0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
probabilityofactivation within10msaftershock
Fig. 8.3 Probability of an area being activated in the first 10 ms after a monophasic 5 ms long shock of varying Voltage is given. The probabilities are calculated from 70 events for each panel, and these repetitions were split over multiple VF episodes (each VF episodes would contain all voltage values). Shock strengths are indicated in the lower right in V/cm with 1 V/cm corresponding to 9.5 V.
8.2 Recruitment and Wave Propagation after the Shock
In this section, results from a different experiment will be presented. In the experiment more and higher voltages were tested. First, the results of the previous section will be confirmed. Then I will take a closer look at the wave propagation just after the shock occurred.
In this experiment6 eight separate episodes of VF were measured with shocks ranging from 0.5 V/cm to 5 V/cm with 1 V/cm corresponding to 9.5 V. Within each episode all voltages were tested at most 10 times in a randomized order. The VF was given a time of 2 s to recover after each shock. In total 70 shocks are analyzed for each voltage. Very occasionally defibrillation occurred and was reinduced again quickly. Since the statistics are over a large number of events, these, however, have no significant effect on the outcome.
Figure 8.3 shows the result of the analysis as done in the previous section. At low voltages up to 1 V/cm (9.5 V) little or no activation can be seen within the first 10 ms after the shock was applied. At larger voltages, the affected areas as well as the probability of activation increase dramatically. This again confirms the expectation
6Experiment performed 2018-08-22.
from activation maps or simulations about which areas are recruited for new waves, although in this heart the activation pattern results have less clear structures.
Instead of a snapshot as seen Figure 8.3 it is now also possible to shift the 10 ms window to later times after the shock. In this case wave propagation originating from the first activated sites becomes visible as an increased activation probability in neighbouring regions with a small delay. As the activity evolves after the shock, the probability of activation will also decrease again in the initially activated areas.
Figure 8.4 captures this analysis. The first panel shows the maximum probability of activation found within any 10 ms window during a time of 100 ms after the shock.
Thus, an area which is commonly activated at a later time after the shock will also have a large probability in the figure. Looking at this analysis, one finds that also in the left side of the plots (right side of heart) areas are eventually activated with a relatively high probability. Part of this area is indicated by a green square in the 5V/cm panel.
To capture the temporal information, Figure 8.4 (b) shows the time span when the maximum activation probability occurred, with 0 ms indicating the probability of activation was maximal within 0 ms to 10 ms and 10 ms indicating a time span from 10 ms to 20 ms and so on. Especially at low voltages the times vary widely since the shock is dominated by the random fluctuations of the VF. However, the later panels with higher voltages show clear structures. First, they clearly show the initially activated areas. Then, waves spreading from these sites create areas of increasing times in the neighbouring regions. These regions also have large values in (a). Further, the area indicated by the green square in the 5V/cm panel of (a) has the highest probability of activation at times around 20 ms to 40 ms. This may be linked to wave sources that are recruited within the right ventricle on the endocardial wall, which require a short time to propagate to the epicard. Another interesting observation is in areas orthogonal to the shock electrodes (top and bottom of the plot). These areas are indicated by green arrows in the 4V/cm panel of (a), and in them the probability of activation never reaches high values even at large voltages.
This phenomenon warrants some more discussion. On the one hand, it is known that less recruitment is expected to occur in these areas. However, one may expect a larger probability of waves propagating into such an area from those sites that are activated close by. One contribution to this low probability of activation may be that different activation paths have different length and propagation times. Further, it may be that no activation sites lie close to these areas inside the heart. Contrary to the orthogonal areas with a low excitation probability, for the main activated sites aligned towards the electrodes activation is expected to occur in large areas on or close to the wall itself. Thus, the relative distance to the closest wave emitting site may be larger.
Also, Niels Otani and colleagues [114] suggested wave emission from the wall in an orthogonal direction to the direction of wave propagation as a mechanism for removing spirals from the heart and thus to defibrillate. A similar pattern as suggested by them may be an additional reason for the differences in the shock effect. The results shown highlight the importance of the shock geometry. The localized nature by which shocks
8.2 Recruitment and Wave Propagation after the Shock 113
0.3 0.4 0.5 0.6 0.7
maximumprobabilityofactivation within10mswithinone110msaftershock
a)
0 5 10 15 20 25 30 35 40
timeofmaximumactivation probability[ms]
b)
Fig. 8.4 a) Probability of an area being activated during any 10 ms time span within 100 ms after a shock of different voltage is given (moving windows). It is thus an extension of Figure 8.3 showing always larger values, since sometimes areas are commonly activated at a later time. b)The time when the maximum activation probability shown in (a) occurred.
Thus, for example a value of 10 ms indicates that it was most likely to find an activation to occur in the time range of 10–20 ms after the shock. This indicates wave propagation and later activation on the left side of the plot (right side of the heart).
affect the tissue are an important information that supports theoretical considerations for designing new shock vectors such as those suggested by Otani.