Deexcitation

Figure 1: Tissue responses to virtual electrode polarization. The virtual anode deexcites the tissue and shortens the action potential and refractory period. The virtual cathode extends the action potential duration and refractory period. If deexcitation fully or partially restores excitability in the area of the virtual anode, and if the virtual cathode is within one space constant, reexcitation will take place

Figure 1: Tissue responses to virtual electrode polarization. The virtual anode deexcites the tissue and shortens the action potential and refractory period. The virtual cathode extends the action potential duration and refractory period. If deexcitation fully or partially restores excitability in the area of the virtual anode, and if the virtual cathode is within one space constant, reexcitation will take place

Further illustration of simultaneous shock-induced prolongation and shortening of the action potential is shown in Fig. 2.69 The top panels of Fig. 2a show maps of postshock (+200 V) transmembrane potential (left), and action potential duration during a control beat (middle) and postshock (right). The action potentials were shortened in areas of negative polarization and prolonged in areas of positive polarization, resulting in dispersion of repolarization. In contrast, Fig. 2b shows a response that appears as action potential prolongation in all areas when the shock voltage was increased to +300 V. There is little difference in the areas of positive polarization (red traces), but dramatic differences in the areas of negative polarization (blue traces). In these areas, action potentials were shorted with a +200 V shock and lengthened due to reexcitation in response to a +300 V shock.

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