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Ziad F. Issa

29 Oct, 8 tweets, 4 min read

#IssaTweetorials
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Impedance & RF ablation:
Part 1: How does impedance affect RF lesion formation?

1/8
During RF ablation, system impedance = impedance of genera¬tor + transmission lines + catheter + electrode-tissue interface + skin patch interface + interposed tissues.

2/8
IMPEDANCE & POWER
The magnitude of RF current delivered by the generator is determined by impedance btwn ABL electrode and ground pad. Ablation at lower impedance yields higher current output (and tissue heating) compared with ablation at a similar power & higher impedance.

3/8
IMPEDANCE OF ELECTRICAL CONDUCTORS
Currently used electrical conductors from the generator to the patient and from the ground pad back to the generator are designed to have low electrical resistance to help minimize power loss within those conductors.

4/8
IMPEDANCE AT SKIN-GROUND PAD INTERFACE
Part of the RF power is dissipated at the skin-ground pad interface. High impedance at that location results in more RF power loss and skin heating at the ground pad, and less energy available for myocardial heating at ABL electrode.

5/8
IMPEDANCE AT SKIN-GROUND PAD INTERFACE
A large ground pad surface area (or adding a second patch) and meticulous skin preparation to optimize skin contact are required at to reduce impedance and minimize power loss, and to effectively dissipate heat and prevent skin burns.

6/8
IMPEDANCE AT ELECTRODE-BLOOD-TISSUE INTERFACE
With ABL electrode in contact with the endocardial wall, part of the electrode contacts tissue and the rest contacts blood, and the RF current flows through both myocardium & blood, then through the thorax to the ground pad.

7/8
RF LESION FORMATION
With normal electrode-tissue contact, only a fraction of all power is effectively applied to the tissue. The rest is dissipated in the blood pool and elsewhere in the patient.

8/8
MODULATING IMPEDANCE TO IMPROVE RF LESION FORMATION
RF current flow into the myocardium can be increased by reducing total circuit impedance and by modulating the ratios of electric impedances between RF electrode, blood pool, and cardiac tissue.

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More from @ZiadIssaMD

Ziad F. Issa

@ZiadIssaMD

8 Oct

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The mimicry of second-degree AV block (2°AVB)

1/8
ECG patterns that mimic 2°AVB are often related to atrial ectopy, concealed junctional ectopy, or AVN echo beats. Distinguishing physiologic from pathologic AVB is important.

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2/8
In 2°AVB, sinus P-P interval is fairly constant (except for some variation caused by ventriculophasic arrhythmia), the nonconducted P wave occurs on time as expected, and P wave morphology is constant. With ectopy, P waves occur prematurely & often have different morphology.

3/8
Early PACs can arrive at the AVN during the refractory period and conduct with long PRI or block (physiologic rather than pathologic block) and can mimic Mobitz I or Mobitz II 2°AVB.

Read 8 tweets

Ziad F. Issa

@ZiadIssaMD

24 Sep

#IssaTweetorials
What is Concealed Conduction?

1/7
Concealed conduction can be defined as "the propagation of an impulse within the conduction system that can be recognized only from its effect on the subsequent impulse, interval, or cycle."

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2/7
Impulse propagation in the conduction system generates too small electrical current to be recorded on ECG. If this impulse travels only a limited distance (incomplete penetration) in the conduction system, it can interfere with formation or propagation of another impulse.

3/7
Irregular Ventricular Response During AF:
AVN is expected to conduct at regular intervals when its RP expires after each conducted AF impulse. Irregular response is caused by incomplete penetration of some AF impulses into AVN, variably resetting its refractoriness.

Read 7 tweets

Ziad F. Issa

@ZiadIssaMD

10 Sep

#IssaTweetorials
#EPeeps

1/10

Q: If you could deliver 30 W of RF energy for 30 sec using any of the ablation (abl) electrodes shown in the figure, which RF ablation catheter creates larger ablation lesion size?

A: Let’s talk about how the RF abl lesion is formed.

2/10
The size of the lesion created by RF is determined by the amount of tissue heated to >>50°C.

Heat is generated when charged ions in tissue oscillate rapidly (following the alternating RF current) converting RF energy to kinetic/thermal energy (Ohmic/Resistive Heating)

3/10
According to Ohm’s law, the amount of power per unit volume (resistive heating) equals the square of current density times the impedance (resistance) of the tissue, which in turn, is a function of the square of RF current density.

Read 10 tweets

Ziad F. Issa

@ZiadIssaMD

20 Aug

#IssaTweetorials
1/
What are the types of CTI-dependent atrial macroreentry?

CTI-dependent macroreentrant atrial tachycardias (MRATs) are confined to the RA & incorporate the CTI as a critical part of the circuit. All these MRATs can be eliminated by CTI ablation.
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2/
CTI-dependent MRATs include:
(1) peritricuspid reentry (clockwise and counterclockwise typical atrial flutter [AFL])
(2) peritricuspid double-wave reentry
(3) lower loop reentry
(4) intra-isthmus reentry.

3/
PERITRICUSPID REENTRY
In typical AFL the wavefront rotates around the tricuspid annulus. A line of conduction block in the RA free wall is usually required to as a critical lateral boundary that prevents short-circuiting of the flutter wavefront around the IVC.

Read 7 tweets

Ziad F. Issa

@ZiadIssaMD

13 Aug

#IssaTweetorials
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Do you know the mechanism of typical atrial flutter (AFL)?
1/
Typical AFL is a macroreentrant circuit with the activation wavefront rotating clockwise or counterclockwise around the tricuspid annulus and using the CTI as an essential part of the circuit.

2/
Conduction across the CTI is relatively slower than the rest of the atrium (likely due to the anisotropic fiber orientation), which provides the protected zone of relatively slow conduction necessary for the flutter reentry circuit.

3/
Key to the development of typical AFL is formation of a line of transverse conduction block in the RA free wall, which acts as a critical lateral boundary that prevents short-circuiting of the flutter wavefront around the IVC and, hence, extinguishes (see video).