(Circulation. 1999;99:319-325.)
© 1999 American Heart Association, Inc.
Basic Science Reports |
Lesion Dimensions During Temperature-Controlled Radiofrequency Catheter Ablation of Left Ventricular Porcine Myocardium
Impact of Ablation Site, Electrode Size, and Convective Cooling
From Medical Department B, National University Hospital, 2100 Copenhagen Ø, Denmark.
Correspondence to Helen Høgh Petersen, Department B 2142, National University Hospital, Blegdamsvej 9, 2100 Copenhagen Ø, Denmark. E-mail hhp{at}dadlnet.dk
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Abstract |
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Background—It is important to increase lesion size to improve the success rate for radiofrequency ablation of ischemic ventricular tachycardia. This study of radiofrequency ablation, with adjustment of power to approach a preset target temperature, ie, temperature-controlled ablation, explores the effect of catheter-tip length, ablation site, and convective cooling on lesion dimensions.
Methods and Results—In vitro strips of porcine left
ventricular myocardium during different levels
of convective cooling and in vivo pig hearts at 2 or 3 left
ventricular sites were ablated with 2- to 12-mm-tip
catheters. We found increased lesion volume for increased catheter-tip
length 
8 mm in vitro (P<0.05) and 6 mm in
vivo (P<0.0001), but no further increase was found for
longer tips. For the 4- to 10-mm catheter tips, we found smaller lesion
volume in low-flow areas (apex) than in high-flow areas (free wall and
septum) (P<0.05). Increasing convective cooling of the
catheter tip in vitro increased lesion volume
(P<0.0005) for the 4- and 8-mm tips but not for the
12-mm tip as the generator reached maximum output. In contrast to
power-controlled ablation, we found a negative correlation between tip
temperature reached and lesion volume for applications in which maximum
generator output was not achieved (P<0.0001), whereas
delivered power and lesion volume correlated positively
(P<0.0001).
Conclusions—Lesion size differs in different left
ventricular target sites, which is probably related to
convective cooling, as illustrated in vitro. Longer electrode tips
increase lesion size for tip lengths 6 to 8 mm. For
temperature-controlled ablation, the tip temperature achieved is a poor
predictor of lesion size.
Key Words: catheter ablation • arrhythmia • tachycardia
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Catheter ablation of ventricular tachycardias in patients with structural heart disease can permanently cure the tachycardia targeted in only
60%
of cases,1 2 3 4 whereas the success rate after ablation of
AV node reentrant tachycardias, accessory pathways, or
fascicular ventricular tachycardias is
95%.5 6 Several features in patients with structural
heart disease may limit the applicability of radiofrequency ablation:
The ventricular tachycardia can be
hemodynamic or electrically unstable; multiple
reentrant circuits can be present; or the critical reentrant
circuit can be located deep in the myocardium. To improve
the success rate in attempts to coagulate the critical parts of the
reentrant circuits, 2 approaches can be taken: improving the accuracy
of the mapping technique and increasing the size of the lesion created
by catheter ablation. Several methods for increasing lesion size have
been investigated, ranging from surgery to ablation with direct
current,7 8 laser,9
microwaves,10 and alcohol injection11 to
radiofrequency ablation with large-tip12 13 14 15 or
saline-irrigated16 17 18 19 electrodes. The chosen ablation
site may have significant influence on lesion size because of various
degrees of cooling of the electrode tip from the intracavitary blood
flow, although this has not yet been evaluated systematically. The purpose of this in vitro and in vivo study was to assess the effect on lesion dimensions of increased catheter-tip length, ablation site, and convective cooling of the electrode tip during temperature-controlled radiofrequency ablation. Another study aim was to evaluate how applied power and reached tip temperature correlated with lesion volume.
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Methods |
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Experimental Preparations
In Vitro
Strips of left ventricular myocardium from freshly excised pig hearts were suspended in a tissue bath with isotonic saline at 37°C controlled by a thermostat. The ablation catheter was mounted in a holder maintaining the tip parallel to the tissue. Catheter pressure was adjusted to 20 g by a dynamometer on the catheter holder. Convective cooling around the tip was created by a pump circulating the saline in the tissue bath, releasing it through a laminator close to the electrode tip. Flow velocity around the tip was set at 0, 0.1, or 0.2 m/s measured by pulse-wave Doppler (Toshiba Sonolayer SSA-270A) with an esophageal probe (Hewlett Packard Omniplane 5-MHz PEF 511-SA) suspended in the tissue bath opposite the laminator (Figure 1
).
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In Vivo
The protocol followed the requirements of Danish legislation for
the care and use of experimental animals. A total of 34 pigs of either
sex weighing between 26 and 40 kg were anaesthetized with midazolam
(bolus, 25 mg; infusion, 3 to 6 mg/h) and ketamine (bolus, 750
mg; infusion, 100 to 200 mg/h) supplemented with fentanyl (0.375 mg/h)
and N2O
(N2O:O2 ratio=75:25). A
bolus of 450 mg amiodarone IV was given before ablation to
reduce the risk of ventricular fibrillation.20
Arterial blood pressure and surface ECG were monitored
continuously. The pigs were intubated and mechanically ventilated. The
left ventricle was catheterized through a hemostatic sheath introduced
into the left femoral artery. An indifferent electrode was attached to
the left lateral chest wall. The position of the ablation electrode was
controlled by fluoroscopy and recording of a stable bipolar
electrogram from the electrode tip. Two hours after ablation, the pigs
were killed by infusion of pentobarbital. The hearts were excised, and
the lesions were identified and inspected for crater formation, which
was defined as disruption of the myocardium.
Ablation Equipment
A generator delivering radiofrequency current as an unmodulated
sine wave of 500 kHz modified to deliver an output of up to 75 W was
used (ATAKR, Medtronic CardioRhythm). The in vivo experiments using 2-
and 4-mm tip lengths were performed with a standard 50-W generator
(ATAKR, Medtronic CardioRhythm). Power supply, impedance, and electrode
tip temperature were continuously monitored during energy application.
Power delivery was discontinued automatically if impedance was outside
the range of 40 to 250 
or if tip temperature was >100°C.
Unipolar ablation was performed with 7F thermocouple-type ablation
catheters (MARINR Medtronic CardioRhythm). Catheters were modified to
conduct up to 100 W.
Tissue Preparation and Lesion Volume Determination
Transmural tissue blocks containing the visible lesion and
5 mm of surrounding tissue were chilled and cut into 1-mm-thick
slices with a guillotine. The slices were incubated with 0.5 mg
nitroblue tetrazolium per 1 mL 0.2-mol/L Sorenson's buffer at 37°C
for 10 minutes. The reaction was stopped with 10% formalin after
staining. The viable area becomes dark purple after staining, whereas
the necrotic area remains unstained. The size of the slices was
amplified 7 times, and the area of necrosis was planimetered
electronically on both sides of each slice. The volume of necrosis in
each slice was calculated as the sum of these 2 areas divided by 2 and
multiplied by slice thickness, which was 1 mm, except for slices
with necrosis visible on only 1 side, which were considered to have a
lesion thickness of 0.5 mm. Lesion volume was calculated as the
sum of the necrotic volume of all slices. Maximum width and depth of
each lesion were measured with a caliper.
Ablation Protocols
All applications were performed with automatic power adjustment
to maintain a preset temperature of the electrode tip
(temperature-controlled radiofrequency ablation). The target
temperature was 80°C for 60 seconds in all applications.
Applications were performed in vitro with tip lengths of 4, 8, and 12 mm and in vivo with tip lengths of 2, 4, 6, 8, 10, and 12 mm. Only 1 catheter-tip length was used in each pig. In vivo, 3 different target sites in the left ventricle were chosen: posterior midseptum, left anterior free wall, and apex.
Statistical Analysis
Values are given as mean±SD. For each energy application, the
average power delivered and the average tip temperature measured in the
60-second energy delivery time were used for analysis.
Individual groups were compared by 1-ANOVA. Relations were assessed
with linear regression. The influence of convective cooling in vitro
and ablation site in vivo was assessed by multiple regression. Values
of P<0.05 were considered statistically significant.
Statistical software SAS 6.12 (SAS Institute) was used.
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Results |
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In Vitro
A total of 122 lesions were produced in vitro (Table 1
).
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Catheter-Tip Length
For each level of convective cooling, lesion volume and width were
significantly larger for the 8- than for the 4-mm-tip catheter
(P<0.05). Lesion dimensions were not further increased for
applications with the 12-mm-tip catheter. Lesion depth was not
significantly increased for increasing catheter-tip length. Average
power consumption increased significantly with increasing catheter-tip
length for all levels of convective cooling (P<0.005),
whereas average reached tip temperature decreased significantly
(P<0.001).
Convective Cooling
Increasing convective cooling by increasing the flow velocity of
the saline in the tissue bath increased lesion volume
(P<0.0005), width (P<0.05), and depth
(P<0.005) significantly for the 4- and 8-mm-tip catheters.
For the 12-mm-tip catheter, maximum generator output was reached when
flow was induced in the tissue bath, and lesion dimensions did not
increase (Table 1
).
Average power consumption was not significantly increased for
increasing levels of convective cooling for any of the 3 catheter-tip
lengths, although there was a trend toward increasing average power
consumption for increasing convective cooling (Table 1).
Relation Between Average Tip Temperature Reached, Average Power
Consumption, and Lesion Volume
To discriminate between "true temperature-controlled"
applications in which the power output of the generator was sufficient
to approach the target temperature and applications in which the
maximum generator output was reached, resembling power-controlled
ablation, data were divided into 2 groups: group 1 included
applications with average power consumption <65 W, or true
temperature-controlled ablation (n=87), and group 2 included
applications with average power consumption 
65 W, or
"pseudo–power-controlled" ablation (n=35).
In group 1, there was a negative correlation between reached tip
temperature and lesion volume (P<0.0001) (Figure 2A) and a positive correlation between
power consumption and lesion volume (P<0.0001) (Figure 2B). In group 2, there was a positive correlation between
reached tip temperature and lesion volume (P<0.0001)
(Figure 2A) and a negative correlation between power consumption
and lesion volume (P<0.05) (Figure 2B).
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In Vivo
A total of 34 pigs were catheterized. Two were excluded, 1 because
of incessant ventricular fibrillation after ablation and 1
because of catheter movement in 2 applications, which precluded exact
lesion identification. In the remaining 32 pigs, 94 applications
produced 85 lesions because 9 applications were excluded, 8 because of
catheter displacement and 1 because of hemorrhage. There were
no cases of premature termination of energy application owing to
impedance rise.
Catheter-Tip Length
For increasing catheter-tip length (Table 2), lesion volume and width increased
significantly up to a catheter-tip length of 6 mm
(P<0.0001), and lesion depth increased significantly up to
a catheter-tip length of 4 mm (P<0.001). No further
significant increase in lesion dimensions was observed for longer
catheter tips. Average power consumption increased significantly for
increasing catheter-tip length (P<0.0001), whereas average
reached tip temperature decreased significantly
(P<0.0001).
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Application Site
For lesions created with the 4- to 10-mm-tip catheters, when both
all applications (n=59) and only applications with no occurrence of
ventricular fibrillation (n=39) were analyzed,
lesions in the apex had significantly smaller volume, higher
reached tip temperature, and lower average power consumption
(P<0.05) than applications in the septum and the free wall,
which did not differ significantly from each other. For applications
performed with the 2- and 12-mm-tip catheters, there was no significant
difference in lesion volume for the different application sites (Figure 3).
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Relation Between Average Tip Temperature Reached, Average Power
Consumption, and Lesion Volume
Lesions were divided as described previously into group 1 (power
consumption <65 W; n=62) and group 2 (power consumption 65 W;
n=23).
In group 1, there was a negative correlation between reached tip
temperature and lesion volume (P<0.0001) (Figure 4A) and a positive correlation between
power consumption and lesion volume (P<0.0001) (Figure 4B). In group 2, there was a positive correlation between
reached tip temperature and lesion volume (P<0.05) (Figure 4A) and no significant correlation between power consumption and
lesion volume (Figure 4B).
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Complications
A total of 21 episodes of ventricular fibrillation
developed in 14 of 32 pigs during energy application. Mean time for the
start of ventricular fibrillation was 32.8±20 seconds
after the onset of radiofrequency energy. In all cases, the 60-second
energy application was completed, and then the pig was resuscitated by
direct-current conversion. These lesions were included in the data
analysis. One other pig was excluded because of incessant
ventricular fibrillation. Ventricular
fibrillation occurred in 11 of 30 apical applications, in 6 of 28
septal applications, and in 4 of 27 free wall applications. These
frequencies were not significantly different. No
ventricular tachycardias were observed in the 2-hour period
from the end of the procedure until the pig was killed.
Crater formation was seen in 12 lesions, and none of these cases was associated with impedance rise. One crater formation observed with the 6-mm-tip catheter was associated with an audible pop. Two craters were observed after ablation with the 4-mm-tip catheter. The remaining 10 craters were seen after ablation with large-tip catheters (4 of 14 for the 6-mm tip in the septum and free wall, 1 of 12 for the 8-mm tip in the free wall, 2 of 16 for the 10-mm tip in the free wall, and 3 of 12 for the 12-mm tip in the apex). For equal catheter-tip lengths, applications with crater formation were not associated with higher-power delivery or higher average tip temperature than the applications without crater formation, except for the 12-mm-tip electrode. This was the only tip length that produced craters in the apex, and these applications were associated with higher average tip temperatures (68±7°C) than the 12-mm-tip applications without crater formation (49±8°C) (P<0.05).
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Discussion |
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Main Findings
We found increased lesion volume for increasing catheter-tip lengths
8 mm in vitro and 6 mm in vivo and no further
increase for longer catheter tips. In vitro, increased lesion volume
for increasing levels of convective cooling was found, and in vivo,
apical lesions were significantly smaller than septal and free wall
lesions for the 4- to 10-mm catheter tips. For true
temperature-controlled ablation, lesion volume and average reached tip
temperature were negatively correlated, whereas there was a positive
correlation between lesion volume and average power consumption. For
applications close to the maximum generator output, there was a
positive correlation between reached tip temperature and lesion
volume.
Ablation Site
In the in vivo experiments, we demonstrated significantly smaller
lesion volume and higher average reached tip temperature in apical
applications than in septal and free wall applications for the 4- to
10-mm catheter tips. The main differences between applications in the
apex and the 2 other application sites are that better electrode-tissue
contact can be established in the apex and lower convective cooling of
the electrode tip will occur in the apex because it is filled with
blood from the very early phase of diastole. Because this
difference in convective cooling is not present during cardiac
arrest, this analysis was also performed with exclusion of
applications in which ventricular fibrillation
occurred.
Better electrode-tissue contact increases lesion volume, but as illustrated in the in vitro experiments, less convective cooling reduces power consumption and decreases lesion size. Because apical lesions were smaller in the in vivo applications, the effect of the convective cooling must be the more important of the 2 factors. This was supported by the in vitro experiments in which controlled catheter-tissue contact was established and convective cooling was varied. This showed that reducing convective cooling around the catheter tip decreased lesion volume.
No differences in lesion volume for different ablation sites for the 2- and 12-mm catheter tips were found. For the 12-mm-tip catheter, output of the radiofrequency generator was insufficient to achieve the target temperature, and catheter-tissue contact could probably be established only along a section of this very long electrode tip. For the 2-mm-tip catheter, it is probable that the small size allowed the tip to be in contact with the endocardium around almost its entire surface, thus protecting it from the effect of the cooling blood in all 3 application sites. This might explain why the lesions created with the 2-mm-tip catheter showed no difference in lesion volume for the 3 different application sites.
To the best of our knowledge, no previous studies have demonstrated the importance of ablation site for lesion dimensions. There are a few clinical observations, however, on how the ablation site affects the reached tip temperature, supporting our observation of differences between separate ablation sites.
Two studies concerning temperature-controlled radiofrequency ablation of accessory pathways in humans21 22 found that applications on the atrial side of the tricuspid annulus (high-flow site) had significantly higher power consumption and lower reached average tip temperature than applications under the mitral valve (low-flow site, because the electrode tip is sheltered by the mitral leaflet). This is consistent with our finding that applications in the apex (low-flow area) had higher reached temperature and lower power consumption. In addition, during power-controlled radiofrequency ablation of accessory pathways in humans, differences between separate application sites have been observed. Langberg et al23 found lower reached tip temperatures for applications on the atrial side of the tricuspid annulus compared with applications on the ventricular side of the mitral annulus. These observations cannot be extended to temperature-controlled ablation, in which the effect of increased cooling is the opposite, because the generator increases power output in high-flow conditions to maintain the target temperature, which causes enlargement of the lesion size.
External Convective Cooling
In the in vitro experiments, we found significantly increased
lesion volume and a trend toward increased power consumption for
increasing levels of convective cooling of the electrode tip for the 4-
and 8-mm-tip electrode but not for the 12-mm electrode tip, in which
the maximum power output of the radiofrequency generator was reached.
This suggests that our finding in vivo of differences in lesion volumes
in different ablation sites is probably related to the differences in
convective cooling of the electrode tip in the beating heart according
to the specific area ablated, although the quality of electrode-tissue
contact also affects lesion size. Only a few other studies have
examined the effect of the convective cooling of the ablation electrode
during temperature-controlled ablation in vitro,24 25 and
they are consistent with our findings.
Tip Length
The present results are in agreement with the findings of
earlier published in vivo12 13 14 and in
vitro15 studies of temperature- and power-controlled
radiofrequency ablation in which increased lesion dimensions were found
for increasing catheter-tip lengths 6 or 8 mm and no further
increase was found for longer catheter tips. However, ablation site and
convective cooling were not considered in these studies.
Lesion Volume Related to Average Delivered Power and Average Tip
Temperature Reached
For true temperature-controlled ablation, there is a negative
correlation between reached tip temperature and lesion volume, whereas
a positive correlation exists between average power consumption and
lesion volume. This implies that during temperature-controlled
ablation, for a given preset target temperature (here 80°C), the tip
temperature actually reached does not give any indication of the lesion
size. This is in accordance with 2 clinical studies26 27
that showed a lack of correlation between reached tip temperature and
success or recurrence rate after temperature-controlled
ablation of accessory pathways.
In the temperature-controlled mode of radiofrequency ablation in which a preset tip temperature is aimed for by regulation of the power output, the power delivered will reflect the electrode-tissue contact and the level of convective cooling caused by intracavitary blood flow in vivo, and the reached tip temperature does not correlate positively with lesion volume, as illustrated in our study. In vivo, this has also been illustrated by Kongsgaard et al,28 who found no correlation between reached peak tip temperature and lesion size during temperature-controlled radiofrequency ablation.
Most experimental studies, however, have been performed in the power-controlled mode. In contrast to the temperature-controlled mode, the power is preset, and the reached tip temperature will reflect the quality of electrode-tissue contact and convective cooling caused by intracavitary blood flow in vivo.
During power-controlled ablation, convective cooling will reduce lesion
dimensions. In power-controlled ablation, increased tip temperature is
associated with larger lesion dimensions, as illustrated by Hindricks
et al29 and Rosenbaum et al.13 This was also
illustrated in those of our experiments in which power consumption
approached the maximum generator output (pseudo–power-controlled
ablation); in this group, reached tip temperature was positively
correlated with lesion volume (Figures 2A and 4A).
Haines and Watson30 have shown that increasing target temperature increases lesion volume, but to the best of our knowledge, the relation with reached tip temperature for a given chosen target temperature has not been studied before. We found that for a given chosen target temperature, the reached tip temperature is not a good predictor of lesion size when there are different levels of convective cooling, as in the beating heart.
Clinical Implications
Assuming that the experimental data can be applied to the human
heart, there are 3 main clinical implications of our study concerning
temperature-controlled radiofrequency ablation. First, in areas with
low convective cooling, such as the apex or ventricular
aneurysms, the lesion is likely to be smaller than expected,
even though the target temperature is reached. Ablation with large- or
irrigated-tip catheters could be considered when the
tachycardia cannot be ablated with a standard catheter
despite high reached tip temperature. Second, in areas with high
convective cooling, ie, ablation of slow pathways in AV node reentrant
tachycardia, right-sided accessory pathways, and especially
anteroseptal pathways, the lesion is likely to be larger than in other
areas when the same target temperature is used. Reducing target
temperature or using smaller tip size could be considered for ablation
of the tachycardia and might reduce the risk of AV block.
Third, once a target temperature is chosen, the reached tip temperature
does not correlate positively with lesion size; thus, high reached tip
temperatures do not indicate large lesion dimensions.
Study Limitations
This study was performed in normal porcine left
ventricular myocardium, and the properties of
diseased myocardium probably differ, affecting heat
transfer in the tissue and thus lesion size. However, there is recent
evidence from dog experiments that the tissue temperatures during
radiofrequency ablation in scarred and normal myocardium do
not differ.31
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Acknowledgments |
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Dr Petersen is the recipient of a research fellowship from the Danish Research Council. Grants for equipment were donated by the Alfred Benzon Foundation. Technical assistance by Lise Schmidt, Michael Below, Letty Klarskov, and Mette Olesen is appreciated.
Received May 6, 1998; revision received September 8, 1998; accepted September 25, 1998.
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myocardium was ablated with 2- to 12-mm-tip catheters. We
found increased lesion volume for tip lengths 6 to 8 mm. For 4-
to 10-mm catheter tips, smaller lesion volume was found in low-flow
than in high-flow areas. Increasing convective cooling in vitro
increased lesion volume for the 4- to 8-mm tips. A positive correlation
was found between power and lesion volume but not between reached tip
temperature and lesion volume. Lesion size differs in different left
ventricular sites, probably as a result of convective
cooling. For temperature-controlled ablation, reached tip temperature
is a poor predictor of lesion size.
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