474
SECTION III: Abnormal Rhythms
REFERENCES
GLOSSARY
Atrioventricular (AV) block:
a conduction
abnormality located between the atria and
the ventricles. Both the severity and the loca-
tion of the abnormality should be considered.
Degree:
a measure of the severity of AV block.
First-degree AV block:
conduction of atrial
impulses to the ventricles with PR intervals
of 0.21 second.
Footprints of the Wenckebach sequence:
the
pattern of clusters of beats in small groups,
with gradually decreasing intervals between
beats, preceding a pause that is less than
twice the duration of the shortest interval.
Heart block:
another term used for AV block.
His-bundle electrograms:
intracardiac recordings
obtained via a catheter positioned across the
tricuspid valve adjacent to the common or His
bundle. These recordings are used clinically to
determine the location of AV block when this is
not apparent from the surface ECG recordings.
Infranodal block:
AV block that occurs dis-
tal to or below the AV node and therefore
within either the common bundle or in both
the RBB and LBB.
Mobitz type I (type I):
a pattern of AV block
in which there are varying PR intervals. This
pattern is typical of block within the AV node,
which has the capacity for wide variations in
conduction time. Wenckebach sequences are
the classic form of type I block.
Mobitz type II (type II):
a pattern of AV block
in which there are constant PR intervals
despite varying RP intervals. This pattern is
typical of block in the ventricular Purkinje
system, which is incapable of significant
variations in conduction time.
RP interval:
the time between the beginning of
the previously conducted QRS complex and
the beginning of the next conducted P wave.
RP/PR reciprocity:
the inverse relationship
between the interval of the last previously
conducted beat (RP interval) and the time
required for AV conduction (PR interval).
This occurs in type I AV block.
Second-degree AV block:
the conduction of
some atrial impulses to the ventricles, with
the failure to conduct other atrial impulses.
Third-degree AV block:
failure of conduction
of any atrial impulses to the ventricles. This
is often referred to as “complete AV block.”
Wenckebach sequence:
the classic form of
type I AV block, which would be expected to
occur in the absence of autonomic influences
on either the SA or AV nodes.
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