1469
Temperature Monitoring During Radiofrequency
Catheter Ablation of Accessory Pathways
Jonathan J. Langberg, MD; Hugh Calkins, MD; Rafel El-Atassi, MD; Mark Borganelli, MD;
Angel Leon, MD; Steven J. Kalbfleisch, MD; and Fred Morady, MD
Downloaded from http://circ.ahajournals.org/ by guest on June 18, 2017
Background. Animal studies have suggested that the temperature of the electrode-tissue interface
during radiofrequency catheter ablation accurately predicts lesion size. The purpose of the current study
was to evaluate the utility of continuous temperature monitoring during radiofrequency catheter ablation
in patients with Wolff-Parkinson-White syndrome.
Methods and Results. Twenty patients with manifest preexcitation were included in the study. The
ablation catheter was positioned on the ventricular side of the mitral annulus for left-sided accessory
pathways and on the atrial side of the tricuspid annulus for right-sided and septal accessory pathways. A
thermistor imbedded in the distal electrode of the ablation catheter allowed continuous temperature
monitoring during each energy application. To define the relation between power and temperature,
radiofrequency current was applied several times at each site using outputs of 20, 30, 40, and 50 W. The
accessory pathways were successfully ablated in each of the 20 patients. Because of marked variability in
the efficiency of heating between sites, power output did not predict temperature. However, at any given
site, there was a positive dose-response relation between power and temperature. Radiofrequency energy
applications on the atrial side of the tricuspid annulus produced lower temperatures than did applications
on the ventricular side of the mitral annulus (49±7 versus 60+16°C, p=0.0001). Transient block in the
accessory pathways occurred at a mean of 50+8°C, whereas permanent block was seen at a mean of
62+15°C (p=0.0001). Less than half of the applications at outputs .40 W produced temperatures
adequate to interrupt accessory pathway conduction. An abrupt rise in impedance caused by coagulum
formation occurred only at temperatures between 95 and 100°C.
Conclusions. Temperature monitoring may facilitate radiofrequency catheter ablation of accessory
pathways. By adjusting power output to ensure that adequate but not excessive temperatures have been
achieved, a rise in impedance can be avoided and the total number of energy applications and procedure
duration may be reduced. (Circulation 1992;86:1469-1474)
KEY WoRDs * tachycardia, supraventricular * Wolff-Parkinson-White syndrome
Ca atheter ablation using radiofrequency energy
has emerged as the technique of choice for
definitive therapy of arrhythmias caused by
accessory atrioventricular pathways.' However, optimal
energy delivery strategies have not been defined.
Application of radiofrequency current to the endocardium results in resistive heating of the tissue
adjacent to the ablating electrode.2 Multiple variables,
including catheter orientation, surface area of contact,
contact pressure, and cavitary blood flow may affect the
efficiency of radiofrequency energy application.3
Therefore, there is a poor correlation between radiofrequency energy delivery parameters (e.g., power and
duration) and resultant lesion size.67 In contrast, the
temperature of the electrode-tissue interface accurately predicts radiofrequency lesion volume in vitro8
From the Department of Internal Medicine, Division of Cardiology, University of Michigan Medical Center, Ann Arbor, Mich.
One of the authors of this article (J.J.L.) serves on the scientific
advisory board of EP Technologies, Inc., the company that provided the prototype catheters used in this study.
Address for correspondence: Jonathan J. Langberg, MD, University of Michigan Medical Center, 1500 East Medical Center
Drive, Bi F245, Ann Arbor, MI 48109-0022.
Received January 21, 1992; revision accepted July 31, 1992.
and in ViVO.6 No prior studies have described the use of
temperature probes in patients undergoing catheter
ablation.
See p 1648
The purpose of this study was to evaluate the utility of
continuous temperature monitoring during radiofrequency catheter ablation of accessory pathways in patients with Wolff-Parkinson-White syndrome.
Methods
Characteristics of Subjects
Catheter ablation was performed under a protocol
approved by the Committee on Human Research at the
University of Michigan. Twenty patients with manifest
preexcitation gave written consent and were studied.
The mean age was 34±14 years (14 male patients and
six female patients). These patients had been symptomatic for 15±13 years and had been treated with 2.5 ±2.4
antiarrhythmic drugs before being referred for ablation.
Electrophysiological Testing
Electrophysiological testing was performed using
three quadripolar electrode catheters positioned in the
high right atrium, His bundle position, and the right
1470
Circulation Vol 86, No 5 November 1992
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ventricular apex. Leads V,, I, and III and the intracardiac electrograms were recorded on a Siemens Elema
Mingograf 7 recorder. Pacing and extrastimulation were
performed with a programmable stimulator at a pulse
width of 2 msec and an intensity of twice diastolic
threshold. Conduction and refractoriness of the atrioventricular node and accessory pathway were assessed,
and the mechanism of tachycardia was defined. In
addition, the accessory pathway was localized to a
general region before detailed mapping with the ablation catheter. Four of the patients had accessory pathways located in the right free wall, six had posteroseptal
pathways, and 10 had left-sided accessory pathways.
Techniques for positioning the ablation catheter have
been described previously.9 A retrograde approach was
used for left-sided accessory pathways. The catheter
was inserted into the femoral artery and advanced
across the aortic valve to the ventricular aspect of the
mitral annulus. In patients with septal or right-sided
accessory pathways, the catheter was introduced via the
femoral vein and positioned on the atrial side of the
tricuspid annulus.
Mapping was performed during sinus rhythm or atrial
pacing. Target sites were selected based on the presence
of discrete atrial and ventricular electrograms with early
onset of local ventricular activation relative to the
surface QRS. Bipolar electrograms were bandpass filtered between 50 and 500 Hz and recorded at a paper
speed of 200 mm/sec. The amplitudes of the atrial and
ventricular components of the signal were measured at
each ablation site. Each bipolar recording was also
analyzed for the presence of an accessory pathway
potential defined as a high-frequency deflection distinct
from the atrial and ventricular components of the signal.
No attempt was made to validate the presence of
accessory pathway potentials with pacing maneuvers.
Each target site electrogram was also analyzed for
stability. The signal was classified as stable if there was
< 10% variation in the atrial or ventricular electrogram
amplitudes between successive beats. To determine if
there was a correlation between temperature and the
magnitude of ST elevation in the endocardial electrogram, unfiltered unipolar electrograms were recorded
with the ablation catheter from each target site in 10 of
the 20 subjects.
Thermistor Ablation Catheter
To accurately measure heating during radiofrequency
energy application, the temperature of the tissue in
contact with the ablating electrode must be measured
rather than the temperature of the electrode itself or the
adjacent cavitary blood. A previous study has shown that
a thennistor in contact with the endocardium and thermally isolated from the delivery electrode accurately
measures electrode-tissue interface temperature.'0
In the present study, a standard 7F bipolar electrode
catheter (EP Technologies, Inc., Mountain View, Calif.)
with a deflectable shaft and a large (4 mm in length)
distal electrode and 2-mm interelectrode spacing was
modified to incorporate a thermistor in the tip of the
distal electrode (Figure 1). The thermistor bead (Thermomedics, Inc.) was exposed to the surface and thermally insulated from the surrounding platinum electrode by a polyamide plastic sleeve. Each catheter was
individually calibrated in a tank of warm saline solution
FIGURE 1. Design of the electrode catheter used for temperature monitoring during radiofrequency catheter ablation of
accessory pathways. A small thermistor (T) insulated by a
plastic sleeve (S) is imbedded in the distal electrode (D). P,
proximal ring electrode.
using a precision fiberoptic thermometer (Luxtron,
Inc.). The thermistor was accurate to within ±20C from
37 through 100°C for each of the catheters used.
Catheter Ablation and Temperature
Monitoring Protocol
Radiofrequency energy was produced by a power
supply that delivered a continuous, unmodulated sine
wave output at 500 kHz (EP Technologies, Inc.). This
device, via an interface with a microcomputer (Toshiba
Electronics, Inc.) continuously measured, displayed,
and recorded power, impedance, and tip temperature
during each energy application. Radiofrequency energy
was delivered between the distal electrode of the ablation catheter and a large adhesive skin electrode that
was placed over the left scapula. To define the relation
between power and temperature, radiofrequency current was applied several times at each site by using
incrementally higher outputs. Initially, 20 W was given
for 20 seconds followed by 30 W and then 40 W for 20
seconds. This duration was chosen because a previous
experimental study showed that lesions reach full size
after approximately 20 seconds." If the maximal temperature at 40 W was <50°C, then a final application of
50 W for 20 seconds was given. Incrementally higher
power outputs were used at each site regardless of the
electrophysiological effects of the applications. Energy
delivery was stopped immediately if the catheter became dislodged or if a rise in impedance was observed.
Statistical Analysis
Continuous variables are expressed as mean+ 1 SD.
ANOVA for repeated measures was used to define the
relation between power and mean temperatures. Fisher's least significant differences method for multiple
comparisons was used to analyze the change in temperature with increasing power at each site. Comparisons
between temperature and discrete variables were performed using the Student's t test for unpaired variables,
and linear regression was used for comparison with
continuous variables. A value of p<0.05 was considered
significant.
Results
A total of 213 applications of radiofrequency energy
were delivered with the thermistor catheter in the 20
patients at 94 different target sites (range, one to nine
sites per patient). All patients had successful outcome
Langberg et al Temperature Monitoring During Ablation
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40
20
30
40
50
60
POWER (WATTS)
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4
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--
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6
8
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DURATMON OF RF APPUCATN (SECN)
FIGURE 2. Graphs demonstrate tip temperature vs. duration
of radiofrequency (RF) energy. Panel A: Application on the
ventricular side of the mitral annulus. Note that temperature
reaches a steady state in .5 seconds. There is a substantial
increase in temperature with increments in power from 20 to
40 W Panel B: An ablation site on the atrial side of the
tricuspid annulus is associated with much lower temperatures,
a shorter rise time, and less of an increase in steady-state
temperatures with increments in power.
defined as the complete absence of accessory pathway
conduction at the conclusion of the procedure. The
mean procedure duration was 70±40 minutes, and the
duration of fluoroscopy was 28 ±22 minutes. There were
no complications.
Relation Between Temperature and Energy
Delivery Parameters
The baseline temperature measured at the target site
before ablation was always between 37 and 39°C. After
the onset of the radiofrequency energy application,
temperature rose exponentially to a steady state. The
mean duration from onset to the time that temperature
had reached 90% of steady state was 2.2±+3 seconds.
Applications that produced higher temperatures tended
to take longer to reach steady state (Figure 2).
At any given power output, the steady-state temperature varied considerably from site to site. For all
radiofrequency energy applications considered as a
group, there was no difference in steady-state temperature between 20, 30, 40, and 50 W (51+13, 56+14,
60+17, 54+-t8°C; p=NS). In contrast, at each individual
target site, there was a positive dose-response relation
between applied power and temperature (Figure 3).
The average increase in temperature at any given site
between 20 and 40 W was 12±11°C (p=0.0001). The
slope of the temperature versus power curve was directly proportional to the steady-state temperature at 20
W (R2= 0.51, p=O.OOl).
FIGURE 3. Graph of dose-response relation between applied
radiofrequency power level and steady-state temperature at
each ablation site. Despite the marked variability in temperature for any given power level, there is a consistent positive
dose-response relation at each site. Note that at poorly
coupled sites (as manifest by low steady-state temperatures at
20 W) there is a shallow slope but much steeper slopes at sites
with better coupling.
An abrupt rise in impedance was noted during all
eight applications of radiofrequency energy that resulted in temperatures between 95 and 100°C (Figure
4). Temperature appeared to plateau just before the
impedance rise.
No impedance rise was seen at temperatures <95°C.
During one application, temperature rose to 114°C
without a measured change in impedance. Withdrawal
and inspection of the catheter after this application
revealed coagulum coating only the distal portion of the
ablating electrode.
Relation Between Temperature and Catheter Position
Ablation of right-sided and posteroseptal accessory
pathways on the atrial side of the tricuspid annulus
resulted in lower temperatures than applications on the
ventricular side of the mitral annulus used to ablate
100
W-
aanon Rooms
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TIME (seconds)
8
10
FIGURE 4. Plot shows abrupt rise in impedance (dashed
line) during the course of radiofrequency energy application.
Note that this occurs only after the tip temperature has
reached a steady-state level of 95°C.
~~ -
Circulation Vol 86, No 5 November 1992
1472
A
W
B
r
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--
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- -
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FIGURE 5. Graphs of time and temperature during radiofrequency (RF) energy application. Panel A: Temperature recorded
during RF energy application that produced transient (approximately 30 seconds) elimination of accessory pathwayfunction. Note
that the delta wave disappears at a temperature of 44°C. Panel B: Application that permanently eliminated accessory pathway
function is associated with a steady-state temperature of 600C.
.
.
2
3
4
DURATION OF RF APPLICATION (SECONDS)
1
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left-sided
accessory
pathways (48+7
versus
60+ 16C,
p=0.0001).
There were no characteristics of the target site electrogram that appeared to correlate with temperature.
There was no difference in steady-state temperature at
20 W between stable and unstable sites (52±10 versus
51+12, p=NS). There was no correlation between the
amplitude of the bipolar atrial or ventricular electrogram at each ablation site and the steady-state temperature at 20 W (R2=0.011, 0.103). The magnitude of ST
segment elevation recorded on the unfiltered unipolar
electrogram did not correlate with steady-state temperature at 20 W whether analyzed in terms of absolute
value (R2=0.011) or as a fraction of the amplitude of the
ventricular electrogram (R2= 0.006).
Temperature and Effects on Accessory
Pathway Conduction
Transient block of accessory pathway conduction was
noted during 76 of 213 applications of radiofrequency
energy with the thermistor catheter. The temperature
associated with initial disappearance of the delta wave
was measured for these applications as well as for
applications that permanently eliminated accessory
pathway function (Figure 5). The mean temperature at
the time of transient interruption of accessory pathway
conduction was 50±8°C compared with 62±150C for
applications that permanently ablated the pathway
(p=0.0001). Only 46% of applications at 20 and 30 W
produced temperatures .480C, the minimum temperature associated with permanent interruption of accessory pathway conduction (Figure 6). At 40 W, 63% of
applications resulted in steady-state temperatures in
excess of 48°C.
Discussion
1
5
does not predict temperature; 3) at any
given site, however, there is a clear-cut dose-response
relation between power and temperature; 4) radiofrequency energy applications on the atrial side of the
tricuspid annulus produce lower temperatures than do
applications on the ventricular side of the mitral annulus; 5) characteristics of the ablation site electrogram,
including stability and amplitude, do not correlate with
temperature; 6) applications of radiofrequency energy
that cause transient block in the accessory pathway have
a mean temperature of 50+8°C, whereas those that
permanently eliminate accessory pathway function are
associated with a significantly higher mean temperature
of 62+15°C; 7) at a power output of 20 or 30 W, a
majority of sites have sufficiently poor coupling between
the ablating electrode and adjacent tissue to prevent
temperatures from exceeding 48°C, the minimum temperature associated with permanent accessory pathway
block.
power output
80
-
60
Z
CD
aW
-W
to
20
Main Findings
Temperature monitoring at the tip of the ablation
catheter during radiofrequency ablation of accessory
pathways reveals the following: 1) temperature at the
catheter-tissue interface rises exponentially to reach a
steady state within a few seconds; 2) because of marked
variability between sites in the efficiency of heating,
30
FIGURE 6.
50
40
POWER
(WATTS)
Bar graph shows percentage of radiofrequency
energy applications at each power
with steady-state
temperatures
level
.480C,
that were associated
the minimum
that
produced permanent block of accessory pathways. Note that
W, onl a minority of sites
resullt in adequate heating.
at 20, 30, and 40
well coupled to
were sufficiently
Langberg et al Temperature Monitoring During Ablation
Downloaded from http://circ.ahajournals.org/ by guest on June 18, 2017
Comparison With Previous Studies
Temperature monitoring has been used to titrate
radiofrequency ablation in the central nervous system
for over three decades.'2 In the present study, temperature correlated with the electrophysiological effect of
radiofrequency energy application. Previous studies in
animals have shown that tip-temperature monitoring
predicts lesion volume more accurately than applied
power or the duration of energy application.6'8 Accessory pathway conduction was permanently interrupted
at a mean of 62°C. Interestingly, several studies in intact
animals have shown 62°C as the threshold temperature,
below which detectable lesions are not consistently
produced.6,13
The higher temperatures associated with ablation of
left-sided accessory pathways compared with those on
the right are probably due to differences in the technique used for positioning the catheter. Greater contact
pressure can be applied at the base of the left ventricle
than on the atrial side of the tricuspid valve. The force
of contact is directly proportional to temperature and
lesion volume during radiofrequency ablation in animals.3,5 In addition, the cavitary blood flow is likely to
be greater on the atrial side of the tricuspid annulus
than between the mitral leaflet and left ventricular
endocardium. Thus, convective heat loss may also contribute to the inefficiency of right-sided radiofrequency
energy applications.
Abrupt impedance rise in this study was invariably
associated with temperatures between 95 and 100°C and
did not occur at lower temperatures. This probably
reflects the behavior of water at the phase transition
(boiling point). The electrode-tissue interface would be
expected to stay at the boiling point as long as some
moisture remained. Because the power supply discontinued radiofrequency current delivery 1-2 seconds
after impedance rise, further increases in temperature
were not observed. Haines and Verow14 made similar
observations in a study of radiofrequency ablation in
canine ventricular myocardium, with impedance rise
due to coagulum formation occurring only at temperatures approaching 100°C.
The rate of temperature rise was considerably faster
in this clinical study than with previous studies in
vitro.6'8 This probably reflects the substantial convective
heat loss engendered by intracavitary blood flow. The
time required for temperature to approach steady state
(2.2+3 seconds) is in remarkably close agreement with
the time to initial accessory pathway block reported in a
series of 195 patients (2.9+3.4 seconds).'5
A recent study has identified several characteristics of
the ablation site electrogram that are predictive of
successful interruption of accessory pathway conduction.'6 However, even at optimal sites, the probability of
success was only 57%. Results of the current study
suggest that inadequate heating may often be responsible for inefficacy despite ideal electrogram characteristics at the ablation site.
Limitations
The temperature measured during radiofrequency
energy application may have been affected by catheter
orientation and contact pressure. Convective cooling by
cavitary blood is more likely to artificially lower temper-
1473
ature readings when the catheter is oriented nearly
parallel to the endocardium. However, such catheter
positions are unlikely to result in significant heating.
There was a distinct difference in mean temperatures
between applications that produced transient as opposed to permanent accessory pathway block. In addition, impedance rise always occurred at a measured
temperature between 95 and 100°C and not at lower
temperatures. These observations suggest that the temperature measurement system was reasonably accurate
at sites with sufficient contact to have a biological effect.
Clinical Implications
Results of this study suggest that temperature monitoring may facilitate radiofrequency catheter ablation of
accessory pathways. By adjusting power output (either
manually or with feedback control), temperatures
>90°C can be avoided, decreasing the possibility of
coagulum formation and impedance rise.
In this study, many ablation catheter positions were
poorly coupled to the endocardium, and power outputs
in the usual range (20-40 W) did not produce adequate
heating. This was particularly problematic for pathways
approached from the atrial side of the tricuspid annulus.
The ablation site electrogram, including the magnitude
of ST elevation, was not helpful in predicting the
efficiency of heating by radiofrequency energy. Temperature monitoring will reliably allow the operator to
determine whether inefficacy is due to inadequate heating or improper positioning of the ablation catheter.
Sites with promising electrograms but poor heating at
moderate power outputs could have energy reapplied at
higher outputs. Conversely, repeat applications at sites
that have already been heated to 260°C without effect
are unlikely to succeed. Thus, temperature monitoring
offers the prospect of decreasing the number of radiofrequency energy applications required to interrupt
accessory pathway conduction, possibly decreasing the
duration of the procedure and exposure to fluoroscopy.
Transient block of accessory pathway conduction was
often noted at -50°C. This probably reflects hyperthermia-induced reversible injury to tissue containing a
portion of the accessory pathway. Radiofrequency applications titrated to achieve temperatures in this range
may be a useful mapping tool while minimizing myocardial necrosis. Further studies are required to better
define the clinical role of temperature monitoring.
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Temperature monitoring during radiofrequency catheter ablation of accessory pathways.
J J Langberg, H Calkins, R el-Atassi, M Borganelli, A Leon, S J Kalbfleisch and F Morady
Downloaded from http://circ.ahajournals.org/ by guest on June 18, 2017
Circulation. 1992;86:1469-1474
doi: 10.1161/01.CIR.86.5.1469
Circulation is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
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