Ablation Energy Sources

3 types of ablation energy sources

RF, PFA, Cryo

Thermal Energy Sources

RF and Cryo

Non-thermal Energy Source

PFA

RF: radiofrequency ablation

uses alternating current to burn/heat tissue to ablate

What electrode configurations for RF?

Unipolar and bipolar

How does RF generate heat?

- RF delivers energy which causes ions to move

- This causes friction and heats the tissue through conduction below the surface

Resistive Heating

ions moving to generate heat

Conductive Heating

Heat conducting below the surface

What increases lesion size in RF?

Higher power and higher tissue temperature

What happens if tissue temp > 100 C?

tissue vaporization and boiling of the blood

What happens with boiled plasma?

Coagulum develops on catheter electrode and increases electrical impedance

aka Charring

Ideal tissue temp

50-90 C - below vaporization but hot enough to damage

Convective Cooling

- heat loss due to blood flow around the catheter

- cools endocardial surface

Where in RF is the highest tissue temp?

slightly below the endocardial surface

- only get convective cooling on the surface

What affects convective cooling?

- unstable catheter position

- poor catheter-tissue contact

- high blood flow in region of catheter position

Advantages of convective cooling

- more power can be delivered into tissue

- increase depth of resistive heating

- avoids high endocardial surface temp

Actively cooled RF

- allows for higher amt of RF power for longer duration

- closed loop and open irrigation methods

Closed Loop Catheter Cooling

Internal

- cooling internally within the catheter and gives us slightly larger lesion volume and depth

Open Irrigation Cooling

Internal and external cooling

- exchange of fluid

- deliver fluid like heparinized saline outside of catheter to cool

Lesion size without active cooling

a bigger electrode would cause more damage

Lesion size with active cooling

more damage with smaller electrode that stays on tissue longer

How to optimize lesion size

- increase electrode temperature

- increase RF power delivery

- increase duration of RF application

- increase electrode length

- increase tissue contact

Impedence Monitoring

Before heating and with good catheter contact around 90-120 ohms

- 5-10 ohm reduction in impedance associated with proper lesion formation

Abrupt rise in impedance

associated with coagulum formation which leads to more resistance

Why is there reduction in impedance with lesion formation?

- Cell membrane has some resistance prior to ablation

- After ablation, cell membrane explodes which leads to decrease in impedance

Steam Pop

- High RF power causing superheating within tissue (>100 C)

- gas bubble forms and can erupt

What can a steam pop cause?

crater formation or myocardial perforation (uncommon)

Unipolar RF Circuit

Generator -> wire -> catheter -> electrode -> tissue -> grounding pad -> generator

Unipolar RF System

Energy flows from ablation electrode, through myocardium, to the dispersive electrode (grounding pad/skin pad or patch)

Highest current density in unipolar RF

Catheter tip/electrode

What do we need to prevent burns in unipolar RF?

good skin contact and large surface area

Type of lesions formed in unipolar RF

narrow and deep

Bipolar RF Circuit

Generator -> wire -> catheter -> electrode I -> tissue -> electrode II -> generator

What system would we want for septal VT/AT?

Bipolar sandwich method

- better for thick tissue

Bipolar RF system

uses two ablation electrodes adjacent to each other

Sandwich Method Bipolar

two ablation catheter tips with myocardial wall sandwiched between (increased transmurality)

- focal lesion with small width and deeper lesion

Adjacent Electrodes in Bipolar System

- one catheter with 2 electrodes, a wide but shallow lesion

- electrodes placed on one side of myocardial wall and transmits power to adjacent electrodes

Cryoablation

freezing tissue

T/F: Cryoablation utilizes some sort of refridgerant within the catheter to freeze tissue

True

2 phases of Cryoablation

1. extracellular ice formation

2. intracellular ice formation

Extracellular Ice Formation

- ice forms extracellularly in response to mild temp (0 to -20 C)

- causes hypertonic stress which causes release of water from cells and shrinkage

Is EC ice formation reversible?

Yes if rewarming achieved within 30-60 seconds

Intracellular Ice Formation

- tissue cooled below -40 C, ice forms in and out of the cell

- intracellular ice propagates from one cell to next through gap junctions

- NONREVERSIBLE

Cryo Lesion Formation

- maximum tissue damage to adjacent electrode

- further from electrode, less complete tissue damage/hypothermia

- thawing of IC ice further damages tissue

Cryoablation Lesion Size Determinants

- Freezing duration

- freezing rate

- thawing rate

- electrode contact

Steps of Cryoablation

Cryomapping

Cryoablation

Cryomapping

- done at -32 C so it is reversible within 80 seconds

- catheter adheres to frozen tissue (good focal lesions)

- allows for PES to test functionality of potential ablation target

Cryoablation step

- once successful cryomapping occurs, cryoablation begins

- IC freezing creates irreversible lesion

Advantages of Cryo

- reversible lesions and modulation of lesion formation

- functional assessment of efficacy and safety of ablation site

- lower risk of coagulum/charring

- cryoadherence results in very focal lesion

- absence of pain perception in non-sedated patients

Cryo for AVNRT

- not a single case of CHB has been reported

- higher risk of recurrence and may have second ablation

- able to test to make sure we ablated slow pathway before irreversible cryoablation

Cryo for PVI

- cryoballon

- bigger risk of phrenic nerve damage because balloon isn't as focal

- could cause esophageal damage or fistula

PFA: Pulsed Field Ablation

uses electricity to kill tissue instead of thermal energy

- form of IRE aimed to achieve atrial myocardial injury and reducing non-target injury

Electroporation

- holes created in membranes of cells due to reaching a certain voltage threshold

Atrial Cardiomyocytes in PFA

- have a low threshold for electroporation meaning PFA only kills these cells not other tissue types

What type of current does PFA use?

Direct current with biphasic waveforms (positive and negative)

Irreversible Electroporation (IRE)

- causes lesion formation by exposing cells to electrical fields that break down the membrane (causing cell death)

What would cause a more thermal effect when using PFA?

higher voltage/electrical field strength and longer pulse duration

Blood vessels and nerves are relatively ______ to IRE injury

resistant

What else can IRE be used for?

non-resectable tumors near blood vessels and nerves

PFA complications

Coronary Spasms

Hemolysis

Coronary Spasms PFA

could cause MI potentially or sx

can give nitro to relax blood vessels

Hemolysis PFA

blood cells explode and die which can damage kidneys

may need acute dialysis

Is PFA shorter or longer than RF?

Shorter because we do not have to wait for heat to conduct down

catheter size also plays a role

QDOT Micro Ablation Catheter

BWI RF catheter

- 6 thermocouples on tip, tip has 3 micro electrodes for high res ECG

QMODE+

high power, short duration (90W @ 4 s)

QMODE

conventional ablation 25-50 W

Tactiflex Ablation Catheter

Abbott RF

- contact force sensing with flexible tip allowing greater stability and ability to visualize tissue contact

- directed irrigation flow

Sphere 9

Medtronic RF and PFA

- nitinol lattice for single spherical electrode for energy delivery

- nine isolated smaller electrodes for mapping and pacing

Farawave Nav

Boston Scientific PFA

- 20 electrodes

- basket and flower shapes for different PV shapes

- magnetic tracking ability for accurate mapping