Indian Pacing Electrophysiol. J. 2003;3(3):117
Factors Influencing Lesion Formation During
Radiofrequency Catheter Ablation
Olaf J. Eick, PhD
Sr. Scientist, Bakken Research
Center Maastricht, the Netherlands
Address for correspondence: Olaf J. Eick, Ph.D., Medtronic Bakken
Research Center, Endepolsdomein 5
6229 GW Maastricht, the Netherlands.
E-mail: Olaf.Eick@medtronic.com
Abstract
In radiofrequency (RF) ablation, the heating of cardiac
tissue is mainly resistive. RF current heats cardiac tissue and in turn
the
catheter electrode is being heated. Consequently, the catheter tip
temperature
is always lower - or ideally equal - than the superficial tissue
temperature. The lesion size is influenced by many parameters such as
delivered RF
power, electrode length, electrode orientation, blood flow and tissue
contact. This review describes the influence of these different
parameters
on lesion formation and provides recommendations for different catheter
types on selectable parameters such as target temperatures, power
limits
and RF durations.
Introduction
Lesion Generation due to Radiofrequency Energy Application –
theoretical considerations
In
RF ablation, the heating of cardiac tissue is mainly resistive. RF
current is applied to the tissue via a metal electrode at the tip of
the catheter, with a large skin electrode serving as indifferent
electrode. The current density patterns in the tissue are determined by
electrode size and geometry, electrode contact and local tissue
properties. Also, of course, the current density will be proportional
to the current (I) delivered by the RF generator, which, for constant
resistance (R) of the electrode-tissue volume conductor is proportional
to the square root of the RF power (P = I2 R).
Current flow through a resistive medium causes heating, which is
proportional to the square of the current or, locally, to the square of
the local
current density. The temperature increase will locally be proportional
to the energy applied per second (local power), and increases
proportional
to the heat capacity of the local medium. In addition, when temperature
differences between adjacent areas develop because of differences in
local
current density or local heat capacity, heat will conduct from "hotter"
to "colder" areas, causing the temperature of the former to decrease
and
that of the latter to increase. Additionally, heat loss to the blood
pool
at the surface and to intramyocardial vessels determine the temperature
profile within the tissue.
The
heating occurs especially in the proximity of the active electrode due
to its relatively small surface area causing locally high current
density as compared to the site of the indifferent electrode.
Typically, living tissue will be permanently destroyed at temperatures
of approximately 45° to 50° C sustained for several seconds 1.
The
tissue surface is cooled by the blood flow and thus the highest
temperature during radiofrequency delivery occurs slightly below the
surface.
Parameters Determining Lesion Formation
Impact of target temperature - In Vitro Study
The
impact of the target temperature on lesion size was evaluated by the
author in an in vitro study. A 4-mm tip catheter (RF Marinr, Medtronic
EPSystems, Minneapolis, MN) was positioned in a parallel orientation to
porcine epicardium either with 0.5 N or 1.0 N contact force. A total of
48 lesions was produced with different target temperatures of 50, 60,
70
and 80° C (Atakr II, Medtronic, 100 W max. output, 60 s duration).
Each setting was repeated 6 times and the average values were used for
evaluation.
The results are given in Table 1.
Table 1:
Ablation and lesion data for different target temperatures of 50, 60,
70 and 80° C. Radiofrequency energy was delivered for 60 s with a
maximal available power of 100 W. Each setting was repeated 6 times and
the average values are given with the corresponding standard deviation.
With
increasing target temperature the delivered power, tip temperature and
lesion dimension increased. Lesions created with a target temperature
of 50° C were very small and nearly unrecognizable.
These results indicate that the lesion size could be well predicted by
measuring tip temperature. However, the in vitro experiments were
performed under stable flow conditions. In the in vivo situation, the
flow is dependent on the ablation site and also varying over the heart
cycle. These cooling effects have a strong impact on the catheter tip
temperature and thus on the delivered power and also on lesion size.
Impact of Flow - In Vitro Study
Petersen et al. evaluated the impact of convective cooling on lesion
dimension 2. In vitro strips of porcine
left ventricular myocardium during different levels of convective
cooling were ablated. All applications were performed with a 4-mm tip
catheter in parallel orientation to the tissue and automatic power
adjustment (Atakr, 75W) to reach and maintain a target temperature of
80° C for 60 seconds. The results are shown
in Table 2.
Table 2:
Ablation and lesion data for different flow rates of 0.0, 0.1 and 0.2
m/s. Radiofrequency energy was delivered for 60 s with a target
temperature of 80° C and a maximal power of 75 W.
With
increasing flow, i.e. increasing convective cooling, lesion depth,
width and volume increased due to increasing power consumption to reach
and maintain target temperature. Note, that the tip temperature was not
different and is thus a poor indicator for lesion size if the flow
condition is not stable.
Impact of Flow - In Vivo Study
Petersen et al. induced a total of 13 lesions in 6 pigs either at the
left
ventricular apex or at the mid-septum of the left ventricle 3. A 4-mm tip catheter was used for RF delivery
with a target temperature of 80° for 60 s (Atakr, 50 W maximum
output). The
results are shown in Table 3.
Table 3:
Ablation and lesion data for 2 different ablation sites, an apical site
with presumably low flow and a septal site with presumably high flow.
Radiofrequency
energy was delivered for 60 s with a target temperature of 80° C and a maximal power of 50 W.
Lesions induced at the apical site were significantly smaller than
those at the septal site. The blood flow is considered to be higher at
the
septum as compared to the apex, thus more power is consumed to maintain
target temperature at the septum resulting in larger lesions. Again the
tip temperature is a poor (or even contradicting) parameter for lesion
size. Comparing the lesion volumes of the in vivo study with those of
the
in vitro study, the flow at the apex might be between 0.0 m/s and 0.1
m/s
whereas the flow at the septum might me be more in the order of 0.2 m/s
or higher.
Catheter Orientation - In Vitro Study
Using the same experimental setup Petersen et al. investigated the
impact of electrode orientation on lesion size 4.
A
4-mm tip catheter was used for ablation with a target temperature of
80°
C for 60 s either in a parallel or perpendicular orientation on porcine
tissue stripes. The contact pressure between electrode tip and tissue
was
10g and the flow 0.1m/s. The results are shown in Table 4.
Table 4:
Ablation and lesion data for 2 different catheter
orientations. Radiofrequency energy was delivered for 60 s with a
target temperature of 80° C.
Perpendicular electrode orientation yielded larger lesion volume using
less power than parallel electrode orientation. The target temperature
was approached in all applications with a mean electrode tip
temperature of 75+2° C.
Changing the 4-mm tip electrode orientation from parallel to
perpendicular decreases the proportion of the electrode tip area that
is in contact with
the tissue and increases the proportion of the tip area that is exposed
to the convective cooling of the surrounding fluid. The larger lesion
volume
in the perpendicular electrode orientation suggests that cooling by
flow
around the electrode has greater impact than contact area. However, one
would expect a significantly higher power delivery in the group with
larger
lesion volume. This can be explained by the fact, that only a minor
fraction
of the energy delivered by the generator is used for the lesion
production
itself. [In a first approximation the power required to produce a
lesion
with a certain volume can be calculated as follows: Power = 1/t *
lesion
volume * density * specific heat * DT = 1/60 * 0.25
* 1.05 * 3.72 * (80-37) = 0.7 W. That means that only 0.7 W are
required to produce a lesion of 250 mm3 with a target temperature of 80°C for 60 s !
Assumption:
Tissue Density = 1.05g/cm3, specific heat = 3.72 J/(g*°C), t = 60
seconds, DT
= target temperature – body temperature = 43°C.] The
major part is dissipated as electrical heating of the intracavitary
blood, convective heat losses from electrode to blood, electrical
heating of
tissue outside the lesion volume and in electrical resistance of
catheter
and skin and fat layers at the indifferent electrode.
Catheter Orientation - In Vivo Study
Chan
et al. published results based on in vivo data that are in conflict
with those described by Petersen 5 and a
likely explanation will be offered at the end of this paragraph.
In
26 dogs 144 lesions were created either in a parallel or perpendicular
orientation in the right atrium with a target temperature of 75° C for 60 s
using different tip lengths. The orientation was confirmed by
fluoroscopy and intravascular ultrasound. For reasons of comparison
only the results for the 4-mm tip catheter are given in Table 5.
Table 5:
Ablation and lesion data for 2 different catheter
orientations. Radiofrequency energy was delivered in vivo in the right
atrium for 60 s with a target temperature of 75° C.
The lesion volume was larger for the parallel orientation as compared
to the perpendicular orientation which is in contradiction with the
results from the in vitro study performed by Petersen et al.. However,
all lesions were markedly smaller than in the previously summarized
studies mainly due to the small lesion depth. The lesions were produced
in the right
atrium and the atrial wall is rather thin. The lesions would have been
deeper in thicker tissue-note that 10 out of 14 for the perpendicular
orientation and 9 out of 14 for the parallel orientation were
transmural.
Chugh et al. who performed similar experiments in the left ventricle
confirmed this 6. They analysed
103 lesions
in 20 dogs produced in the left ventricle also with a target
temperature
of 70° C
for 60 s using different tip lengths. Table 6 summarizes the
results
for the 4-mm long catheter tip.
Table 6: Ablation
and lesion data for 2 different catheter orientations. Radiofrequency
energy was delivered in vivo in the left ventricle for 60 s with a
target
temperature of 70° C.
The lesion depth was markedly higher than that of the atrial
applications and similar to the values that Petersen et al. reported.
Although there was a trend that the lesions produced in the parallel
orientation were
larger the difference did not reach statistical significance.
Based on these studies one may conclude that lesion depth is only
little affected by catheter tip orientation using 4-mm long tip
catheters but that lesions are slightly longer in the parallel
orientation as compared to the perpendicular orientation.
Impact of Electrode Tip length
Petersen et al. evaluated the impact of electrode tip length on lesion
size in 34 pigs 2. Lesions were produced
in the left ventricle in the posterior mid-septum, the left anterior
free wall and in the apex using 2, 4, 6, 8, 10 and 12-mm long catheter
tips. A target temperature of 80° C for 60 s
was chosen with a maximum available power output of 75 W (Atakr II).
The results are given in Table 7.
Table 7:
Ablation and lesion data for applications with different catheter tip
lengths
of 2, 4, 6, 8, 10 and 12-mm. Radiofrequency energy was delivered in
vivo
in the left ventricle for 60 s with a target temperature of 80° C and a maximal power of 75 W.
The lesion volume increased with increasing tip length for tip lengths
between 2 and 10-mm. The lesion volume produced with an 8-mm long tip
was about twice as big than that with a 10-mm tip catheter and even 3
times
as big than that produced with a 4-mm tip catheter. Further increase in
tip length did not result in further increase in lesion volume. With a
very
long catheter tip a large part is exposed to the blood flow and more
energy
is dissipated into the blood stream. Note that the average temperature
decreased with increasing tip length and was thus a poor (or no)
indicator
for lesion volume. It is the amount of power that is effectively
delivered
to the tissue that determines lesion size. In addition, the depth did
not
differ between 4 and 8-mm tip catheters, the produced lesions are only
wider
but not necessarily deeper. Also, the applied average power was “only”
49
W for lesions created with an 8-mm tip catheter. Limiting the wattage
to
50 W may reduce the likelihood of coagulum formation without
compromising
lesion size.
Langberg et al., who also produced lesions in the left ventricle using
either 4-mm, 8-mm or 12-mm tip catheters, confirmed these results in
part 7. The target temperature was also 80° C for 60 s
with a maximum available power of 100 W. The results are given in
Table
8.
Table 8:
Ablation and lesion data for applications with different catheter tip
lengths
of 4, 8 and 12-mm. Radiofrequency energy was delivered in vivo in the
left ventricle for 60 s with a target temperature of 80° C and a maximal power of 100 W.
The power required to achieve a steady state temperature of 80° C was
directly proportional to electrode size. The lesions produced by the
8-mm tip
electrode were nearly twice as deep and four times as large as those
made with a conventional 4-mm tip electrode. Lesions produced by the
12-mm
tip electrode were intermediate in size and sometimes associated with
charring and crater formation. Langberg et al. stated furthermore that
ablations with larger tip electrodes caused a drop in arterial pressure
and more ventricular ectopy than those with a 4-mm tip electrode.
The
main difference to the results published by Petersen et al. is that
lesions produced with the 8-mm tip catheter were also much deeper than
those produced with the 4-mm tip catheter in the Langberg study. In
concordance to the Petersen study, the use of a very large electrode
did not further increase lesion size, and the tip temperature was even
negatively correlated with lesion size.
The clinical relevance of the fact that 8-mm tip catheters produce
larger lesions was demonstrated by Tsai et al.8.
In a prospective, randomized study they compared 4-mm with 8-mm tip
electrodes for linear ablation of typical atrial flutter in 104
patients. They reported that the 8-mm electrode catheter achieved
higher complete isthmus block rate (92% vs. 67%, p<0.05) with fewer
pulses (2+1 vs. 3+1, p<0.05), shorter procedure time
(24+15 vs. 31+12 minutes, p<0.05), and shorter
fluoroscopy time (14+10 vs. 23+15 minutes, p<0.05).
Impact of RF Duration
Simmers et al. evaluated the relation between RF duration and lesion
size and published their results in 1994 9.
In 11
dogs a total of 46 lesions was produced, 31 at left ventricular sites
and
15 at right ventricular sites. They applied a constant power of 25 W
for
either 5, 10, 20, 30 and 60 seconds using a 7F, 4-mm tip electrode
catheter
(Mansfield-Webster, Watertown, MA, USA). The results are given in Table
9.
Table 9:
Lesion data for applications with different durations of radiofrequency
delivery of 5, 10, 20, 30 and 60 s. Radiofrequency energy was delivered
in vivo at ventricular sites with a constant power of 25 W.
These results indicate that the lesion is predominantly generated
within the first 10 seconds of energy delivery and reaches a maximum
after 30 s. Further extension of RF delivery during power controlled RF
delivery does not seem to further increase lesion size.
Impact of Electrode-Tissue Contact - In Vitro Study
It
is difficult to evaluate the impact of electrode-tissue contact in the
in vivo situation since the contact pressure between electrode and
tissue
cannot be assessed by fluoroscopy and a direct indicator for tissue
contact
is lacking at present. Some studies have been published where the
influence of the electrode-tissue contact has been investigated in a
well controllable in vitro environment. However, the conclusions out of
these studies need to be drawn carefully to avoid misleading
interpretations.
The
author produced in vitro ablations on porcine myocardium with a 7F,
4-mm tip electrode with different electrode-tissue contact forces and a
target temperature of 70° C for 30 s
(50 max. output) 10,11.
A thermostat maintained a moderate flow. Figures 1 and 2
illustrate
the results. With increasing contact force between tissue and electrode
the electrode tip temperature increased and the average power decreased
(Figure 1). Figure 2 illustrates the behavior of applied
power and lesion depth. With increasing contact force the lesion depth
increased until a plateau was reached. Further increase in contact
force decreased
lesion depth (although tip temperature was further increasing).
Figure 1:
Graphical representation of the average catheter tip
temperature and power for different contact forces during temperature
controlled radiofrequency delivery for 30 s with a target temperature
of 70° C and a maximal power of 50 W.
Figure 2:
Graphical representation of the average power and the
resulting lesion depth for different contact forces during temperature
controlled radiofrequency delivery for 30 s with a target temperature
of 70° C and a maximal power of 50 W.
It
is the amount of RF delivered effectively into the tissue that
determines tissue heating and thus lesion generation. With increasing
electrode-tissue contact a higher amount of RF power can be effectively
brought into the tissue resulting in increasing lesion depth. At a
certain moderate contact force further increase in contact
force results in progressively smaller lesions because less amount of
RF power is required to reach
target temperature. Between 0.2 and 0.9 N (which is likely to be the
bandwith in the clinical environment) the lesion depth is not much
affected
by the contact force and increasing contact force is balanced by
decreasing
applied power. These experiments were performed under stable flow
conditions
and in conclusion it seems likely that the flow around the electrode is
of greater impact on lesion size than that of the electrode-tissue
contact.
Impact of Irrigation
Irrigation has been introduced to avoid overheating at the
tissue-electrode interface, thus allowing the delivery of higher
amounts of RF power for a longer duration to create relatively large
lesions.
Irrigated Power controlled RF delivery
Skrumeda et al. compared lesions created with a standard 4-mm tip
catheter (RF Marinr) with those created with an irrigated tip catheter
(RF Sprinklr) in animal experiments 12.
Three ablation
protocols were conducted in canine left ventricles. Protocol I:
standard
ablation was performed in temperature controlled mode at 70° C and 90° C (120
sec). Protocol II: irrigated ablation was conducted with 30 and 50 W
(30 and
120 sec). Protocol III: irrigated ablation was performed at 20 W with
very long RF durations (5 and 10 min). The maximal available RF power
output was 50 W. The results are given in Table 10.
Table 10: Ablation
and lesion data for applications with standard tip electrodes and
irrigated tip electrodes. The results for different target
temperatures, power
settings and durations of radiofrequency delivery and the incidence of
craters and coagulum formation are given.
With a standard electrode, lesions were larger with a target
temperature of 90°
C as compared to those created with a target temperature of 70° C, however,
a coagulum was observed in 95% of applications with a target
temperature
of 90°
C.
The largest irrigated lesions were formed using 50 W (986+357
mm3)
but were associated with craters in 54% and coagulum in 27% of the
applications, respectively. Large lesions without craters and coagulum
were created
with irrigation using 20 W for 10 minutes (602+175 mm3).
Skrumeda
et al. concluded that irrigated ablation created larger lesions than
standard ablation and that large lesions may be created without craters
using moderate power and long duration !
Irrigated Temperature Controlled RF delivery
Petersen et al. compared lesions produced by standard temperature
controlled RF delivery (TC) with those produced by either power
controlled RF delivery (PC) with a high irrigation flow rate (20
ml/min) or temperature controlled RF delivery with a low irrigation
flow rate (1 ml/min) 16. The results are
given in Table 11.
Table 11:
Ablation and lesion data for applications with standard tip electrodes
either
in temperature controlled or power controlled mode and irrigated tip
electrodes with a low irrigation flow rate of 1ml/min and different
target temperatures.
Petersen et al. demonstrated that lesion size and tissue temperatures
were significantly higher during irrigated tip ablation compared to
standard temperature controlled RF delivery (p<0.05). Lesion volume
correlated positively with tissue temperature (r=0.87). The maximum
recorded tissue temperature was always 1 mm from the ablation
electrode. Crater formation only occurred at tissue temperatures of
greater than 100°
C.
Based on the results of this in vitro study it may be concluded that
irrigated temperature controlled RF delivery yields relatively large
lesions without crater formation if a moderate target temperature
between 60 and 70°
C and a low
irrigation flow rate of 1ml/min are chosen. A target temperature of
greater
than 70°
C may result in tissue overheating and crater formation.
Impact of irrigation flow rate
Weiss et al. investigated the influence of different flow rates on
lesions produced on the thigh muscle in six sheep 17.
A total of 43 lesions was created with an irrigated tip catheter (RF
Sprinklr) in a power controlled mode with 30 W target power for 30 s. A
constant
contact pressure of 0.1 N was maintained with a perpendicular
orientation
to the tissue. The results are given in Table 12.
Table 12: Ablation
and lesion data for applications with irrigated tip electrodes with
different irrigation flow rates of 5, 10 and 20 ml/min. Besides the
catheter tip temperature, the temperature at a depth of 3.5 and 7.0 mm
is given.
The
tissue temperatures at 7-mm depth, the lesion depth and width were not
significantly different between the 3 different flow rates. The
diameter measured at the surface was significantly smaller following RF
applications with an irrigation flow rate of 20 ml/min due to increased
cooling at the surface, which resulted also in lower tissue
temperatures at a depth of 3.5 mm. Neither audible pops nor thrombus
formation was observed in all applications.
Based on these results a flow rate of 10 ml/min may be recommended when
operating an irrigated catheter in the power controlled mode with a
target
power of about 30 W. The application of more than 30 W may require a
higher
flow rate to avoid excessive heat development at the superficial tissue
layers.
Irrigation during Atrial Flutter Ablation
Jais
et al. published the results of a prospective randomized comparison of
irrigated tip versus conventional tip catheters for ablation of atrial
flutter 13.
Cavotricuspid ablation was performed with a conventional (n=26) or an
irrigated
tip catheter (n=24). RF was applied for 60 seconds with a
temperature-controlled mode: 65°C to 70°C up to 70 W with a
conventional catheter or
50°C up to 50 W (with a 17 ml/min saline flow rate) with the
irrigated
tip catheter. Complete bidirectional isthmus block was achieved for all
patients. Four patients crossed over from conventional to irrigated tip
catheters. The number of applications, procedure duration, and x-ray
exposure
were significantly higher with the conventional than with the irrigated
tip catheter: 13±10
versus 5±3
pulses, 53±41
versus 27±16
minutes, and 18±14
versus 9±6
minutes, respectively. No significant side effects occurred, and the
coronary angiograms of the first 30 patients after ablation was
unchanged.
Jais
et al. concluded that irrigated tip catheters were found to be more
effective than and as safe as conventional catheters for flutter
ablation, facilitating the rapid achievement of bidirectional isthmus
block.
Irrigation for difficult Accessory Pathways
Yamane et al. used an irrigated tip catheter for the ablation of
accessory pathways resistant to conventional catheter ablation 14.
Among 314 accessory pathways in 301 consecutive patients, conventional
ablation failed to eliminate accessory pathway conduction in 18
accessory pathways in 18 patients (5.7%), 6 of which were located in
the left free wall, 5 in the middle/posterior-septal space, and 7
inside the coronary sinus (CS) or its tributaries. Irrigated tip
catheter ablation was subsequently performed with temperature control
mode (target temperature, 50°C), a moderate saline flow rate (17
ml/min), and a power limit of 50 W (outside CS) or 20 to 30 W (inside
CS) at previously resistant sites. Seventeen
of the 18 resistant accessory pathways (94%) were successfully ablated
with a median of 3 applications using irrigated tip catheters. A
significant increase in power delivery was achieved (20.3±11.5
versus 36.5±8.2
W; P<0.01) with irrigated tip catheters, irrespective of the
accessory pathway location, particularly inside the CS or its
tributaries. No serious complications occurred.
Yamane et al. concluded that irrigated tip catheter ablation is safe
and effective in eliminating accessory pathway conduction resistant to
conventional catheters, irrespective of the location.
Irrigation for Ventricular Tachycardia (VT)
Nabar et al. used irrigated tip catheters for ablation of ventricular
tachycardias that were resistant to conventional catheter ablation 15.
Eight patients (6 men, age 59±12 years)
in whom the clinical target VT (cycle length 430±97 msec)
could not be ablated using a conventional 4-mm tip RF ablation catheter
underwent additional attempts to ablate this VT using an irrigated tip
catheter. Ablation of the clinical target VT using an irrigated tip
catheter was
attempted from the left ventricle in 6 (septal, posterobasal, and
inferior:
2 each) and from the right ventricle in 2 patients (both septal), by
entrainment, activation, or pace mapping. A mean of 6±5 (range
2 to 15) pulses was delivered. Target VT ablation was successful in 5
patients (63%). After successful ablation, at a mean follow-up of 6.5±4 months
and while taking antiarrhythmic drugs, all 5 patients were free of VT
recurrences. Nabar et al. concluded that the clinical target VT could
be ablated using an irrigated tip catheter in 5 (63%) of the 8 patients
in whom ablation using a conventional RF catheter was unsuccessful.
Recommendations
4-mm Tip Catheters
The
target temperature for 4-mm tip catheters should be less than 80° C. Since
tissue temperature can be markedly higher than tip temperature a higher
target temperature may increase the incidence of tissue overheating
associated with crater formation and coagulum formation. The lesion
size is poorly
correlated to tip temperature in the in vivo situation. In high flow
areas
the tip is cooled and more RF power is delivered to the tissue to reach
target temperature resulting in relatively large lesions and vice
versa.
Consequently, in high flow areas in the heart the difference between
tip
temperature and tissue temperature is large and a lower target
temperature
should be considered (e.g., 60° C) whereas
in
low flow areas the tissue temperature is much better reflected by the
tip
temperature and a higher target temperature could be considered (e.g.,
80° C).
The
duration could be limited to 30 seconds for non-irrigated 4-mm tip
electrodes. The lesion is formed within the first 30 seconds
predominantly. A longer duration does not create larger lesions.
8-mm Tip Catheters
A
larger portion of 8-mm tip catheters is exposed to the blood and thus
cooled by the blood flow and a relatively large difference between tip
temperature and tissue temperature can be expected. Consequently, a
moderate target temperature (e.g., 60° C) should
be chosen and the RF power may be limited to 50-60W to avoid tissue
overheating and coagulum formation.
Irrigated Tip Catheters
Irrigation Flow rate
An
irrigation flow rate of 10ml/min may be selected in a power controlled
mode with a delivered power of up to 30 W. The irrigation flow rate
should be increased to 15-20 ml/min when more than 30 W are delivered
to avoid excessive heat development at the superficial tissue layers.
RF Power and RF duration
The
RF duration in power controlled mode with irrigated tip catheters
should be considered to be longer than 30 s. Instead of increasing the
power
to achieve the desired effect (which increases the likelihood of crater
formation) the duration could be increased. Skrumeda demonstrated
lesions
of similar size with 20 W for 300 s as with 50 W for 30 s.
Consequently, a moderate power of 20-35W with relatively long RF
duration of 60-300 seconds
should be considered to achieve relatively large lesions with a limited
risk of crater formation.
References
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