Page 392 - Cardiac Nursing

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368 P A R T III / Assessment of Heart Disease
which is done to (1) confirm the presence of preexcitation, (2) pathway with a long refractory period and a slow-conducting path-
identify the mechanism of the associated tachyarrhythmia, (3) lo- way with a short refractory period. In AVNRT, a reentry circuit is
calize the site of the accessory pathway, (4) confirm participation set up in the AV node, using one pathway (usually the slow path-
of the accessory pathway in maintenance of the tachycardia, (5) way) for the anterograde limb and the other pathway (usually the
determine the functional behavior of the accessory pathway, and fast pathway) as the retrograde limb (Fig. 16-11).
(6) determine the effects of different drugs on conduction veloc- Normally, the sinus impulse conducts down the fast pathway
ity and refractoriness in both pathways. If the arrhythmia is due into the ventricles, resulting in a normal PR interval of 0.12 to
to reentry, therapy is directed toward changing the conduction 0.20 second. If a PAC occurs before the fast pathway with its long
time or the refractory period in the AV node or in the accessory refractory period has recovered, the impulse conducts down the
pathway, or both, so that reentry is abolished. Prolonging the re- slow pathway because of its shorter refractory period, resulting in
fractory period in the AV node or in the bypass tract or inducing a PAC with a long PR interval. The long conduction time through
block in either of these pathways can interrupt reentry and stop the slow pathway allows the fast pathway time to recover, making
the tachycardia. If AF is the mechanism, treatment is aimed at it possible for the impulse to conduct backward into the fast path-
preventing the occurrence of the arrhythmia and slowing conduc- way. The returning impulse can then reenter the slow pathway
tion through the accessory pathway. Radiofrequency catheter ab- and initiate a circuit in the AV node, resulting in AVNRT. Figure
lation of the bypass tract offers a cure for tachycardias associated 16-11 illustrates the most common mechanism of AVNRT. The
with accessory pathways and has become the therapy of choice. resulting rhythm is usually a narrow QRS tachycardia because the
The reported success rate for accessory pathway ablation is ventricles are activated through the normal His–Purkinje system.
95%. 67,68 (See Chapter 18 for more information about electro- P waves are either not visible at all or are seen peeking out at the
physiology studies and ablation in management of arrhythmias. end of the QRS complex because the atria are activated in a ret-
The section below on supraventricular tachycardias covers drug rograde direction at the same time as the ventricles are being de-
therapy of specific tachycardias in more detail.) polarized in an anterograde direction (Fig. 16-12). In the presence
of preexisting bundle-branch block or rate-dependent bundle-
branch block, the QRS in AVNRT is wide.
Supraventricular Tachycardia In approximately 10% of cases of AVNRT, the fast pathway is
used as the anterograde limb and the slow pathway is used as the
The term SVT is used for all tachycardias that either originate retrograde limb of the circuit. 22,69 This results in a long R-P
from supraventricular tissue (i.e., SA node reentry, atrial flutter, tachycardia in which the P wave appears in front of the QRS be-
junctional tachycardia) or incorporate supraventricular tissue in a cause atrial activation is delayed owing to slow conduction back-
reentry circuit (i.e., AVNRT and CMT using an accessory path- ward through the slow pathway. These P waves are inverted in in-
69
way). Usually, SVT is used to describe narrow QRS tachycardias ferior leads because the atria are depolarized in a retrograde
because the narrow QRS denotes normal intraventricular conduc- direction.
tion through the His–Purkinje system from a supraventricular fo- AVNRT is an AV nodal active SVT because the AV node’s par-
cus. It is possible for an SVT to conduct with bundle-branch ticipation is required to maintain the tachycardia. Therefore, any-
block, which would result in a wide QRS but would not change thing that blocks the AV node, such as vagal stimulation or drugs
the fact that the rhythm is supraventricular in origin. Thus, SVT like adenosine, -blockers, or calcium channel blockers, can ter-
can be used for wide QRS rhythms that are known to be coming minate the rhythm. AVNRT is usually well tolerated unless the
from above the ventricles. rate is extremely rapid. Many people with this arrhythmia learn to
SVT can be classified into those that are AV nodal passive and stop it by coughing or breath holding, which stimulates the vagus
those that are AV nodal active. AV nodal passive SVTs are those in nerve. Adenosine is the preferred agent for terminating AVNRT
which the AV node does not play a part in the maintenance of the except in patients with asthma, but it does not prevent its recur-
tachycardia but serves only to conduct passively the supraventricu- rence. Radiofrequency ablation of the slow pathway has become
25
lar rhythm into the ventricles. AV nodal passive SVTs include AT, the treatment of choice for all forms of AVNRT, with almost
atrial flutter, and AF, all of which arise from within the atria and 100% success. 68,69,72
do not need the AV node’s participation to sustain the atrial ar-
rhythmia. AV nodal active tachycardias require participation of the Circus Movement Tachycardia
AV node in the maintenance of the tachycardia. The two most CMT (also called AV reentry tachycardia [AVRT]) is an SVT that
common causes of a regular, narrow QRS tachycardia are AVNRT occurs in people who have accessory pathways, also called bypass
and CMT using an accessory pathway, both of which require the tracts, that allow impulses to conduct directly from atria to ven-
AV node as part of the reentry circuit that sustains the tachycardia. tricles (see section titled “Preexcitation Syndromes”, earlier). Ap-
AF is usually easily recognized owing to its irregularity, but AT, proximately 30% of regular, narrow QRS tachycardias are due to
atrial flutter, junctional tachycardia, AVNRT, and CMT can all CMT using an accessory pathway. 22,59,69
present as a regular, narrow QRS tachycardia whose mechanism In CMT, the reentry circuit involves the atria, AV node, ven-
cannot always be determined from the ECG. Because AVNRT tricle, and accessory pathway. The term orthodromic is used to de-
and CMT are responsible for most regular, narrow QRS tachy- scribe the most common type of CMT, in which the AV node is
cardias, these two are discussed in detail here.
used as the anterograde limb and the accessory pathway is used as
the retrograde limb of the circuit. This results in a narrow QRS
AV Nodal Reentry Tachycardia tachycardia because the ventricles are depolarized through the
AVNRT is the most common mechanism of SVT and is responsi- His–Purkinje system. If bundle-branch block is present, the QRS
ble for up to two thirds of regular, narrow QRS tachycardias. 69–71 is wide. Because the atria and ventricles depolarize separately, the
This rhythm involves dual AV nodal pathways: a fast-conducting P waves in CMT, if visible, are often seen in the ST segment or