Radiofrequency Catheter Ablation of
Supraventricular Tachycardia
Hugh
Calkins,* Ajit
Kumar V.K.,** Johnson Francis***
*Professor of Medicine Johns
Hopkins Hospital, Baltimore, Maryland, USA.
**Associate Professor of Cardiology, Sree Chitra Tirunal Institute of
Medical
Science and Technology, Trivandrum, India.
***Associate Professor of Cardiology, Medical College Calicut, Kerala,
India.
Address for correspondence: Hugh Calkins, MD, Professor of
Medicine, The Johns Hopkins Hospital, 592 Carnegie, 600 North Wolfe
Street, Baltimore, Maryland 21287-6568, USA
Email: hcalkins@jhmi.edu
Keywords: supraventricular tachycardia; atrioventricular
nodal re-entry tachycardia; atrioventricular re-entry tachycardias
Introduction
Supraventricular tachycardias are quite common in clinical practice.
Medical treatment of supraventricular tachycardia often involves
regular intake of drugs for several years. Problems of drug therapy
include poor efficacy and bothersome side effects including
proarrhythmia. This has lead to the development of non-pharmacological
therapies. Arrhythmia surgery initially demonstrated that many types of
supraventricular arrhythmias could be cured. However during the past
decade
arrhythmia surgery has been largely replaced by catheter ablation.
Catheter
ablation can be defined as the use of an electrode catheter to destroy
small
areas of myocardial tissue or conduction system, or both, that are
critical
to the initiation or maintenance of cardiac arrhythmias. Arrhythmias
most
likely to be amenable to cure with catheter ablation are those which
have
a focal origin or involve a narrow, anatomically defined isthmus.1 This review
aims to provide an update on the technique and results associated that
can be achieved with catheter ablation of supraventricular
tachycardias.
Classification of
Supraventricular Tachyarrhythmias
Supraventricular tachyarrhythmias have been classified into regular and
irregular (atrial fibrillation).2 Atrial
fibrillation is less amenable to cure with catheter ablation than are
the regular forms of supraventricular tachyarrhythmias such as
atrioventricular nodal reentrant tachycardia (AVNRT) and
atrio-ventricular reciprocating tachycardia.(AVRT). Non-pharmacological
treatment of atrial fibrillation has been reviewed recently.3 Regular supraventricular tachycardias (SVT)
can be
cured by catheter ablation with high success rates (95–99%) and low
complication rates (< 1%)2. Regular
SVTs can be subdivided into:
1. Atrioventricular re-entry tachycardias (AVRT), using the ventricle
as part of the circuit; these tachycardias are dependent on the
presence of an accessory atrioventricular (AV) pathway.
2. Atrioventricular nodal re-entry tachycardia (AVNRT), where the
re-entry circuit is within the AV node and the ventricle plays no part
in maintaining the arrhythmia.
3. Atrial tachycardia, where the re-entry circuit does not involve any
part of the AV junction e.g. atrial flutter, ectopic atrial tachycardia.
Technique of Ablation
Catheter ablations are performed in cardiac electrophysiology
laboratories specially equipped with recorders, programmed stimulators
and ablators. Ever since 1989, radiofrequency current has been used for
ablation, while earlier attempts were with direct current. The
procedure is typically performed using conscious sedation. Two to five
multipolar electrode catheters are inserted percutaneously under local
anaesthesia into a femoral, brachial, subclavian, or internal jugular
veins and positioned in the heart under fluoroscopic guidance. Each
electrode catheter has four or more electrodes. The most distal
electrode pair is usually used for pacing and the delivery of
critically timed extra stimuli, while all of the electrodes are used to
record electrograms from localised regions within the heart. Up to 50 W
of radiofrequency energy is delivered for 30-60 seconds as a
continuous, unmodulated, sinusoidal waveform with a frequency of
approximately 500,000 Hz, between the 4 mm tip of a
deflectable ablation catheter and a ground plate positioned on the
patient's
back or chest. The temperature of the ablation electrode can be
monitored
and the power output automatically adjusted to achieve a targeted
electrode
temperature of between 60-70°C. Knowledge of the electrode
temperature
at a particular ablation site is useful in determining whether an
unsuccessful
application of radiofrequency energy failed because of inaccurate
mapping
or inadequate heating.4 If the failure was
as
a result of inadequate heating, additional applications of energy at
the
same site with improved catheter stability may succeed. Automatic
adjustment
of power output using closed loop temperature control has been shown to
reduce the incidence of coagulum development. This reduces the number
of
times the catheter has to be withdrawn from the body to have a coagulum
removed from the electrode tip. Thermal injury is the main mechanism of
tissue
destruction during radiofrequency catheter ablation. Elevation of
tissue
temperature results in desiccation and the denaturation of proteins,
and
coagulation of tissue and blood. Irreversible tissue injury occurs at
temperatures
above 50°C. When temperature reaches 100°C, plasma proteins
denature
to form a coagulum. The coagulum causes a sharp rise in the impedance
and
a corresponding fall in the current density, thereby limiting further
lesion
growth. Methods for improved cooling of the electrode have been
developed
to allow delivery of higher radiofrequency power without the formation
of
coagulum. These include the use of larger (8 mm) electrodes, which
receive
greater convective cooling by the blood, and saline irrigated electrode
tips,
in which the electrode is actively cooled. Biplane fluroscopic imaging
gives
a three dimensional orientation of catheter position, enabling better
localization.
Non fluroscopic imaging systems using magnetic fields and ultrasound
are
being developed. These modalities avoid the inherent risk of radiation
exposure
to the patient and the treating physician.
Ablation of
atrioventricular re-entry tachycardias (AVRT)
AVRT
utilizes accessory pathways which are anomalous extra nodal connections
between the epicardial surface of the atrium and ventricle along the
atrioventricular groove. Accessory pathways can be classified based on
their location along the mitral or tricuspid annulus, type of
conduction (decremental or non-decremental), and whether they are
capable of antegrade conduction, retrograde conduction, or both.
Accessory pathways which are capable only of retrograde conduction are
concealed while those capable of antegrade conduction are manifest,
demonstrating pre-excitation on a standard ECG. AVRT is the most common
arrhythmia
in patients with a manifest accessory pathway, occurring in 75% of
cases.
An
electrophysiological study is performed to localise the accessory
pathway and determine its conduction characteristics prior to ablation.
Accurate localisation of an accessory pathway is critical to the
success of catheter ablation procedures. Preliminary localisation of
the accessory pathway can be determined based on the delta wave and QRS
morphology on the surface
ECG in patients with manifest pre-excitation. Mapping of concealed
accessory
pathways and more accurate localisation of manifest accessory pathways
require
analysis of the retrograde atrial activation sequence and/or antegrade
ventricular activation sequence. Right sided and posteroseptal
accessory pathways are typically localised and ablated using a
steerable electrode catheter with a 4 mm distal electrode positioned
along the tricuspid annulus or in the
coronary sinus os from the inferior vena cava. The location of left
sided
accessory pathways can be determined using a multipolar electrode
catheter
positioned in the coronary sinus, which runs parallel to the left
atrioventricular
groove or with a steerable catheter positioned in the left atrium or
ventricle.
Once localised to a region of the heart, precise mapping and ablation
is
performed using a steerable 4 mm tipped electrode catheter positioned
along
the mitral annulus using either the transeptal or retrograde aortic
approach.
These two approaches for ablation of left sided accessory pathways are
associated with a similar rate of success and incidence of
complications.5 The decision over which
approach to employ is usually based on physician preference, although
the transeptal approach may be preferable in the elderly and in young
children. In rare instances, left sided accessory pathways can only be
ablated via the coronary sinus.
Appropriate sites for radiofrequency energy delivery during ablation of
manifest accessory pathways are characterised by early ventricular
activation, the presence of an accessory pathway potential, and
stability of the local electrogram6.
Appropriate sites for energy delivery in patients with retrograde
conduction accessory pathways mapped during ventricular pacing or
orthodromic AVRT are characterised by continuous electrical activity,
the presence of accessory pathway potential, and electrogram stability.
Once an appropriate target site is identified, radiofrequency energy is
delivered for 30-60 seconds with a target electrode temperature of
60-70°C. At successful ablation sites, interruption of conduction
through the accessory pathway usually occurs within 10 seconds, and
often within two seconds of the onset of radiofrequency energy delivery.
Efficacy of catheter ablation of accessory pathways varies from 89-99%.7,8, 9, 10
The success rate is highest for left free wall accessory pathways and
lowest for posteroseptal and right free wall accessory pathways.
Recurrence of conduction occurs in approximately 7%.7,
8,9, 10
Recurrence is more common following ablation of posteroseptal and right
free wall pathways. Accessory pathways which recur can usually be
successfully reablated. Complications associated with catheter ablation
may result from vascular access (haematomas, deep venous thrombosis,
perforation of the aorta, arteriovenous fistula, pneumothorax),
catheter manipulation (valvar damage, microemboli, perforation of the
coronary sinus or myocardial wall, coronary dissection and/or
thrombosis), or delivery of radiofrequency energy (atrioventricular
block, myocardial perforation, coronary artery spasm or occlusion,
transient ischaemic attacks or cerebrovascular accidents). Complete
heart block occurs in about 1% of patients, most commonly after
ablation of septal and posteroseptal accessory pathways. The overall
incidence of complications varies between 1-4%. Procedure related death
is estimated to be less than 0.2%.9
Ablation of
Atrioventricular Nodal Re-entrant tachycardia
Atrioventricular nodal re-entrant tachycardia (AVNRT) occurs in
patients with dual AV nodal physiology. They have two functionally
distinct pathways known as the slow and fast pathway. The slow pathway
has a shorter refractory period. During the common form of AVNRT, an
atrial premature beat travels down the slow pathway and gets conducted
back by the fast pathway to depolarise the atrium. In the uncommon
variety, the reverse pattern occurs. The fast pathway is located
anteriorly along the septal portion of the tricuspid annulus, near the
compact atrioventricular node, whereas the atrial insertion of the slow
pathway is located more posteriorly along the tricuspid annulus, closer
to the coronary sinus ostium.
AVNRT can be cured by ablating either the fast pathway or the slow
pathway. However, it has clearly been shown that catheter ablation of
the slow pathway using a “posterior approach” is associated with
greater safety and efficacy. For this reason, the “anterior approach”
is merely of historical interest. Fast pathway ablation is by an
“anterior” approach. Catheter ablation is performed by locating an
electrogram with a large His potential and then withdrawing the
ablation catheter into the right atrium until the atrial signal is
at least twice that of the ventricular signal (A:V ratio > 2) with a
His potential no larger than 50 µV. Radiofrequency energy is then
applied during sinus rhythm for 30-60 seconds while watching for
prolongation
in the PR interval. Energy delivery is immediately terminated if
atrioventricular block occurs. Successful ablation of AVNRT is
characterised by lengthening of the PR interval and the inability to
induce the tachycardia. There is
elimination or pronounced attenuation of retrograde conduction during
ventricular
pacing. The atrioventricular block cycle length and the
atrioventricular node
effective refractory period are not usually altered during ablation by
the
anterior approach. Anterior approach is successful in about 90% of
cases.
Inadvertent atrioventricular block occurs in approximately 7% of
patients. Recurrence rate is about 9%.1
Slow
pathway ablation is by the “posterior” approach. The ablation catheter
is directed into the right ventricle low near the posterior septum and
is then withdrawn until an electrogram is recorded with a small atrial
electrogram and a large ventricular electrogram (A:V ratio < 0.5).
Specific
ablation sites along the posterior portion of the tricuspid annulus can
be selected based either on the appearance of the local atrial
electrogram
or based strictly on anatomic factors. While using the electrogram
guided
approach, fractionated atrial electrograms with a late "slow potential"
are targeted. In the anatomic approach, the initial applications are
delivered
at the level of the coronary sinus ostium and subsequent applications
at
more superior sites. Junctional beats occurring during the application
of
radiofrequency energy are a marker for successful ablation. Successful
ablation
is characterised by an increase in the atrioventricular block cycle
length
and in the atrioventricular node effective refractory period and
elimination
of inducible AVNRT. The posterior approach for ablation of AVNRT is
effective
in greater than 95-97% of patients. 9,11, 12 Atrioventricular
block
occurs in 0.5-1% and is the most common complication. Recurrence
following
successful ablation using the posterior approach is approximately 3%.
Posterior
approach is the preferred approach to ablation of AVNRT due to the
higher
efficacy, lower incidence of atrioventricular block and arrhythmia
recurrence,
and the greater likelihood of maintaining a normal PR interval during
sinus
rhythm.
Atrial Tachycardias
Catheter ablation of sustained regular atrial tachycardias can also be
performed with high safety and efficacy. Focal atrial tachycardia,
whether reentrant, automatic, or triggered can be ablated with success
rates greater than 90%. The site for ablation is generally chosen based
on early activation. Typical atrial flutter is also amenable to cure
with catheter ablation. This macroreentrant arrhythmias is localized to
the right atrium. Atrial flutter can be cured by creating a continuous,
transmural, linear lesion between the tricuspid annulus and the
inferior vena cava. This procedure is generally referred to as an
“isthmus” ablation. Catheter ablation of atrial flutter can be
performed with an efficacy greater than 90% and a < 5% risk of
recurrence of atrial flutter. Non-isthmus dependant atrial flutter can
also be treated with
catheter ablation. These regular atrial tachycardias, with rates
between 250 and 350 bpm often involve scars created during prior
cardiac surgery. The procedure can be accomplished by defining a
critical region of conduction which is necessary for maintenance of the
arrhythmia. The creation of a linear
lesion in this region can result in cure of the tachycardia. Catheter
ablation
of non-isthmus dependant atrial flutter can be accomplished with an
efficacy of greater than 80% and a < 5% risk of major complications.
Conclusion
Because of the remarkable safety and efficacy of catheter ablation, it
is now considered as first line therapy for most types of
supraventricular arrhythmias. These include AVNRT, AVRT, atrial
tachycardia, and atrial flutter. It is anticipated that the dream of
curing atrial fibrillation with ablation techniques will also be
realized sometime during the next five years.
References
1. Calkins H.
Radiofrequency catheter ablation of supraventricular arrhythmias. Heart
2001;85:594-600
2. Schilling R.J. Which patient should be referred
to an electrophysiologist: supraventricular tachycardia. Heart 2002;
87:299-304
3. Anfinsen O. Non-pharmacological Treatment of
Atrial Fibrillation. Indian Pacing and Electrophysiology Journal; 2002;
2:4
4. Dinerman J, Berger RD, Calkins H. Temperature
monitoring during radiofrequency ablation. J Cardiovasc Electrophysiol
1996;7:163-173
5. Lesh MD, Van Hare G, Scheinman MM, et al.
Comparison of the retrograde and transeptal methods for ablation of
left free-wall accessory pathways. J Am Coll Cardiol
1993;22:542-549
6. Calkins H, Kim YN, Schmaltz S, et al.
Electrogram criteria for identification of appropriate target sites for
radiofrequency catheter ablation of accessory atrioventricular
connections. Circulation 1992;85:565-573
7. Jackman WM, Wang X, Friday KJ, et al. Catheter
ablation of accessory atrioventricular pathways (Wolff-Parkinson-White
syndrome) by radiofrequency current. N Engl J Med 1991;324:1605-1611
8. Calkins H, Sousa J, El-Atassi R, et al.
Diagnosis and cure of the Wolff-Parkinson-White syndrome or paroxysmal
supraventricular tachycardias during a single electrophysiology test. N
Engl J Med 1991;324:1612-1618
9. Calkins H, Yong P, Miller JM, et al, for the
Atakr Multicenter Investigators Group. Catheter ablation of accessory
pathways, atrioventricular nodal reentrant tachycardia, and the
atrioventricular junction: final results of a prospective, multicenter
clinical trial. Circulation 1999;99:262-270
10. Calkins H, Yong P, Miller JM, et al, for the
Atakr Multicenter Investigators Group. Catheter ablation of accessory
pathways, atrioventricular nodal reentrant tachycardia, and the
atrioventricular junction: final results of a prospective, multicenter
clinical trial. Circulation 1999;99:262-270
11. Haissaguerre M, Gaita F, Fischer B, et al.
Elimination of atrioventricular nodal reentrant tachycardia using
discrete slow potentials to guide application of radiofrequency energy.
Circulation 1992; 85:2162-2175.
12. Jackman WM, Beckman KJ, McClelland JH, et
al. Treatment of supraventricular tachycardia due to atrioventricular
nodal reentry by radiofrequency catheter ablation of slow-pathway
conduction. N Engl J Med 1992;327:313-318
