(Circulation. 1995;92:1188-1192.)
© 1995 American Heart Association, Inc.
Articles |
Correlation of Temperature and Pathophysiological Effect During Radiofrequency Catheter Ablation of the AV Junction
From the Cardiovascular Division, Department of Internal Medicine, University of Virginia School of Medicine, Charlottesville.
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Abstract |
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Background Accelerated junctional rhythms have been observed before the development of AV nodal block during radiofrequency (RF) catheter ablation of the AV junction. However, the time course and temperatures required to induce an accelerated junctional rhythm and AV nodal block during this procedure have not yet been characterized.
Methods and Results Nineteen patients underwent RF ablation of the AV junction with a thermistor ablation catheter. RF energy was initially delivered at 10 W for 9 seconds and then increased by 5-W increments for 9 seconds at each power level up to a maximum power of 50 W. If a junctional rhythm was observed during the power titration, a 30- to 60-second RF application was then delivered at the same power level. The power was then further increased to a maximum of 50 W if AV nodal block was not observed after 20 seconds of RF delivery. The procedure was successful in all 19 patients. A median of one RF application (range, one to eight applications) was required to produce permanent AV nodal block. An accelerated junctional rhythm was observed during 89% of successful attempts versus 70% of unsuccessful deliveries (P=NS). The median time to onset of the junctional rhythm was significantly shorter during successful compared with unsuccessful applications (1.8 versus 7.7 seconds, respectively; P<.001). Similarly, the mean time to appearance of AV nodal block was significantly shorter during successful compared with unsuccessful attempts (19.6±9.4 versus 36.8±19.0 seconds, respectively; P<.01). The catheter tip temperatures associated with the development of an accelerated junctional rhythm were significantly lower than those associated with the appearance of AV nodal block (51±4°C versus 58±6°C, respectively; P<.001). Mean temperatures in the range of 60±7°C were required to produce permanent AV nodal block.
Conclusions The development of an accelerated junctional rhythm within 5 seconds and the appearance of AV nodal block within 30 seconds of RF onset were both highly characteristic of successful target sites during RF ablation of the AV junction. The accelerated junctional rhythm and AV nodal block were both highly temperature dependent. The temperatures associated with the onset of AV nodal block were significantly higher than the temperatures resulting in an accelerated junctional rhythm.
Key Words: catheter ablation • atrioventricular node • radiofrequency
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Introduction |
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Radiofrequency (RF) catheter ablation has become the preferred technique for creating permanent AV block in patients with atrial arrhythmias refractory to medical treatment. The reported success rate in recent series for producing complete AV nodal block with this technique has varied from 81.5% to 100%.1 2 3 4 5 Occasionally, ablation of the AV junction from the left ventricle has been required when the standard approach from the right side has failed.2 Previous studies have used electrogram criteria to identify the optimal ablation site for inducing permanent AV nodal block—typically an equal atrial and ventricular amplitude signal with a small-amplitude His deflection recorded on the ablation electrode to find the best ablation site.1 2 3 4 5 A potential problem with using only electrogram amplitudes to guide ablation of the AV junction is that patients are frequently in atrial fibrillation at the time of the procedure, making electrogram interpretation difficult. Other investigators have used a combination of electrograms and anatomic guidance to select the optimal ablation site.1 In one preliminary report, the occurrence of accelerated junctional beats at the onset of RF energy delivery was found to be the best predictor of complete AV nodal block.6
Tissue injury by RF ablation is presumed to be thermally mediated.7 The time course and temperatures associated with an accelerated junctional rhythm and complete AV nodal block during RF ablation of the AV junction have not previously been investigated. The aims of the current study were to investigate the time course and temperatures associated with the development of an accelerated junctional rhythm and permanent AV nodal block and to determine whether these parameters could be used as predictors of a successful RF ablation.
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Methods |
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Study Population
The study population consisted of 19 patients referred for RF catheter ablation of the AV junction. All patients were enrolled in the Food and Drug Administration–approved Evaluation of the Safety and Efficacy of the EP Technologies Radiofrequency Powered Cardiac Ablation System for Treatment of Supraventricular Arrhythmias multicenter trial. Written informed consent was obtained from all patients. The study protocol was approved by the Institutional Review Board at the University of Virginia. Patients were enrolled in the study between December 1993 and September 1994.
RF Catheter Ablation
The RF ablation catheter used in the
study consisted of a
quadripolar electrode catheter with a 7F deflectable tip and a
4-mm-long distal ablation electrode (EP Technologies Inc). The distance
between each of the four electrodes was 2 mm. The catheter had a
thermistor incorporated into the tip of the distal electrode. The
thermistor was exposed to the surface at the apex of the electrode but
was thermally insulated from the surrounding electrode by a polyamide
plastic sheath. The thermistor was accurate to within ±2°C in the
range of 37°C to 100°C.
RF energy was supplied by a 500-kHz RF
generator that produced a
maximum output of 50 W (EP Technologies Inc). Power, impedance, and
temperature were continuously measured and displayed during each RF
application. Power delivery was automatically discontinued if measured
impedance exceeded 300 
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Ablation Protocol
The patients were sedated with intravenous
midazolam
and fentanyl. With patients under local anesthesia, a 6F
quadripolar catheter was inserted percutaneously into
the right or left femoral vein and advanced into the right
ventricular apex under fluoroscopic guidance. This
electrode was used for temporary ventricular pacing if
required during the procedure. The RF ablation catheter was inserted
percutaneously into the right femoral vein and advanced
across the tricuspid valve into the right ventricle under fluoroscopic
guidance. The ablation catheter was initially positioned so that the
distal ablation bipolar electrode pair recorded the
largest-amplitude His bundle potential. The catheter was then withdrawn
until the distal electrode pair recorded a small His bundle
deflection and approximately equal-amplitude atrial and
ventricular potentials. In patients who were in atrial
fibrillation at the time of the procedure, the ablation catheter was
initially positioned so that the distal electrode pair recorded a
maximum His bundle potential. The catheter was then slowly withdrawn
until the His and ventricular deflections decreased in
amplitude and the atrial electrograms increased in amplitude but there
was still a visibly distinct His bundle potential.
RF energy was
delivered between the distal electrode of the ablation
catheter and a large adhesive skin patch placed on the patient's arm
or leg. RF applications were performed at each ablation site in the
following manner: RF energy was initially delivered at 10 W for 9
seconds and then increased by 5-W increments for 9 seconds at each
power level up to a maximum power of 50 W. If an accelerated junctional
rhythm was observed during the power titration, a 30- to 60-second RF
application was then delivered at the same power level. During the 30-
to 60-second application, the power level was further increased to a
maximum output of 50 W if complete AV nodal block was not observed
after 20 seconds of RF delivery. The mean temperature during the
1-second time period before the onset of accelerated junctional beats
or AV nodal block was recorded during the 30- to 60-second RF
application. The time from the initiation of RF energy delivery to the
onset of an accelerated junctional rhythm or AV nodal block was also
measured. Peak temperature, mean temperature, and power were also
measured for each 30- to 60-second RF application. If complete AV nodal
block was produced, the patient was observed 30 minutes after the
ablation, and if complete AV nodal block persisted, a rate-responsive
dual- or single-chamber permanent pacemaker was implanted. If the RF
application failed to produce AV nodal block or if AV nodal conduction
returned during the waiting period, the ablation was repeated.
Successful RF applications were defined as those that resulted in
permanent AV nodal block. Unsuccessful RF applications were defined as
those that resulted in no AV or transient AV nodal block. In patients
in atrial fibrillation, an accelerated junctional rhythm was identified
by a 
100-ms decrease in the mean RR interval with regularization of
the rhythm (
60-ms variation in the RR interval between beats after
the onset of RF delivery). After the procedure, patients were observed
in a telemetry unit for at least 24 to 48 hours and discharged from the
hospital. Serial total creatine kinase (CK) and CK-MB isoenzyme were
obtained before the ablation and approximately 4 and 12 hours after the
procedure.
Statistical Analysis
All normally distributed values are
reported as mean±SD. Values
with a nonnormal distribution are reported as median
values±interquartile range. Continuous variables were
analyzed with ANOVA or Student's paired and unpaired
t tests. Categorical variables were analyzed by
Fisher's exact test. A value of P<.05 was considered
significant.
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Results |
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TopAbstractIntroductionMethods Results Discussion References |
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The study population consisted of 11 men and 8 women. The mean age was 69±7 years (range, 55 to 78 years). Eleven patients underwent AV junctional ablation for chronic atrial fibrillation; 7 patients, paroxysmal atrial fibrillation; and 1 patient, multifocal atrial tachycardia. The AV junctional ablation was successful in all 19 patients. The median number of 30- to 60-second RF applications per patient was one (range, one to eight). Fifteen minutes after the ablation, 7 patients had an intrinsic escape rhythm
2000 ms with a
mean cycle length of 1256±244 ms (range, 988 to 1622 ms). The mean
total CK level was 71±49 U/L before the procedure and 146±107
U/L 12
hours after ablation (P<.01). However, no patient had a
significant CK-MB isoenzyme rise after ablation. During a mean
follow-up period of 5.4 months (range, 1 to 10 months), there were no
late recurrences of AV nodal conduction.
AV Nodal Block
Of the 39 total 30- to 60-second RF
deliveries, 19 (49%) resulted
in the production of permanent AV nodal block, 9 (23%) were
associated with transient AV nodal block, and 11 (28%) failed to
achieve AV nodal block. The mean time from initiation of RF delivery to
appearance of AV nodal block was significantly shorter during
successful applications compared with unsuccessful attempts (19.6±9.4
versus 36.8±19.0 seconds, respectively; P<.01; Fig
1
). Because AV nodal block could be identified only
after cessation of the accelerated junctional rhythm, these values
represent the maximal time to onset of block (Fig 2). The
sensitivity, specificity, and positive
predictive value of early AV nodal block (30 seconds of RF onset) as
a predictor of a successful ablation were 87%, 63%, and 81%,
respectively.
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The catheter tip temperatures recorded at the appearance
of AV
nodal block during applications resulting in permanent AV nodal block
were not significantly different from those measured during RF
deliveries associated with transient AV nodal block (58±6°C versus
59±6°C, respectively; Fig 1
). The peak temperatures
reached during
successful applications were not significantly different from those
reached during unsuccessful attempts (77±15°C versus
72±13°C,
respectively; Fig 1). However, the mean temperatures measured
during
successful RF deliveries tended to be higher than during unsuccessful
deliveries (60±7°C versus 56±7°C, respectively;
P=.06), although there were no significant differences in
the RF power levels used (37±10 versus 41±11 W, respectively;
Fig 3).
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Accelerated Junctional Rhythm
An accelerated junctional
rhythm was observed during 17 of 19
(89%) successful deliveries compared with 14 of 20 (70%) unsuccessful
applications (P=NS). Fig 4 illustrates the
characteristics of the junctional rhythm during successful RF
deliveries compared with unsuccessful attempts. The catheter tip
temperature associated with the onset of an accelerated junctional
rhythm was not significantly different between successful and
unsuccessful applications (51±4°C versus 52±5°C,
respectively).
The cycle length of the junctional rhythm was not significantly
different between successful and unsuccessful applications (554±190
versus 648±170 ms, respectively). In contrast, the time to onset of a
junctional rhythm was significantly shorter during successful ablation
attempts compared with unsuccessful attempts (median, 1.8 versus 7.7
seconds; interquartile range, 1.5 to 4.5 seconds versus 5.2 to 8.9
seconds, respectively; P<.001; Fig 5). The
sensitivity, specificity, and positive predictive value of an early
junctional rhythm (5 seconds of RF onset) as a predictor of a
successful ablation were 86%, 79%, and 80%, respectively. The
temperatures associated with the onset of a junctional rhythm were
significantly lower than those associated with the appearance of AV
nodal block (51±4°C versus 58±6°C, respectively;
P<.001; Fig 6).
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Discussion |
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TopAbstractIntroductionMethods Results Discussion References |
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The present study demonstrated that the development of an accelerated junctional rhythm within 5 seconds and the appearance of AV nodal block within 30 seconds of RF onset were both characteristic of successful target sites during RF ablation of the AV junction. This study also found that the catheter tip temperatures associated with the onset of AV nodal block were significantly higher than those associated with the development of an accelerated junctional rhythm. Mean temperatures in the range of 60±7°C were required to ensure permanent AV nodal block. Unsuccessful RF applications were characterized by either no junctional rhythm or the relatively late onset of a junctional rhythm (>5 seconds) and AV nodal block (>30 seconds) after initiation of RF delivery. In addition, the mean catheter tip temperatures tended to be higher during successful RF applications compared with unsuccessful deliveries, even though there were no significant differences in the power levels used. This latter finding implies better electrode-tissue contact during successful RF applications.7
Previous Studies
Previous in vitro studies have found that
temperatures 50°C
are required to produce irreversible loss of myocellular excitability
and conduction block in ventricular
myocardium.8 9 In addition, two clinical
studies have reported that the mean temperatures associated with
permanent block of accessory pathway conduction were 62±15°C and
63±12°C, respectively.10 11 However, no
previous study
has investigated the temperatures associated with the development of an
accelerated junctional rhythm or AV nodal block. The current study
found that significantly more tissue heating was required to induce AV
nodal block than an accelerated junctional rhythm during RF ablation of
the AV junction. Junctional rhythms have been observed during both RF
ablation of the AV junction and ablation of posteroseptal
"slow" AV nodal pathway
sites.1 2 3 4 5 12
In the latter
case, the development of a junctional rhythm is typically not
associated with the appearance of AV nodal block. A previous study
reported that junctional rhythms leading to AV nodal block had faster
rates and were more likely to exhibit VA dissociation than those not
associated with AV nodal block.12 In the current study, no
significant differences in the cycle lengths of the junctional rhythms
were found between those resulting in permanent AV nodal block and
those associated with transient or no block. Because many of the
patients were in atrial fibrillation at the time of the ablation, VA
conduction could not be reliably assessed in the current study.
Pathophysiology
It has been shown both in vitro and in vivo
that the rate of
temperature rise at the RF ablation electrode-tissue interface is
rapid, and steady-state temperatures are reached within a few
seconds.7 13 14 However, because heating
of deeper tissue
layers depends on the slower process of heat conduction away from the
electrode-tissue interface, the rate of tissue temperature rise is
progressively slower with increasing distance from the ablation
electrode.7 13 14 In addition,
steady-state tissue
temperatures fall in an inverse proportion to distance from the
ablation electrode.7 Consequently, a critical determinant
of success during ablation of the AV junction is the distance between
the ablation electrode and the compact AV node. The greater the
distance the ablation electrode is positioned from the compact AV node,
the longer the time required to reach a target temperature and the
lower the steady-state temperature achieved. This will result in either
a delay in onset of the junctional rhythm and AV nodal block or no
pathophysiological effect at all.
An important determinant of tissue heating is electrode-tissue contact. As a result of greater convective cooling by the circulating blood pool, sites with poor electrode-tissue coupling would be expected to have less efficient myocardial heating than sites with good electrode-tissue contact. This may also result in a slower rate of rise of tissue temperatures, leading to a delay in onset of a junctional rhythm or AV nodal block at poorly coupled sites.
The precise origin and mechanism of the junctional rhythms observed during RF ablation have not been well defined. In vitro experiments have demonstrated heat-induced abnormal automaticity of ventricular myocardium at temperatures >45°C.8 The junctional rhythms may therefore be caused by heat-induced automaticity of transitional cells or perinodal atrial or ventricular myocardium.
Clinical Implications
The current study suggests that
development of an accelerated
junctional rhythm within 5 seconds of RF initiation may be a useful
marker of a successful target site during RF ablation of the AV
junction. In addition, the study found that the temperatures associated
with the development of AV nodal block were significantly higher than
the temperatures resulting in an accelerated junctional rhythm. This
finding suggests that even if a relatively rapid-onset junctional
rhythm is observed after initiation of RF delivery, further increases
in tissue heating are still necessary to ensure permanent AV nodal
block. If, at maximal power levels, mean temperatures within a range of
60±7°C cannot be attained, then the ablation catheter may need to
be
manipulated, or a long sheath may need to be used to obtain better
electrode-tissue contact and hence more tissue heating. If, despite
these maneuvers, adequate temperatures are still not attained, a left
ventricular approach should be
considered.2
Study Limitations
The times to onset of a junctional rhythm
and AV nodal block
reported in the current study may, in part, be a function of the
step-up increase in power used in the study. A protocol where high
power is applied initially may have resulted in more rapid onset of
electrophysiological effects. In addition,
because ablation sites were selected on the basis of 9-second RF
applications, the results of this study may not be applicable to other
ablation protocols where longer RF applications are initially used.
However, the use of short test pulses may reduce the number of longer
RF deliveries; hence, the amount of myocardial necrosis may be
minimized. In the current study, the median number of 30- to 60-second
RF applications was one, and no patient had a significant CK-MB
isoenzyme rise after ablation. However, serum CK-MB activity after RF
ablation may not be an accurate marker of myocardial injury because of
RF-induced inactivation of the enzyme.15
The development of a junctional rhythm precluded precise identification of the initial time to onset of AV nodal block. Therefore, the time to onset of AV nodal block and the temperature associated with the initial appearance of block reported in the current study may be overestimations of the actual time and temperature required to produce AV nodal block during RF ablation of the AV junction.
Conclusions
The development of an accelerated junctional
rhythm within 5
seconds and the appearance of complete AV nodal block within 30 seconds
of RF onset were both characteristic of successful target sites during
RF ablation of the AV junction. The accelerated junctional rhythm and
AV nodal block were both highly temperature dependent. The temperatures
associated with the development of AV nodal block were significantly
higher than the temperatures resulting in an accelerated junctional
rhythm AV. Temperatures in the range of 60±7°C were necessary to
ensure permanent AV nodal block.
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Footnotes |
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Reprint requests to Sunil Nath, MD, Cardiovascular Division, Department of Internal Medicine, PO Box 158, University of Virginia Health Sciences Center, Charlottesville, VA 22908. Email sn2v@virginia.edu.
Received December 19, 1994; revision received March 6, 1995; accepted March 10, 1995.
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