(Circulation. 1995;91:2264-2273.)
© 1995 American Heart Association, Inc.
Articles |
Comparison of In Vivo Tissue Temperature Profile and Lesion Geometry for Radiofrequency Ablation With a Saline-Irrigated Electrode Versus Temperature Control in a Canine Thigh Muscle Preparation
From the Cardiovascular Section/Department of Medicine, University of Oklahoma Health Sciences Center and the Department of Veterans Affairs Medical Center, Oklahoma City.
Correspondence to Warren M. Jackman, MD, Department of Medicine/Cardiovascular Section, University of Oklahoma Health Sciences Center, PO Box 26901, Rm 5SP300, Oklahoma City, OK 73190-3048.
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Abstract |
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Background It is thought that only a thin layer of tissue adjacent to the electrode is heated directly by electrical current (resistive heating) during radiofrequency ablation. Most of the thermal injury is thought to result from conduction of heat from the surface layer. The purpose of this study was to determine whether lesion depth could be increased by producing direct resistive heating deeper in the tissue with higher radiofrequency power, allowed by cooling the ablation electrode with saline irrigation to prevent the rise in impedance that occurs when the electrode-tissue interface temperature reaches 100°C.
Methods and Results In 11 anesthetized dogs, the thigh muscle was
exposed and bathed with heparinized canine blood (36°C to 37°C). A
7F catheter, with a central lumen, a 5-mm tip electrode with six
irrigation holes, and an internal thermistor, was positioned
perpendicular to the thigh muscle and held at a constant contact weight
of 10 g. Radiofrequency current was delivered to 145 sites (1) at high
constant voltage (66 V) without irrigation (CV group, n=31), (2) at
variable voltage (20 to 66 V) to maintain tip-electrode temperature at
80°C to 90°C without irrigation (temperature-control group,
n=39),
and (3) at high CV (66 V) with saline irrigation through the catheter
lumen and ablation electrode at 20 mL/min (CV irrigation group, n=75).
Radiofrequency current was applied for 60 seconds but was terminated
immediately in the event of an impedance rise 
10 
. Tip-electrode
temperature and tissue temperature at depths of 3.5 and 7.0 mm were
measured in all three groups (n=145). In 33 CV irrigation group
applications, temperature was also measured with a separate probe at
the center (n=18) or edge (n=15) of the electrode-tissue
interface. In
all 31 CV group applications, radiofrequency energy delivery was
terminated prematurely (at 11.6±4.8 seconds) owing to an impedance
rise associated with an electrode temperature of 98.8±2.1°C. All 39
temperature-control applications were delivered for 60 seconds without
an impedance rise, but voltage had to be reduced to 38.4±6.1 V to
avoid temperatures >90°C (mean tip-electrode temperature,
84.5±1.4°C). In CV irrigation applications, the tip-electrode
temperature was not >48°C (mean, 38.4±5.1°C) and the
electrode-tissue interface temperature was not >80°C (mean,
69.4±5.7°C). An abrupt impedance rise with an audible pop and
without coagulum occurred in 6 of 75 CV irrigation group applications
at 30 to 51 seconds, probably owing to release of steam from below the
surface. In the CV and temperature-control group applications, the
temperatures at depths of 3.5 (62.1±15.1°C and
67.9±7.5°C) and
7.0 mm (40.3±5.3°C and 48.3±4.8°C) were always lower
than the
electrode temperature. Conversely, in CV irrigation group applications,
electrode and electrode-tissue interface temperatures were consistently
exceeded by the tissue temperature at depths of 3.5 mm (94.7±9.1°C)
and occasionally 7.0 mm (65.1±9.7°C). Lesion dimensions were
smallest in CV group applications (depth, 4.7±0.6 mm; maximal
diameter, 9.8±0.8 mm; volume, 135±33 mm3),
intermediate
in temperature-control group applications (depth, 6.1±0.5 mm; maximal
diameter, 11.3±0.9 mm; volume, 275±55 mm3), and
largest
in CV irrigation group applications (depth, 9.9±1.1 mm; maximal
diameter, 14.3±1.5 mm; volume, 700±217 mm3;
P<.01, respectively).
Conclusions Saline irrigation maintains a low electrode-tissue interface temperature during radiofrequency application at high power, which prevents an impedance rise and produces deeper and larger lesions. A higher temperature in the tissue (3.5 mm deep) than at the electrode-tissue interface indicates that direct resistive heating occurred deeper in the tissue (rather than by conduction of heat from the surface).
Key Words: catheter ablation • radiofrequency • arrhythmia • tachycardia
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Introduction |
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TopAbstractIntroductionMethods Results Discussion References |
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Radiofrequency catheter ablation has become a principal form of therapy for paroxysmal supraventricular tachycardia and idiopathic ventricular tachycardia1 2 3 4 5 6 7 8 9 10 and is being used in increasing numbers of patients for treatment of ventricular tachycardia associated with coronary artery disease and other forms of heart disease. Radiofrequency current delivered through a standard 7F 4-mm catheter tip electrode has been highly successful for ablation of arrhythmogenic tissue located within a few millimeters of the ablation electrode, such as accessory AV pathways (in patients with Wolff-Parkinson-White syndrome) and the atrial end of the slow AV-nodal pathway. However, in 1% to 10% of patients with accessory pathways and 30% to 50% of patients with ventricular tachycardia associated with a healed myocardial infarction, the arrhythmogenic tissue cannot be destroyed with a conventional ablation electrode.11 12 13 14 15 16 17 Deeper lesions may be necessary for ablation in these patients.18 19 For any given electrode size and tissue contact area, radiofrequency lesion size is a function of radiofrequency power level and exposure time.20 21 At higher power, however, the exposure time is frequently limited by an impedance rise that occurs when the temperature at the electrode-tissue interface reaches 100°C.22 23 The impedance rise can be prevented by maintaining the electrode-tissue interface temperature at <100°C.
Two approaches have been used to prevent the impedance rise and maximize power delivery. In one approach, a thermistor is used in the ablation electrode to monitor tip-electrode temperature. The radiofrequency generator output is adjusted to deliver the greatest level of power that does not result in an increase in electrode temperature beyond a target value, such as 80°C.24 This "temperature-control" approach is based on the findings that radiofrequency current delivered at conventional power directly heats only a thin layer of tissue adjacent to the electrode (since current density decreases at the second power of the distance from the electrode) and that most of the thermal lesion results from heat conduction from the thin surface layer.25 26 27 28 29 30 31 These observations imply that the depth of the lesion is a function of the surface area that is heated, the temperature at the surface, and the time of surface heating.
The second approach to increase lesion size uses a very large ablation electrode (8F, 8 to 10 mm in length).32 The larger electrode-tissue contact area results in a greater volume of direct resistive heating.29 In addition, the larger electrode surface area exposed to the blood results in greater convective cooling of the electrode by the blood. This cooling effect helps to prevent an impedance rise, allowing longer application of radiofrequency current at higher power, which produces a larger, deeper lesion. The two principal limitations of a large ablation electrode (8 to 10 mm in length) are the reduction in mobility and flexibility of the catheter (which may impair positioning of the ablation electrode) and a reduction in the resolution of recordings from the ablation electrode (making it more difficult to identify the optimal ablation site).
An alternative approach, originally proposed by Wittkampf et al,33 34 is to irrigate the ablation electrode with saline for convective cooling to maintain a low electrode-tissue interface temperature and prevent an impedance rise. Since convective cooling from the bloodstream is not required, an irrigated electrode may be capable of delivering higher radiofrequency power at sites of low blood flow, such as within a ventricular trabecular crevasse. Other theoretical advantages of an irrigated electrode versus a very large ablation electrode include a more flexible catheter and higher-resolution recordings for more accurate mapping.
The purpose of this study is to compare the tissue temperature profile in vivo and lesion geometry between radiofrequency ablation at high power in which a saline-irrigated electrode (active cooling) is used with radiofrequency ablation in which variable output to maintain constant electrode temperature (80°C to 90°C), ie, temperature-control, is used.
A novel canine thigh muscle preparation was used to allow in vivo measurement of tissue temperature at various depths during the application of radiofrequency current. The hypothesis being tested is that direct resistive (electrical) heating is not necessarily limited to a thin surface layer. If radiofrequency power can be increased sufficiently by preventing an impedance rise (by maintaining a low temperature at the electrode-tissue interface), direct resistive heating may occur several millimeters below the surface and significantly increase lesion depth.
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Methods |
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Experimental Preparation
The experimental protocol was approved by the University of Oklahoma Committee on Use and Care of Animals. Eleven mongrel dogs weighing between 18 and 22 kg were anesthetized with 25 mg/kg sodium pentobarbital and ventilated mechanically with room air. General anesthesia was maintained with supplemental doses of sodium pentobarbital. Each dog was placed on its left side. The right carotid artery was cannulated for continuous monitoring of arterial pressure. A 15-cm skin incision was made over the right thigh muscle (Fig 1
). The skin, overlying connective tissue, and thin
superficial muscle were gently dissected, exposing the surface of the
thicker underlying muscle. The fascia on the surface of the muscle was
usually thin and transparent. Any thicker fascia was removed. The edges
of the skin were raised to form a cradle that was filled with
heparinized canine blood at 36°C to 37°C (Fig 1). The
blood was
exchanged at a flow rate of 20 mL/min.
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A specially designed 7F
deflectable quadripolar electrode catheter with
a central lumen (Cordis Webster, Inc) was used. The tip electrode was 5
mm long and had six 0.4-mm-diameter irrigation holes located radially
around the electrode, 1 mm from the tip (Fig 2). The tip
electrode also contained a thermistor, 2.5 mm from the tip, allowing
measurement of the tip-electrode temperature between 20°C and 120°C
with an accuracy of ±2°C. The catheter was positioned perpendicular
to the thigh muscle, and the tip electrode was held in contact with the
thigh muscle at a constant weight of 10 g by use of a custom balance
(Fig 2). Tissue temperatures were measured with fluoroptic
thermal
probes (Luxtron model 3000; measurement range, 0°C to 125°C;
accuracy, ±0.2°C). The fluoroptic temperature measurement system
was
equipped with a radiofrequency-shielded package designed to prevent
fluctuations of averaged temperatures >0.2°C root-mean-square (RMS)
in stray radiation fields of up to 10 mW/cm2 (1966 American
National Standards Institute standard). Two thermal sensor probes were
bundled together with shrink tubing. One sensor tip extended 3.5 mm
from the end of the shrink tubing, and the other sensor tip extended
7.0 mm. The sensor probes were inserted into the muscle (3.5 and 7.0 mm
from the surface) directly adjacent to the ablation electrode (Fig
2).
In 3 of the 11 dogs, an additional fluoroptic temperature probe was
positioned at the center or edge of the electrode-tissue interface.
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The ablation electrode was irrigated through the catheter lumen with room temperature (20°C to 22°C) heparinized (2 U/mL) normal saline at 20 mL/min using a Harvard infusion pump (model 55-2083). The saline irrigation was started 3 to 5 seconds before the onset of the application of radiofrequency current and was maintained until 5 seconds after the completion of the application of energy. Radiofrequency current (550 to 650 kHz) was produced by a constant voltage (CV) generator (American Cardiac Ablation Co, model LIZ-88) and was delivered between the catheter tip electrode and an adhesive electrosurgical dispersive pad applied to the shaved skin of the abdominal wall. During each application of radiofrequency current, the RMS voltage and current, the impedance, and the temperatures measured from the thermistor in the ablation electrode and the tissue probes were continuously monitored and recorded on optical disk (Bard LabSystem).
Ablation Protocol
Five to eight applications of
radiofrequency current were
delivered to separate sites on the right thigh muscle. The skin
incision was closed, the dog was turned onto its right side, and five
to eight applications of radiofrequency current were delivered at
separate sites on the left thigh muscle.
Radiofrequency current was
delivered using three approaches: (1) The
constant voltage (CV) group received high CV (66 V) without irrigation
(n=31 applications); (2) the temperature-control group received
variable voltage (20 to 66 V) to maintain tip-electrode temperature at
80°C to 90°C without irrigation (n=39 applications); and (3)
the CV
irrigation group received high CV (66 V) with saline irrigation (n=75
applications). In all three groups, radiofrequency current was applied
for 60 seconds but was terminated immediately in the event of an
impedance rise of 10 . The ablation tip electrode was examined for
coagulum after each application of radiofrequency energy. Before the
next application, the tip-electrode surface and irrigation holes were
cleaned by use of a gauze pad soaked in heparinized saline and a
0.3-mm-diameter drill bit.
After Ablation
Two hours after the ablation procedure was
completed, 2%
triphenyl tetrazolium chloride (30 mL IV) was administered. This dye
stains intracellular dehydrogenase, which distinguishes viable and
necrotic tissue. The dogs were killed and their thigh muscles were
excised, fixed in a 10% formalin solution, and sectioned.
Measurements of Lesion Size
The maximal depth (A), maximal
diameter (B), depth at the
maximal diameter (C), and lesion surface diameter (D) were measured.
The lesion volume (VL) was calculated with the following
formula for an oblate ellipsoid by subtracting the volume of the
ellipsoid extending above the surface of the muscle ("missing
cap").35
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Statistical Analysis
The electrical parameters of
radiofrequency delivery,
tip-electrode temperature, tissue temperatures at 3.5- and 7.0-mm
depths, and lesion dimensions were compared among the three groups by
ANOVA. Any significant differences were measured by Scheffé's
method for pairwise comparisons. The values are expressed as mean±SD.
The significance of the relations between lesion size and total
radiofrequency energy, tip-electrode temperature, and tissue
temperature was assessed by linear-regression analysis. A value of
P<.05 was considered statistically significant.
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Results |
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TopAbstractIntroductionMethods Results Discussion References |
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Radiofrequency Energy Parameters During Ablation
The mean voltage, current, impedance, power, duration, and energy of the applications of radiofrequency current are compared among the three groups in Fig 3
. The impedance was not
significantly different for the three groups (mean, 84 to 86 ). In
the CV group, application of 66 V resulted in a mean current of
0.79±0.06 A and a mean power of 52.3±3.8 W. An impedance rise
>10
occurred in all 31 CV group applications at a mean of 11.6±4.8
seconds, which prematurely terminated each application of
radiofrequency current. All the impedance rises were associated with a
film of coagulum on the electrode. The impedance rise was gradual in 28
CV group applications and was abrupt and associated with an audible pop
in the remaining 3 applications. The short duration of the applications
resulted in a mean energy of only 605±260 J.
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In the temperature-control group applications, the radiofrequency voltage was regulated to maintain a tip-electrode temperature of 80°C to 90°C. This prevented an impedance rise, and all 39 applications were delivered for the full 60 seconds. Although the initial voltage was relatively high (48.2±4.1 V), the steady state voltage was only 38.4±6.1 V; this voltage was obtained 10 to 15 seconds after the onset of the application. The mean current at steady state was 0.46±0.09 A and resulted in a mean power of only 18.3±6.4 W. The mean energy for the temperature-control applications was 1098±380 J.
In the CV
irrigation (saline irrigation) group, 69 of 75
radiofrequency applications were maintained for the full 60 seconds
without an impedance rise. Six of 75 applications (8%) resulted in a
small (
10 ), abrupt impedance rise at 30 to 51 seconds. All 6
impedance rises were accompanied by an audible pop, and none of the 6
were associated with coagulum formation on the electrode. The mean
current for the CV irrigation applications was 0.77±0.07 A, which
resulted in a mean power of 50.6±4.7 W. This power was not
significantly different from that in the CV group, but the sustained
duration of the applications resulted in a mean energy of 2934±389 J,
which was markedly greater than the energy in the CV and
temperature-control groups.
Tip-Electrode Temperature and Tissue Temperatures
The peak
tip-electrode and peak tissue temperatures at depths of
3.5 and 7.0 mm are compared for the three groups in Fig 4. In
the CV group, the tip-electrode temperature
reached 100°C (mean, 98.8±2.1°C) for all 31 applications of
radiofrequency current (Fig 4A). A >10- impedance
rise occurred in all applications at an electrode temperature of 95°C
to 105°C (Fig 5). Due to the early rise in impedance,
the peak tissue temperature at 3.5 mm was 62.1±15.1°C and the peak
temperature at 7 mm was only 40.3±5.3°C (Fig 4B and
4C). A
temperature of 50°C, which was thought to correspond to the
temperature required to produce necrosis,28 29 was
reached
in 23 of 31 CV applications at 3.5 mm but in only 2 of 31 applications
at 7 mm.
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For the temperature-control group, the tip-electrode
temperature was
maintained at 80°C to 90°C (mean, 84.5±1.4°C; Figs
4A
and 6).
The peak temperature at 3.5 mm was 67.9±7.5°C. Although this value
was not statistically greater than the value for the CV group, all 39
applications for the temperature-control group resulted in a tissue
temperature of 50°C at 3.5 mm. The peak temperature at 7 mm was
48.3±4.8°C for the temperature-control group applications (or
significantly greater than CV group applications), with a temperature
50°C in 11 of the 39 applications.
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For the CV irrigation group
applications, the peak tip-electrode
temperature ranged from 30°C to 48°C (mean, 38.4±5.1°C;
Figs 4A and 7). The ability to sustain the
radiofrequency
current at high power for 60 seconds resulted in a peak temperature of
94.7±9.1°C at 3.5 mm and of 65.1±9.7°C at 7.0 mm. These
tissue
temperatures were significantly greater than in applications for the CV
and temperature-control groups (Fig 4). The temperature
exceeded 50°C
at both the shallow and deep sites (3.5 and 7.0 mm) in all 75
applications.
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An additional optical temperature probe was positioned at
the center of
the electrode-tissue interface in 18 CV irrigation group radiofrequency
current applications and at the edge of the electrode-tissue interface
in 15 CV irrigation group applications (Fig 2). The
electrode-tissue
interface temperature did not reach 100°C in any of these 33
applications. The peak temperature at the center of the
electrode-tissue interface was 69.4±5.7°C, and the peak temperature
at the edge of the electrode-tissue interface was 51.1±2.9°C. At
3.5
mm, these interface temperatures exceeded the electrode
temperature but were less than the tissue temperature at 3.5 mm
(Fig 7). In 4 applications, the electrode-tissue interface
temperature
was lower than the tissue temperature at 7 mm.
In all three groups, the
tissue temperature at the 3.5- and
7.0-mm depths was still increasing at the end of every radiofrequency
application, including those applications that extended for the full 60
seconds (Figs 6 and 7).
Lesion Geometry
All lesions were sharply demarcated and
ellipsoid. The size
and depth of the center of the ellipsoids were different for the three
groups (Figs 8 and 9). In the CV group,
the center of the ellipsoid was located at the surface of the tissue,
such that the maximal diameter of the lesion was the same as the
surface diameter. In the temperature-control group, the center of the
ellipsoid (the maximal lesion diameter) was displaced at 1.2±0.5 mm
below the surface. The lesion surface diameter was 10.3±0.9 mm, and
the largest diameter was 11.3±0.9 mm. In the CV irrigation group,
almost all the ellipsoid was below the surface, such that the maximal
lesion diameter was located 4.1±0.7 mm from the surface. The lesion
surface had a significantly smaller diameter than the maximal diameter
(10.1±1.3 versus 14.3±1.5 mm, P<.01). The lesion
depth,
maximal diameter, depth at maximal diameter, and volume were
significantly greater in lesions in the CV irrigation group than those
in the temperature-control group and were significantly greater in
lesions in the temperature-control group than those in the CV group
(Fig 9). Interestingly, the lesion surface diameter was not
significantly different among the three groups.
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A shallow crater (1 to
2 mm in depth and 1 to 3 mm in diameter) was
found on the surface of two lesions from the CV group and three from
the CV irrigation group. A small, abrupt impedance rise associated with
an audible pop occurred during each of the 5 radiofrequency
applications, producing these lesions. Four other applications of
radiofrequency current (one in the CV group and three in the CV
irrigation group, Fig 10) resulted in an abrupt
impedance rise with an audible pop, but neither a crater nor a tear was
found when these four lesions were examined.
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Relation Between Tissue Temperatures and Lesion Dimensions
The relations between each of the measurements of lesion size and
total radiofrequency energy, tip-electrode temperature, and tissue
temperatures at depths of 3.5 and 7.0 mm were examined. Lesion depth
correlated best with tissue temperature measured at 7 mm (Fig
11A). Linear regression for that relation predicts that
necrosis extending to 7 mm would correspond to a temperature of
48.4°C at that depth. The largest lesion diameter corresponded best
with tissue temperature recorded at 3.5 mm (Fig 11B). In CV
irrigation
group applications, there was no correlation between either the depth
or the maximal diameter of the lesion and either the electrode
temperature or the electrode-tissue interface temperature.
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Discussion |
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TopAbstractIntroductionMethods Results Discussion References |
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The application of radiofrequency energy at 66 V without irrigation (CV group) resulted in a quick impedance rise at a mean of 11.6±4.8 seconds. The short application time resulted in the smallest lesions among the three groups. In the temperature-control group, adjusting the voltage to maintain the tip-electrode temperature at 80°C to 90°C prevented an impedance rise, which allowed the sustained application of radiofrequency energy. However, to prevent the electrode temperature from approaching 100°C, the voltage had to be reduced to a relatively low level (38.4±6.1 V; power, 18.3±6.4 W), which limited lesion size and depth. In the CV irrigation group, saline irrigation prevented the tip-electrode temperature and the electrode-tissue interface temperature from reaching 100°C, which allowed the sustained application of radiofrequency current at higher voltage (66±0 V; power, 50.6±4.7 W). This sustained application at high power resulted in a significant increase in lesion size and depth. The mean lesion depth for the CV irrigation group was 9.9±1.1 mm, compared with 6.1±0.5 mm for the temperature-control group and 4.7±0.6 mm for the CV group.
In previous studies, lesion size approached maximum after application
of radiofrequency for 20 to 60 seconds.21 28 In
the
present study, the tissue temperatures at depths of 3.5 and 7.0 mm
continued to increase at the end of each radiofrequency application in
all three groups (Figs 5 through
7 and 10). For the 7.0-mm depth
in the
temperature-control group, at the end of 60 seconds, the temperature
was increasing at a slow rate (Fig 6), which suggests that
extending
the radiofrequency application beyond 60 seconds probably would not
have increased lesion depth substantially. In contrast, at 60 seconds
in the CV irrigation group at 7.0 mm, the temperature continued to
increase at a steep rate (Figs 7 and 10), which
suggests that longer
applications of radiofrequency current with saline irrigation may
result in significantly deeper lesions.
Although the electrode temperature and electrode-tissue interface
temperature did not reach 100°C for any of the applications in the CV
irrigation group, an impedance rise occurred in 6 of 75 applications
after 30 to 51 seconds (Fig 10). All 6 of the impedance rises
were
small, abrupt, and associated with an audible pop. There was no
coagulum on the electrode. A small crater was found on the surface of 3
of these 6 lesions. These observations suggest that the impedance rise
associated with radiofrequency applications in the CV irrigation group
may have resulted from a sudden release of steam from below the surface
of the lesion rather than from boiling at the electrode-tissue
interface.36 The tissue temperature at a depth of 3.5 mm
immediately preceding the abrupt impedance rise with an audible pop was
97°C to 120°C (Fig 10), which is consistent with
superheating of
the tissue and steam formation. Prevention of tissue superheating
during radiofrequency application with irrigation would require the
measurement of tissue temperature, since the electrode-tissue interface
temperature does not reflect the temperature of the underlying
tissue.
For 28 of 31 radiofrequency applications in the CV group, the impedance
rise was gradual and was associated with an electrode temperature of
100°C and coagulum formation on the electrode (Fig 5).
These
observations are consistent with the findings of earlier investigators
that suggested that, in the absence of active electrode cooling, the
impedance rise results from boiling at the electrode-tissue
interface.22 23 24
Tissue temperatures during radiofrequency ablation with saline
irrigation (CV irrigation group) were significantly higher than in the
CV and temperature-control groups (without irrigation), even though the
tip-electrode temperature was lowest, ranging from 30°C to 48°C
(Figs 4 through
7). The
electrode-tissue interface temperature during
radiofrequency ablation with irrigation was higher than the temperature
recorded with the thermistor positioned in the electrode. However, the
electrode-tissue interface temperature was always lower than the tissue
temperature measured at 3.5 mm and was occasionally lower than the
tissue temperature at 7.0 mm. These data suggest that the temperature
at the electrode-tissue interface results from the competing effects of
heating from the underlying tissue and convective cooling by
irrigation. The higher tissue temperature than electrode-tissue
interface temperature indicates that resistive (electrical) heating
extended several millimeters below the surface rather than just
extending along a thin layer of myocardium surrounding the electrode.
Previous studies have shown that the size of radiofrequency lesions
correlates well with tip-electrode
temperature.27 28 29 This
relation is significantly altered when saline irrigation is used to
cool the ablation electrode. With electrode cooling, neither electrode
temperature nor electrode-tissue interface temperature can be used to
predict lesion size.
The differences in lesion geometry among the three groups may be explained by differences in (1) the rate at which heat is generated within the tissue, (2) the duration of time in which heat is generated, and (3) the rate of heat removal from the tissue surface by convective cooling from the blood and saline irrigation. In the CV group, heat was generated quickly for a short period, preventing significant convective cooling by the blood. This resulted in a shallow lesion, with maximal diameter at the surface. In the temperature-control group, the rate of heat generation was slowed by the reduction in radiofrequency power, which allowed some convective cooling by the blood. The longer application time resulted in a deeper and larger lesion, with the maximal diameter located slightly below the surface owing to surface cooling by the blood. In the CV irrigation group, the rate of heat generation was high and the application time was long. However, the surface convective cooling was greatly increased by the saline irrigation. This resulted in a deeper and larger lesion, with the maximal diameter located farther from the surface, at a mean depth of 4.1 mm. The size of the lesion surface was not significantly larger than in the CV and temperature-control groups, despite the greater depth and diameter of the intramural lesion. The small size of the surface lesion may be clinically beneficial for reducing the risk of mural thrombus and thromboembolic complications.
Lesion depth correlated closely with peak tissue temperature at
a depth of 7.0 mm. Linear-regression analysis suggests that, in
this canine thigh muscle preparation, a lesion will extend to 7.0 mm
when the temperature at that depth reaches 48.4°C (Fig
11A). This
temperature correlates closely with the 48°C to 50°C temperature
that was shown to produce necrosis in myocardium in previous
studies.28 29 Maximal lesion diameter correlated well
with
peak tissue temperature at a depth of 3.5 mm (Fig 11B).
Study Limitations
This study was performed in a canine thigh
muscle preparation
instead of the endocardium of a beating heart to control tip-electrode
contact pressure and to allow measurement of temperatures at various
tissue depths beneath the electrode. The flat surface of the thigh
muscle also allows accurate determination of lesion size and geometry.
In contrast, delivering radiofrequency current to the trabeculated
endocardium often results in an irregular lesion shape and greater
variation in lesion size than was found in this study.
Because the intramural blood flow may be less in the resting thigh muscle than in the myocardium, the thigh muscle may provide less heat sink and therefore may result in greater lesion size. However, a previous study28 has suggested that the tissue temperature profile during radiofrequency ablation may be independent of intramyocardial perfusion. In addition, in the present study, the size of the lesions in the CV and temperature-control groups is similar to the lesions described in other studies of the canine heart.21 32 37
Clinical Implications
The results of this study indicate that
radiofrequency current can
be delivered for a prolonged period at higher power by cooling the
ablation electrode with saline irrigation. The higher power produces a
wider, deeper lesion without increasing the diameter of the endocardial
surface of the lesion. The use of an irrigated electrode may allow the
ablation of arrhythmogenic tissue farther from the endocardium (deeper)
and may significantly improve the efficacy of ablation in ventricular
tachycardia associated with structural heart disease and some
difficult-to-reach locations of accessory pathways.
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Acknowledgments |
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This study was supported by grants from the NIH (R01-HL-39670) and from the Oklahoma Center for the Advancement of Science and Technology (HRI-104). The authors thank Dr Fred H.M. Wittkampf for his thoughtful review of the manuscript and helpful suggestions.
Received August 16, 1994; revision received November 7, 1994; accepted November 20, 1994.
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