Direct measurements of membrane time constant during defibrillation strength shocks.

ABSTRACT

BACKGROUND: Defibrillation shocks impose significant energy demand on implantable cardioverter-defibrillators (ICDs). Several modeling studies have been devoted to optimizing shock parameters, and a large number of these studies treat the heart as a simplified lumped network. The time constant of membrane polarization (tau(m)) is a key variable for such modeling efforts. OBJECTIVE: The purpose of this study was to perform direct measurements of transmembrane potential (V(m)) during defibrillation strength shocks and estimate tau(m) of membrane polarization. METHODS: A portion of the left ventricular epicardial surface of Langendorff-perfused rabbit hearts was stimulated using uniform electric fields produced by two parallel line electrodes. The V(m)s were recorded from di-4-ANEPPS-stained hearts using a multisite optical mapping system. The hearts were paced with 20 S1 pulses from the apex, and shocks (S2 5, 10, 20 V/cm) were applied via the line electrodes during the action potential of the 20th S1 at two different coupling intervals (S1S2 120 and 180 ms). Residual responses were obtained by subtracting responses to the 19th S1 from the responses to the 20th S1S2 pair and used for time-constant analysis by fitting a monoexponential function. RESULTS: tau(m) exhibited a large variation and ranged from approximately 1 to 30 ms. Furthermore, tau(m) varied with electric field strength, S1S2 interval, position of the tissue from stimulating electrodes, and polarity of the response. To a large extent, the effects of all these factors were captured in a single parameter-the change in transmembrane voltage (DeltaV(m)) in response to the applied field (E). tau(m) showed a monotonically decreasing trend with DeltaV(m) for all Es and S1S2s. CONCLUSION: Time constant of membrane polarization varies significantly during defibrillation strength shocks and shows a strong dependence on DeltaV(m).