High Voltage Portfolio with the Progeny Platform
Lessons in LesionsPSST 10819_11/2008Arrhythmia in Practice
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Scientific September 20081ObjectivesControlable/un-controlableHow
does cooling impact lesion size and safety?How does power affect
lesions size and safety?How does catheter tip size affect lesion
formation?Relation between catheter tip temperature and tissue
temperatureChar and thrombus formationAffects of contact on lesion
formationWhat does this 1st bullet mean? It seems incompletePSST
10819_11/2008Content provided by Boston Scientific September
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Scientific September 2008Discovery of Cardiac AblationLesion
Science 3Severe complications related to DC ablation led to the
search for an alternative energy sourceFirst accidental ablation
with DC (direct current) energy197919811985TODAYFirst
radiofrequency (RF) ablation in humansFirst DC ablation in a
humanPSST 10819_11/2008Content provided by Boston Scientific
September 2008Arrhythmia in Practice TodayContent provided by
Boston Scientific September 20083It probably wont surprise you that
the first cardiac lesion was created accidentally. In 1979 a
patient was undergoing defibrillation. One defibrillator electrode
came in electrical contact with a catheter electrode that was
positioned at the bundle of His. The result was complete AV
block
In 1981 Dr. Mel Scheinman (UCSF) performed the first DC ablation
in humanspatient with severe chronic rheumatoid arthritis, sever
chronic obstructive lung disease and recurrent episodes of afib,
often accompanied by pulmonary edemarefractory to all available
drugs (including amiodarone)poor surgical candidate; would not
likely survive the postoperative periodprocedure was
successfulalthough AV conduction ultimately resumed, the heart rate
was easily controlled with low dose verapamillived until 1986 when
he died of congestive heart failure
Though effective, the severe complications related to DC
ablation (such as cardiac tamponade, hypotension following shock
delivery, and induction of ventricular arrhythmias) lead to search
for alternative energy source.
Radiofrequency (RF) energyImproved energy form for
ablationLocalized thermal effect on the tissueNo muscle
stimulation
Lesion Science 4Alternating electrical current (AC) at a
frequency of 500kHz similar to commercial electricity @ 50 Hz
PSST 10819_11/2008Content provided by Boston Scientific
September 2008Arrhythmia in Practice TodayContent provided by
Boston Scientific September 20084DorraGoals of Cardiac
AblationSelective neutralization of tissue within the heart that
causes or helps to sustain an arrhythmia
Apply sufficient energy to cause thermal injury and turn
electrically active cardiac tissue into electrically inactive scar
tissue (lesion)Tissue temperature of >50C is required for lesion
formation
Understanding lesion science helps the clinician to deliver
optimal therapy and may ultimately improve patient outcomes
Lesion Science 5123These 2 seem to say the same thingPSST
10819_11/2008Content provided by Boston Scientific September
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Scientific September 20085DorraThe GOAL of cardiac ablation is the
selective neutralization of tissue within the heart that causes or
helps to sustain an arrhythmia
This is accomplished by applying sufficient energy to cause
thermal injury and turn electrically active cardiac tissue into
electrically inactive scar tissueSpecifically, tissue temperatures
above 50oC are required to produce irreversible necrosis, making
the tissue unable to support conduction
The study of the biophysics of energy delivery and subsequent
mechanisms of tissue injury is called lesion science
Understanding lesion science allows the clinician to deliver
optimal therapy and may ultimately improve patient outcomes
Success FactorsLesion Science 6PSST 10819_11/2008Content
provided by Boston Scientific September 2008Arrhythmia in Practice
TodayContent provided by Boston Scientific September 20086Many
factors contribute to a successful ablation procedure. Some factors
are more easily controlled than others.
First and foremost, you have to know where to create the lesion.
An accurate diagnosis requires a thorough understanding of the
physiology and anatomy of the arrhythmia.
Next, you have to navigate the ablation catheter into position,
be able to keep it in position and deliver RF energy as appropriate
for the anatomy. It helps to have a clear EP or anatomical endpoint
to know when you have created an effective lesion.
Above all, be aware of variations in the anatomy and avoid
complications such as collateral damage, perforation and
thromboembolism
Resistive Heating: the light bulb analogyLight BulbRF
Ablation
Lesion Science 7PSST 10819_11/2008Content provided by Boston
Scientific September 2008Arrhythmia in Practice TodayContent
provided by Boston Scientific September 20087A good example of
resistive heating is a light bulb. The lower resistance power lines
leading to and from the light bulb filament are not heated. The
filament offers significant resistance to the electrical current
that passes through it. The resistance causes the filament to heat
up and glow, providing light.
Similarly, heat is generated as electrical current passes
through the highly resistive cardiac tissue. Application of
sufficient energy causes irreversible thermal injury to the tissue,
forming electrically inactive scar tissue (or, a lesion).
The RF energy delivery circuitClosed loop electrical
circuitcatheterheart tissuedispersiveelectrode
generatorLesion Science 8PSST 10819_11/2008Content provided by
Boston Scientific September 2008Arrhythmia in Practice TodayContent
provided by Boston Scientific September 20088RF energy is a high
frequency form of AC current. Like any household electrical
circuit, the RF delivery circuit must be a closed loop.Energy is
delivered by an RF generator into a catheter. Current flows through
catheter tip electrode, into and through the body tissues. It is
collected by the dispersive electrode (or electrodes) on the body
surface. Sometimes you will hear a dispersive electrode called a
return patch or ground pad. The dispersive electrode is connected
to the generator, completing the circuit and closing the loop.
Resistive heatingRF current is delivered via the catheter tip
INTO the tissue Because tissue has a very high resistance, the
passage of current generates heat in the tissueHaines, DE et. al. :
Pacing Clinical Electrophysiol 1989 12:962-976Haines D. The
Biophysics of Radiofrequency Catheter Ablation in the Heart. PACE
Vol 16, Part II, March 1993; 586-591RF ablation is NOT burning with
a hot tip!Blood
TissueLesion Science 9PSST 10819_11/2008Content provided by
Boston Scientific September 2008Arrhythmia in Practice TodayContent
provided by Boston Scientific September 20089Note: Catheters are
NOT a branding irons! The catheter tip gets hot ONLY because of its
close proximity to the heated tissue.
Lesions are formed when heat is generated within the cardiac
tissue. Lets look at how it works.The catheter material has very
LOW electrical resistance (meaning that the current flows easily
through it)Body tissue has very HIGH electrical resistanceAs RF
current passes from the LOW resistance catheter material through
the HIGH resistance tissue (in this case, the myocardium), heat is
generatedThe process of generating heat by passing an electric
current through a resistive material is called resistive
heating
Conductive HeatingHaines, DE et. al. : Pacing Clinical
Electrophysiol 1989 12:962-976Haines D. The Biophysics of
Radiofrequency Catheter Ablation in the Heart. PACE Vol 16, Part
II, March 1993; 586-591Strickberger SA, Hummel J, Gallagher M, et
al. Effect of accessory pathway location on the efficiency of
heating during RF catheter ablation. AM Heart J 129:54-58.
1995.Blood
TissueHeat conducted fromwarm electrode into cooler bloodHeat
conducted fromwarm tissue into cooler bloodHeat conducted
fromresistively heated tissue into catheter tip Catheter tip heats
upLesion Science 10Heat conducted from resistively heated tissue
into surrounding tissue, expanding lesionPSST 10819_11/2008Content
provided by Boston Scientific September 2008Arrhythmia in Practice
TodayContent provided by Boston Scientific September 200810It is
important to note that only a narrow rim of tissue (2-3mm) in close
contact with the catheter electrode is heated directly through
resistive heating. The majority of a lesions volume is formed by
thermal conduction, not resistive heating. Lets see why.
There are 2 major factors which occur at the tip
electrode/tissue interface that have an important influence on the
size of the lesion and the power/time it takes to create it. They
are: thermal conduction, and convective cooling.
The second law of thermodynamics states, in part, that the
transfer of heat is normally from a high temperature object to a
lower temperature object. Applying this theory to ablation, when
heat is generated at the surface of the tissue, it will
spontaneously conduct to the deeper, cooler layers of tissue. Hence
the term thermal conduction.
A commonplace example of thermal conduction is the handle of a
pot on the stove. The handle is cool when you first place the pot
on the stove. Once heat is applied to the pot and the contents
begin to warm, heat is gradually conducted to the pot handle.
In the case of RF delivery and subsequent lesion formation,Note
that only a narrow rim of tissue (2-3mm) in close contact with the
catheter electrode is heated directly through resistive heating.All
heating of deeper tissue layers occurs passively through thermal
conduction from the tissue that has been heated via resistive
heating.Most of a lesions volume is formed by thermal
conduction.Thermal conduction is also responsible for heating the
electrode tip and surrounding blood pool.
Blood
TissueHeat conducted fromwarm catheter tip into cooler bloodHeat
conducted fromwarm tissue into cooler bloodHeat conducted
fromresistively heated tissue into catheter tip Heat conducted from
resistively heated tissue into surrounding tissue, expanding
lesionCatheter tip heats up
Convective CoolingHaines, DE et. al. : Pacing Clinical
Electrophysiol 1989 12:962-976Haines D. The Biophysics of
Radiofrequency Catheter Ablation in the Heart. PACE Vol 16, Part
II, March 1993; 586-591Strickberger SA, Hummel J, Gallagher M, et
al. Effect of accessory pathway location on the efficiency of
heating during RF catheter ablation. AM Heart J 129:54-58.
1995.Convective Cooling via (37C) Blood Flow Lesion Science 11PSST
10819_11/2008Content provided by Boston Scientific September
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Scientific September 200811The major factor opposing thermal
conduction into deeper tissue layers is convective cooling.
Convection is defined as the process by which heat removed from a
medium by rapidly and actively mixing it.
In the case of RF ablation, convective cooling removes heat from
the tip/tissue interface as the intracavity blood flows around the
interface. If the blood is moving rapidly over the tissue surface,
a large amount of the heat produced at the ablation site may be
dissipated into the blood.
Convective cooling influences lesion creation. Generally, a
higher the rate of blood flow increases the cooling effect, which
is why higher RF power levels are required to produce lesions in
regions of high local blood flow (such as the ablation of the slow
pathways in AVNRT, right-sided accessory pathways, and especially
anteroseptal pathways). Conversely, lower RF power levels may be
appropriate for ablation sites shield from high blood flow (such as
locations under the valves, behind the leaflets or at the
apex).
In fact, the efficiency of energy coupling to tissue can be as
low as 10% depending on electrode size, catheter stability, and
catheter position relative to blood flow.
Power vs Temperature ControlPSST 10819_11/2008Content provided
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TemperaturesPSST 10819_11/2008Content provided by Boston Scientific
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AblationPSST 10819_11/2008Content provided by Boston Scientific
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to Tissue ContactPSST 10819_11/2008Content provided by Boston
Scientific September 2008Arrhythmia in Practice TodayContent
provided by Boston Scientific September 2008Lesion ScienceMultiple
Factors Influence Lesion Size & Quality:
RF Power Delivered to the Targeted TissueDurationBlood Flow Over
the Tip/Tissue InterfaceElectrode Geometry*Electrode Tip/Tissue
Contact*Type of TissueTip Orientation to the Tissue
* Effected by Catheter EngineeringPSST 10819_11/2008Content
provided by Boston Scientific September 2008Arrhythmia in Practice
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Science
It will be important to remind physicians that there are
multiple factors that impact the size and quality of RF lesions -
Catheter tip geometry, the amount of power delivered into the
targeted tissue, ablation duration, etc.
While contact is obviously an important factor, it is not the
only factor. Educating your physicians about lesion science and the
factors that influence the size and quality of the lesion is an
important strategy. Doing so will help build a strong foundation
for BSCs Intellatip MiFi launch(es) in 2013.
18Lesion Growth with TimeLesion Size (mm)Time (sec)Steady State
= MAXIMUM Lesion Size heat generated within lesion = heat
transferred away from lesion
Haines D. Biophysics of Radiofrequency Lesion Formation.
Catheter Ablation of Cardiac Arrhythmias (2006)~1/2 max lesion size
created in the first 5-10 seconds of energy deliveryLesion Science
19PSST 10819_11/2008Content provided by Boston Scientific September
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Scientific September 200819We know that lesions do not form
instantaneously they form over time based on the RF energy
delivered and rate of thermal conduction.This graph illustrates
lesion growth as a function of time. It is called a monoexponential
relationshipNotice that half a lesions maximum size is created in
the first 5-10 seconds of energy delivery. This is because tissue
close to the surface heats up very quickly (direct, resistive
heating)As time progresses, more thermal energy is transferred to
deeper tissue layers (thermal conduction).
The Concept of Steady StateIf RF delivery is held at a
consistent level and uninterrupted, the entire electrode-tissue
system will eventually reach a steady state, meaning that the
amount of energy entering the tissue at the thermal source equals
the amount of energy being dissipated at the tissue margins beyond
the lesion border. This is an important concept because it means
that the lesion has reached its maximum size for that particular
level of power delivery.If RF delivery is interrupted before a
steady state is achieved, thermal conduction will cause the tissue
temperature to continue to rise in deeper tissue for a short period
of time. This effect is called thermal latency and has important
clinical implications, depending on the location of the lesion
(proximity to the esophagus).Temperature pattern within a
lesionLesion Science 20
Conductive HeatingConvective CoolingPSST 10819_11/2008Content
provided by Boston Scientific September 2008Arrhythmia in Practice
TodayContent provided by Boston Scientific September 200820This
clip shows how thermal conduction causes heat to be transferred
from the surface, deeper into the tissue.
Because of convective cooling on the surface, you can see that
the highest temperature within a lesion is just below the
surface.
You can also how the thermal energy emanates from the source and
spreads in a radial fashion. Notice how the heating dissipates at a
certain point within the tissue. Benchtop studies1 have shown that
there is a fairly predictable decrease in the tissue temperature as
the distance from the source increases.
This is why you dont see a lesion form from the point of contact
with the myocardium to the return patch on the patients back!
***************************************1Haines DE, Watson DD:
Tissue heating during RF catheter ablation: A thermodynamic model
and observations in isolated perfused and superfused canine right
ventricular free wall. Pacing Clin Electrophysiol 1989;
12:962-976
Consequences of overheating Heat is conducted to the catheter
tip and deeper tissueCatheter tip heats up; Surrounding blood is
heated and coagulatesCoagulum forms and adheres to the catheter
tipFlow of current from the catheter tip to tissue is impededSystem
impedance rapidly increasesPower delivery is terminatedResistive
heat is generated in the tissue as current flows through itRF power
is deliveredBlood becomes denaturedLesion Science 21PSST
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Scientific September 200821Theoretically, in an ideal model with no
convective cooling and stable tip/tissue contact, it seems that
delivery of a very high level of energy should yield a very deep
lesion. So what limits the amount of power delivered into the
tissue?
There is a biologic phenomena at work called coagulum
formation.Blood boils at 100C, denaturing the protein structures in
the blood and causing coagulum to formCoagulum quickly accumulates
on the tip electrode and may form charThis impedes the flow of
electrical current and causes a sharp rise in system impedance.The
rise in system impedance limits or the power delivered into the
tissue and consequently, lesion formation
In addition to causing an impedance rise, coagulum formation
increases the risk of char embolism and subsequent stroke.
Excessive heating at the tip/tissue interface should be
avoided.
Consequences of overheating Heat is conducted to the catheter
tip and deeper tissueExcess power delivery causes tissue
overheating Power delivery is not terminated automatically and must
be terminated manuallyResistive heat is generated in the tissue as
current flows through itRF power is deliveredVaporization of fluid
causes steam pops and risk of ruptureLesion Science 22Add a video
of steam popPSST 10819_11/2008Content provided by Boston Scientific
September 2008Arrhythmia in Practice TodayContent provided by
Boston Scientific September 200822Theoretically, in an ideal model
with no convective cooling and stable tip/tissue contact, it seems
that delivery of a very high level of energy should yield a very
deep lesion. So what limits the amount of power delivered into the
tissue?
There is a biologic phenomena at work called coagulum
formation.Blood boils at 100C, denaturing the protein structures in
the blood and causing coagulum to formCoagulum quickly accumulates
on the tip electrode and may form charThis impedes the flow of
electrical current and causes a sharp rise in system impedance.The
rise in system impedance limits or the power delivered into the
tissue and consequently, lesion formation
In addition to causing an impedance rise, coagulum formation
increases the risk of char embolism and subsequent stroke.
Excessive heating at the tip/tissue interface should be
avoided.
Impedance rise associated with high temp015304560Time
(sec)405060708090100Temperature ( C)100150200250300Impedance
(ohms)Tip temperatureImpedanceHaines D: Biophysics of Ablation:
Application to Technology. J Cardiovasc Electrophysiol 2004;
15:S2-S11Lesion Science 23PSST 10819_11/2008Content provided by
Boston Scientific September 2008Arrhythmia in Practice TodayContent
provided by Boston Scientific September 200823This phenomena was
demonstrated in a experiment using a canine model. Haines and
Verow1 obtained in vitro and in vivo data on the relationship
between the tip electrode temperature and the resultant effect on
impedance. This and other data have consistently shown an sudden
impedance rise at the peak temperature of 100C
*******************************1Haines DE, Watson DD: Tissue
heating during RF catheter ablation: A thermodynamic model and
observations in isolated perfused and superfused canine right
ventricular free wall. Pacing Clin Electrophysiol 1989;
12:962-976Consequence of excessive intramural heatingNormal
Lesion
Thiagalingam A, DAvila A, McPherson C, Malchano Z, Ruskin J,
Reddy V. Impedance and Temperature Monitoring Improve the Safety of
Closed-Loop Irrigated-Tip RF Ablation. J Card Electrophysiol
18:3:318-325, 2007.Lesion with Steam PopLesion Science 24PSST
10819_11/2008Content provided by Boston Scientific September
2008Arrhythmia in Practice TodayContent provided by Boston
Scientific September 200824The second consequence of excessive
heating is a phenomena known as steam pops or pop lesions.
Excessive heating may cause vaporization of water content to occur
intramurally, causing a gas bubble to develop within the tissue
under the electrode. Continued application of RF will cause the
bubble to expand and its pressure to increase, which may lead to
eruption of the gas bubble through the weakest path. When the gas
is suddenly vented to the endocardial or epicardial surface (or
both) it causes an audible pop and may potentially cause
perforation and tamponade.
*************************1Nakagawa H, Yamanashi WS, Pitha JV, et
al.: Comparison of in vivo tissue temperature profile and lesion
geometry for RF ablation with a saline-irrigated electrode versus
temperature control in a canine thigh muscle preparation.
Circulation 91:2264-2273, 1995.Key points of lesion
scienceRadiofrequency (RF) energy causes thermal lesion formation
through resistive heating of myocardial tissue.Catheters are NOT a
branding irons! The catheter tip gets hot ONLY because of its close
proximity to the heated tissue. Heat is transferred to deeper
layers of tissue (and the tip electrode and surrounding blood pool)
via thermal conductionTissue temperatures of >50C or higher are
required for lesion formation
Lesion Science 25PSST 10819_11/2008Content provided by Boston
Scientific September 2008Arrhythmia in Practice TodayContent
provided by Boston Scientific September 200825Key points of lesion
scienceBlood flow cools the electrode tip and tip/tissue interface
through convective cooling the major factor opposing thermal
conduction/lesion formationThe hottest spot in a lesion is below
the tissue surfaceExcessive heating (>100C) at the tip/tissue
interface causes coagulum formation, resulting in a system
impedance rise, limiting RF deliveryBuildup of intramural gases,
due to excessive heating, accompanied by continued RF delivery may
cause a steam pop
Lesion Science 26PSST 10819_11/2008Content provided by Boston
Scientific September 2008Arrhythmia in Practice TodayContent
provided by Boston Scientific September 200826ConclusionRF energy
is the primary technology used in cardiac ablationhigh success
rateslow complication rateseasy to use
Monitoring of tip electrode temperature may help prevent
coagulum formation and steam pops
Ultimately, lesion size is directly proportional to tissue
heatingLesion Science 27PSST 10819_11/2008Content provided by
Boston Scientific September 2008Arrhythmia in Practice TodayContent
provided by Boston Scientific September 200827ConclusionBut, many
factors influence tissue heatingRF power level and duration of
deliveryblood flow over the electrode-tissue interfacecatheter tip
geometrycatheter tip/tissue contact
Understanding lesion science allows delivery of optimal therapy
and may ultimately improve patient outcomes
Lesion Science 28PSST 10819_11/2008Content provided by Boston
Scientific September 2008Arrhythmia in Practice TodayContent
provided by Boston Scientific September 200828
