Transmural recording of shock potential gradient fields, early postshock activations, and refibrillation episodes associated with external defibrillation of long-duration ventricular fibrillation in swine

Abstract

Background: Knowledge of the shock potential gradient (nablaV) and postshock activation is limited to internal defibrillation of short-duration ventricular fibrillation (SDVF).

Objective: The purpose of this study was to determine these variables after external defibrillation of long-duration VF (LDVF).

Methods: In six pigs, 115-20 plunge needles with three to six electrodes each were inserted to record throughout both ventricles. After the chest was closed, the biphasic defibrillation threshold (DFT) was determined after 20 seconds of SDVF with external defibrillation pads. After 7 minutes of LDVF, defibrillation shocks that were less than or equal to the SDVF DFT strength were given.

Results: For DFT shocks (1632 +/- 429 V), the maximum minus minimum ventricular voltage (160 +/- 100 V) was 9.8% of the shock voltage. Maximum cardiac nablaV (28.7 +/- 17 V/cm) was 4.7 +/- 2.0 times the minimum nablaV (6.2 +/- 3.5 V/cm). Although LDVF did not increase the DFT in five of the six pigs, it significantly lengthened the time to earliest postshock activation following defibrillation (1.6 +/- 2.2 seconds for SDVF and 4.9 +/- 4.3 seconds for LDVF). After LDVF, 1.3 +/- 0.8 episodes of spontaneous refibrillation occurred per animal, but there was no refibrillation after SDVF.

Conclusion: Compared with previous studies of internal defibrillation, during external defibrillation much less of the shock voltage appears across the heart and the shock field is much more even; however, the minimum nablaV is similar. Compared with external defibrillation of SDVF, the biphasic external DFT for LDVF is not increased; however, time to earliest postshock activation triples. Refibrillation is common after LDVF but not after SDVF in these normal hearts, indicating that LDVF by itself can cause refibrillation without requiring preexisting heart disease.

Figure 1

Diagram of a septal plunge needle. Black dots represent electrodes with the most endocardial electrode labeled 1 and the most epicardial labeled 6.

Figure 2

The dV/dt of the recordings from a three-electrode RV plunge needle during a successful defibrillation shock after SDVF. Earliest activation after the isoelectric window is indicated by ES. The flat horizontal line represents time during which the gain was decreased during the shock. Approximately 50 ms was allowed owing to uncertainty in triggering of the defibrillator.

Figure 3

A 3D map of fitted potentials (A) and ∇V (B) recorded during a 200-J shock in one animal. The top of the heart represents the base, and the bottom represents the apex. The right side contains the LV needles, and the left side contains the RV needles. Color bars indicate the scale used to display the recorded voltage (A) and ∇V (B). The most positive potentials are on the right, and the most negative potentials are on the left, corresponding to the right and left lateral defibrillation pad locations. The ∇V is lowest in the posterior LV and highest in the right ventricular outflow tract (RVOT).

Figure 4

Example of postshock activity after defibrillation of LDVF. In A, each panel shows in red electrode sites at which dV/dt is ≤−0.5 V/s at any time during a 5-ms interval. Frames advance in 5-ms increments from left to right (time stamp represents time from successful shock delivery). The heart is oriented in the anatomical position with the base at the top and the apex at the bottom. The black line separates RV and LV. A: Focal activity after successful defibrillation. Earliest activity arose focally within the anterior septum and spread throughout the ventricles. In B, the panels represent 10-ms time steps. B: Reentry after a failed defibrillation shock. The earliest activity arose in the posterior RV endocardium and formed a reentrant circuit within the posterior RV (black arrows).

Figure 5

Example of spontaneous refibrillation after LDVF. In A, each panel shows in red electrode sites at which dV/dt is ≤−0.5 V/s at any time during a 10-ms interval. Frames advance in 10-ms increments from left to right. The earliest activity arises from a focus in the LV near the septum and spreads outward in all directions. In B, the panels represent 20-ms time steps. The earliest activity arises from the apical LV followed by reentry within this region.