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Ziad F. Issa

10 Dec, 9 tweets, 5 min read

#IssaTweetorials

What is the mechanism of aberrant conduction?

1/9

“Aberration” describes transient bundle branch block (BBB) and does not include persistent QRS abnormalities caused by persistent BBB, preexcitation, or the effect of drugs.

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2/9

Acceleration-dependent BBB (aka “phase 3 block” or “voltage-dependent block”) occurs when an impulse arrives at tissues that are still refractory due to incomplete repolarization (during phase 3 of the action potential [AP]).

3/9

Aberration secondary to phase 3 block tends to be in the form of RBBB when premature excitation (and Ashman phenomenon) occurs during normal baseline heart rates and in the form of LBBB when it occurs during fast heart rates.

4/9

Phase 3 block constitutes the physiological explanation of several phenomena:
(1) aberration caused by premature excitation.
(2) Ashman phenomenon.
(3) acceleration-dependent aberration.

5/9

Pathologic vs physiologic phase 3 block
Acceleration-dependent aberration is a marker of a diseased HPS when it:
(1) occurs at relatively slow heart rates (<70 bpm).
(2) displays LBBB.
(3) appears with gradual acceleration of the heart rate.

6/9

Pause-dependent block (aka “phase 4 block” or “bradycardia-dependent block”) occurs when conduction of an impulse is blocked in tissues well after their normal refractory periods have ended.

7/9

“Pause-dependent” or “bradycardia-dependent” block is caused by “phase 4 block.” Long intervals between activations allow for spontaneous depolarization and inactivation of Na+ channels & impaired conduction. Other proposed mechanisms include "source-to-sink mismatch".

8/9

Phase 4 block often follows a delay caused by a compensatory pause after a PAC or PVC, spontaneous slowing of the sinus rate, or overdrive suppression of sinus rhythm upon termination of a fast supraventricular rhythm.

9/9

Concealed transseptal conduction underlies aberration in several situations:
1) Perpetuation of aberrant conduction during tachyarrhythmias.
2) Unexpected persistence of acceleration-dependent aberration.
3) Alternation of aberration during atrial bigeminal rhythm.

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More from @ZiadIssaMD

Ziad F. Issa

@ZiadIssaMD

17 Nov

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Impedance and RF ablation:
Part 2: How does RF ablation affect impedance?

1/9
As tissue temperature rises during RF energy application, ions within the tissue being heated become more mobile, resulting in a decrease in impedance to current flow.

2/9
There are currently 2 methods to measure impedance: Generator Impedance (GI) & Local Impedance (LI).

3/9
Lack of impedance drop during RF energy application can reflect inefficient energy delivery to the tissue due to poor tissue contact, lack of catheter stability, or inadequate power delivery.

Read 8 tweets

Ziad F. Issa

@ZiadIssaMD

29 Oct

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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.

Read 8 tweets

Ziad F. Issa

@ZiadIssaMD

8 Oct

#IssaTweetorials

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

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#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.