Intracardiac atrial defibrillation

Abstract

Intravascular ventricular defibrillation and intravascular atrial defibrillation have many similarities. An important factor influencing the outcome of the shock is the potential gradient field created throughout the ventricles or the atria by the shock. A minimum potential gradient is required throughout the ventricles and probably the atria in order to defibrillate. The value of this minimum potential gradient is affected by several factors, including the duration, tilt, and number of phases of the waveform. For shock strengths near the defibrillation threshold, earliest activation following failed shocks arises in a region in which the potential gradient is low. The defibrillation threshold energy can be decreased by adding a third and even a fourth defibrillation electrode in regions where the shock potential gradient is low for the shock field created by the first two defibrillation electrodes and giving two sequential shocks, each through a different set of electrodes. However, the addition of more electrodes and sequential shocks complicates both the device and its implantation. Because patients are conscious when the atrial defibrillation shock is given, they experience pain during the shock, which is one of the main drawbacks of intravascular atrial defibrillation. Unfortunately, the pain threshold for defibrillation shocks is so low that a shock less than 1 J is uncomfortable and is not much less painful than shocks several times stronger. Therefore, even though electrode configurations exist that have lower atrial defibrillation threshold energy requirements than the atrial defibrillation threshold with standard defibrillation electrode configurations used in implantable cardioverter-defibrillators (ICDs) for ventricular defibrillation, they are not clinically practical because their shocks are almost as painful as with the standard ICD electrode configurations. Such electrode configurations would make the ICD more complicated, leading to greater difficulty and longer time required for implantation.

Figure 1

A) Atria of sheep with the location of defibrillation electrodes and recording sites. Locations of defibrillation coils in the superior vena cava (SVC), right atrial appendage (RAA), and distal coronary sinus (DCS) are shown. Positions of the 336 epicardial recording sites covering the epicardial surface of both atria and 24 endocardial recording sites on the RA septum are indicated by the electrode number. B) Early activation sites after failed defibrillation shocks. Early sites clustered in the region of assumed low potential field gradient for the electrode configuration used for the shock. Adapted from Cooper et al., with permission.

Figure 2

Simplified model to illustrate the effects of changing electrode position and size. The figures on the left (A and C) show potentials established with the electrode configuration while the figures on the right (B and D) show potential gradient. Panel B demonstrates that electrodes placed asymmetrically about the heart show high potential gradient close to the electrodes (right hand side of circle), but low gradient regions at locations far from the electrodes (left hand side of circle). Panel D demonstrates that placing another electrode in the low gradient region reduces the size of the low gradient region on the left hand side of the circle, but that a low gradient region emerges between electrodes that share a common polarity. Adapted from Ideker et al., with permission

Figure 3

A) Diagram of the standard ventricular triad defibrillation configuration and an atrial triode electrode configuration B) Compared ADFT delivered energies for the configurations shown in A). Abbreviations are defined in the text. From Benser et al., with permission.