CHAPTER 2 • ELECTRICAL ACTIVITY OF THE HEART
25
gate. Therefore, timing is critical because the
action potential exiting the block must find
the tissue excitable to continue propagation
and thereby establish a reentry circuit.
Because both
conduction velocity and
refractory state of the tissue are important
for reentry to occur, alterations in conduc-
tion velocity and tissue refractoriness can
either precipitate or abolish reentry circuits.
Arrhythmias caused by reentry can be par-
oxysmal in nature (sudden onset and disap-
pearance) because the conditions necessary to
establish and maintain reentry are altered by
normal variations in conduction velocity and
refractoriness brought about by autonomic
and other influences. Changes in autonomic
nerve function, therefore, can significantly
affect reentry mechanisms, either precipitat-
ing in susceptible individuals or terminating
reentry circuits. Antiarrhythmic drugs that
alter the ERP or conduction velocity can be
used to prevent or abolish reentry.
Reentry can occur either globally (e.g.,
between the atria and ventricles) or locally
(e.g., within a small region of the ventricle
or atrium) as shown in Figure 2.12. Global
reentry between the atria and ventricles often
involves
accessory
conduction
pathways
such as the bundle of Kent. Accessory path-
ways allow impulses to be conducted by one
or more routes in addition to the normal
AV nodal pathway. In the example shown in
Figure 2.12, the impulse travels through the
accessory pathway, depolarizes the ventricular
tissue, and then travels backward (retrograde)
through the AV node to reexcite the atrial tis-
sue and thereby establish a counterclockwise
global reentry circuit. (The reentry circuit
can also occur clockwise in direction.) Global
reentry between the atria and ventricles results
in supraventricular tachyarrhythmias
(e.g.,
Wolff-Parkinson-White syndrome). Local sites
of reentry within a small region of the ventricle
or atrium can precipitate ventricular or atrial
tachyarrhythmias, respectively.
CASE 2-1
A patient is diagnosed with a
supraventricular tachycardia caused
by reentry within the AV node. Explain
how using a drug that increases the ERP
of the AV nodal tissue can be used to
abolish this tachyarrhythmia.
FIGURE 2.12 Global and local reentry. Global reentry can occur between the atria and ventricles utilizing
an accessory pathway in addition to the AV node. One such pathway is the Bundle of Kent between the
RA and RV, which can lead to retrograde action potential conduction (in this illustration) through the AV
node and cause premature excitation of atrial muscle and a supraventricular tachycardia. Local reentry
circuits can occur within either the ventricles or atria and produce tachyarrhythmias.
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