PVC
- LBBB pattern
- retrograde p wave may be seen
An atrial pacemaker EKG will show
spike followed by p wave and normal QRS
A sequential pacemaker EKG will show
two spikes, one before the p wave and one before the QRS
Cardiac resynchronization therapy is for pts with
heart failure associated with wide QRS and LV dysfunction
