Page 379 - Cardiac Nursing

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C HAPTER 1 6 / Arrhythmias and Conduction Disturbances 355
rhythm as well, but it can also be proarrhythmic in some of embolization when sinus rhythm is restored and the atria be-
patients. gin contracting again. If the duration of fibrillation is not
Radiofrequency ablation of the AV node with insertion of known or is known to be greater than 48 hours, anticoagulation
a ventricular pacemaker can be used in patients who are re- with warfarin to an International Normalized Ratio between 2
fractory to or intolerant of drug therapy. In this case, AF con- and 3 (target is 2.5) for at least 3 weeks should be done prior to
tinues but AV node ablation causes complete heart block and electrical or chemical cardioversion and continued for another 4
prevents the atrial impulses from reaching the ventricles, thus, weeks. If immediate cardioversion due to hemodynamic insta-
the need for a ventricular pacemaker. Ablation of the acces- bility is needed and the duration of flutter is more than 48
sory pathway in patients with WPW syndrome who develop hours, a transesophageal echocardiography should be performed
AF is often necessary to prevent an extremely rapid ventricu- to determine if clots are present in the atria. Unfractionated he-
lar rate when AF conducts to the ventricle via the accessory parin should be given concurrently with cardioversion (unless
pathway. contraindicated) to keep the activated partial thromboplastin
time at 1.5 to 2 times control. Heparin should be continued until
Rhythm Control. Restoration of sinus rhythm should im- oral anticoagulation results in International Normalized Ratio of
prove hemodynamics, relieve symptoms associated with AF, and 2 to 3; and oral anticoagulation should be continued for at least
was previously thought to prevent embolization. Until recently, 4 weeks. 34
medical therapy for AF was aimed at restoring and maintaining
sinus rhythm using aggressive drug therapy and repeated elec- Supraventricular Tachycardia
trical cardioversions. The expected advantages of rhythm con- The term SVT could be applied to any rhythm at a rate faster
trol were not confirmed in trials comparing rate control to than 100 beats per minute that originates above the ventricle.
rhythm control, 37,38 and currently there is a general preference Technically, sinus tachycardia, AT, atrial flutter, AF, junctional
for rate control unless it is a first episode of AF, or the patient tachycardia, AVNRT, and CMT utilizing an accessory pathway
prefers rhythm control or remains very symptomatic with rate in WPW syndrome can all be called SVT. The other commonly
control. used term to describe the reentrant tachycardia associated with
Elective cardioversion can be performed in patients for whom WPW is AV reciprocating (or reentrant) tachycardia; in this
rhythm control is considered the therapy of choice, in patients chapter the term CMT is used to avoid confusion between the
who are intolerant of drug therapy, or in whom drug therapy has terms AVNRT and AV reciprocating (or reentrant) tachycardia.
been unsuccessful. Cardioversion to sinus rhythm can be done If the type of tachycardia is clear from the ECG, then it should
pharmacologically or electrically. The ACC/AHA guidelines rec- be called by its appropriate name and the term SVT should be
ommend the following drugs for pharmacological cardioversion: reserved for regular, narrow QRS tachycardias in which the ex-
flecainide, dofetilide, propafenone, ibutilide, or amiodarone. 34 act mechanism cannot be identified from the ECG. For exam-
The use of class IC antiarrhythmics (flecainide, propafenone) is ple, rhythms like sinus tachycardia, atrial flutter, AF, and some
not recommended in patients with acute MI. In patients with HF, ATs are easily recognized by seeing atrial activity that is charac-
amiodarone and dofetilide are recommended to maintain sinus teristic of the rhythm on the ECG. However, some ATs,
rhythm. Hospitalization is recommended for patients started on AVNRT, and CMT often appear as regular narrow QRS tachy-
antiarrhythmic drug therapy for restoration or maintenance of si- cardias in which atrial activity cannot be seen, making identifi-
nus rhythm due to a 10% to 15% incidence of adverse cardiac cation of the correct mechanism difficult or impossible from the
events during initiation of therapy. 41 The side effects of greatest ECG. Thus, the term SVT is an umbrella term for rhythms that
concern are bradycardia and proarrhythmia due to QT interval originate above the ventricle (resulting in a narrow QRS com-
prolongation. plex) but whose exact mechanism cannot be determined from
Nonpharmacologic therapies used for the treatment of AF in- the surface ECG.
clude implantable atrial defibrillators and radiofrequency catheter SVT is characterized on the ECG by the following:
ablation. Atrial defibrillators detect the onset of AF and deliver a
shock between two intracardiac leads to terminate AF. 42 Ablation Rate: Greater than 100 beats per minute; can be as fast as 280
to create linear lesions within the atria (similar to the surgical beats per minute
Maze procedure) has been reported to be successful, as well as fo- Rhythm: Regular
cal ablations around the orifice of the pulmonary veins in the left P waves: Usually not visible, making the exact mechanism of the
atrium to isolate the pulmonary veins form the left atrium. 43,44 tachycardia uncertain
See Chapter 18 for more on the use of ablation in managing ar- PR interval: Not measurable if P waves cannot be seen
rhythmias. A new procedure called PLAATO (percutaneous left QRS complex: Usually narrow; may be wide if aberrant ventric-
atrial appendage transcatheter occlusion) is being used in an effort ular conduction occurs
to prevent embolic stroke in patients with nonrheumatic AF. 45 Conduction: Conduction through the atria varies depending on
The PLAATO procedure is performed in a cardiac catheterization the mechanism of tachycardia. Atria may depolarize in a retro-
laboratory by way of a right heart catheterization and transseptal grade direction when the mechanism is AVNRT or CMT.
puncture to place an occluder device into the left atrial appendage Conduction through ventricles is normal unless bundle-
to seal it off and prevent embolization of clots that tend to form branch block is present or there is anterograde conduction
in the appendage. through an accessory pathway.
Examples: Two examples of SVT. (A) SVT at a rate of
Anticoagulation. Anticoagulation is needed prior to elec- 187, found to be AVNRT during electrophysiology study. (B)
trical or chemical cardioversion in patients who have been in Narrow QRS tachycardia at a rate of 187, mechanism is un-
atrial flutter or fibrillation longer than 48 hours due to the risk known.