Lesion Science in Cardiac Ablation

1
Lesion Science in Cardiac Ablation

• The biophysics of energy delivery and subsequent
  mechanisms of tissue injury during cardiac
  ablation

2
Goal of cardiac ablation

• Selective 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 gt50C is required for
  lesion formation
• Understanding lesion science helps the clinician
  to deliver optimal therapy and may ultimately
  improve patient outcomes

3
What makes an ablation procedure successful

• Know where to ablate
• Catheter placement
• Get it there
• Keep it there
• Deliver therapy
• Know when youre done
• Avoid complications

4
Discovery of cardiac ablation

• 1979 first accidental ablation with DC (direct
  current) energy
• 1981 first DC ablation in a human
• Severe complications related to DC ablation led
  to the search for an alternative energy source

5
Radiofrequency (RF) energy

• Ideal energy form for ablation
• able to focus the energy in a small area
• thermal effect of energy dissipates quickly as it
  moves away from its electrode source through
  tissue
• high frequency current heats tissue but does not
  cause muscle stimulation
• Form of alternating electrical current (AC) which
  is similar to current from a wall socket, but at
  a higher frequency (500kHz vs 60Hz)
• 1985 first radiofrequency (RF) ablation in humans

6
The RF energy delivery circuit
Closed loop electrical circuit
dispersive electrode
heart tissue
generator
catheter
7
Lesions are formed via resistive heating
Catheters are not branding irons!
RF current is delivered via the catheter tip
electrode INTO the tissue
Blood Tissue
Because tissue has a very high resistance, the
passage of current generates heat in the tissue
Haines, DE et. al. Pacing Clinical
Electrophysiol 1989 12962-976 Haines D. The
Biophysics of Radiofrequency Catheter Ablation in
the Heart. PACE Vol 16, Part II, March 1993
586-591
8
Resistive Heating the light bulb analogy
Light Bulb
RF Ablation
9
Competing factors at work thermal conduction
Heat conducted from warm electrode into cooler
blood
Heat conducted from warm tissue into cooler blood
Electrode heats up
Blood Tissue
Heat conducted from warm tissue into cooler
electrode
Heat conducted from warm
tissue into surrounding tissue,
expanding lesion
Haines, DE et. al. Pacing Clinical
Electrophysiol 1989 12962-976 Haines D. The
Biophysics of Radiofrequency Catheter Ablation in
the Heart. PACE Vol 16, Part II, March 1993
586-591 Strickberger SA, Hummel J, Gallagher M,
et al. Effect of accessory pathway location on
the efficiency of heating during RF catheter
ablation. AM Heart J 12954-58. 1995.
10
Competing factors at work convective cooling
Convective Cooling via (37C) Blood Flow
Haines, DE et. al. Pacing Clinical
Electrophysiol 1989 12962-976 Haines D. The
Biophysics of Radiofrequency Catheter Ablation in
the Heart. PACE Vol 16, Part II, March 1993
586-591 Strickberger SA, Hummel J, Gallagher M,
et al. Effect of accessory pathway location on
the efficiency of heating during RF catheter
ablation. AM Heart J 12954-58. 1995.
11
Lesion size, growth
Steady State Maximum Lesion Size heat
generated within lesion heat transferred away
from lesion
Lesion Size
1/2 max lesion size created in the first 5-10
seconds of energy delivery
Time
Haines D. Biophysics of Radiofrequency Lesion
Formation. Catheter Ablation of Cardiac
Arrhythmias (2006)
12
Temperature pattern within a lesion
13
Consequences of overheating tissue
RF power is delivered
Coagulum forms and adheres to the tip electrode
Heat is generated in the tissue as current flows
through it
Flow of current from tip to tissue is impeded
Heat is conducted back from the tissue to the
electrode and surrounding blood pool
System impedance rapidly increases
Tip electrode heats up Surrounding blood pool
approaches 100C
Power delivery is limited
Blood boils and becomes denatured
14
Impedance rise associated with high temp
Haines D Biophysics of Ablation Application to
Technology. J Cardiovasc Electrophysiol 2004
15S2-S11
15
Consequence of excessive intramural heating
Lesion with Steam Pop

• Normal 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
183318-325, 2007.
16
Key points of lesion science

• Radiofrequency (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 conduction
• Tissue temperatures of gt50C or higher are
  required for lesion formation
• Blood flow cools the electrode tip and tip/tissue
  interface through convective cooling the major
  factor opposing thermal conduction/lesion
  formation
• The hottest spot in a lesion is below the tissue
  surface
• Excessive heating (gt100C) at the tip/tissue
  interface causes coagulum formation, resulting in
  a system impedance rise, limiting RF delivery
• Buildup of intramural gases, due to excessive
  heating, accompanied by continued RF delivery may
  cause a steam pop

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Conclusion

• RF energy is the primary technology used in
  cardiac ablation
• high success rates
• low complication rates
• easy to use
• Monitoring of tip electrode temperature may help
  prevent coagulum formation and steam pops
• Ultimately, lesion size is directly proportional
  to tissue heating
• But, many factors influence tissue heating
• RF power level and duration of delivery
• blood flow over the electrode-tissue interface
• electrode geometry
• electrode tip/tissue contact
• Understanding lesion science allows delivery of
  optimal therapy and may ultimately improve
  patient outcomes