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AV (atrio-ventricular) block occurs when an atrial impulse is
conducted with delay or fails to conduct to the ventricle at a time when
the AV node should not be refractory.
degree AV block usually results from conduction
delay within the AV node but rarely from intra-atrial
delay, or delay within the His-Purkinje system
(often seen in conjunction with bundle branch block) may be responsible.
The ECG reveals a PR interval greater than 200 milliseconds.
Asymptomatic patients require no therapy. In symptomatic
patients, therapy is directed toward the underlying etiology, and
treatment includes atropine, or cardiac pacing.
Trifascicular heart block
Trifascicular heart block is the triad of first degree heart block,
right bundle branch block, and either left anterior or left posterior
heart block seen on an electrocardiogram (EKG).
The relevance of this entity is that it's impossible to tell how
serious the disease of the electrical conduction system of the heart is
by examination of the EKG.
AV block occurs when some of the atrial impulses are not conducted to
the ventricle at times when the AV node should not be refractory. Two types
of second-degree AV block are recognized, and dis?tinctions between type
I and type II are important, as they carry differ?ent prognostic implications.
type I (Wenckebach's) block has progressive delay in AV con?duction prior
to block. The site of conduction block almost always is within the AV
node. The EGG reveals progressive PR prolongation
prior to a non-conducted P wave resulting in QRS complexes in regular
groupings (grouped beating). The RR interval progressively shortens prior
to a blocked P wave. Symptomatic type I AV block is managed initially
with atropine, 0.~2.0 mg IV For persistent symptoms, therapy should be
instituted (as in Sec. lilA).
type II block is characterized by AV conduction block without preceding
conduction delay. The site of block is localized most often to the His-Purkinje
Type II block, especially in the setting of a bundle
branch block, often antedates the development of complete heart block.
The EGG reveals no change in PR interval preceding
a non-conducted P wave.
Response to pharmacologic therapy is poor and often
temporary Consequently, permanent pacemaker Insertion frequently
3. Third degree
(complete) AV block is the absence of conduction of atrial impulses
to the ventricles. The site of block may be the AV node (as occurs in congenital
heart block), or within the His-Purkinje system (typical for acquired heart
block). In complete heart block, the atrial
rate typically exceeds the ventricular rate, with
no fixed relation between atrial and ventricular activity (AV dissociation),
and the ventricular rate usually is regular, owing to the regularity of
the escape rhythm focus. Complete heart block
should be distinguished from competitive AV dis?sociation. The latter results
from an AV nodal or ventricular focus exceed?ing the sinus or atrial rate
(i.e. the ventricular rate exceeds the atrial rate)
and usually is benign. Response
to pharmacologic therapy is poor and often temporary. Persistent
symptoms, transient episodes with an under-lying etiology likely to recur,
or the failure to identify a reversible cause,
necessitates pacemaker therapy.
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