Epicardial potential gradients induced by defibrillation shocks to hearts immersed and non-immersed in a conducting solution

Studies of defibrillation threshold (DFT) or local epicardial potential gradients (LPG) are performed in open chest preparations or in isolated hearts immersed in solution (I) or exposed to air [non-immersed (NI)]. However, DFT depends on LPG in the low intensity region. In this study the authors determined differences in LPG produced by I and NI. Monophasic shocks (n=123) from a voltage-controlled defibrillator were delivered to 4 perfused rabbit hearts through epicardial patch electrodes either NI or I in Kreb´s solution interleaved in random order. Shock intensities (SI) ranged from 40-160 V. LPG were determined using two, perpendicular pairs of bipolar electrodes placed in a low LPG region (bipole spacing=5 mm). Impedance was significantly higher in NI than in I hearts (99.9±1.5 vs 76.5±1.33 Ω, p<0.0001), the NI/I ratio was 0.76. LPG differed as a function of SI and were higher in NI (n=57 episodes) than in I hearts (n=66) (p<0.00001), the NI/I ratio was 0.67. These results show that changes in cardiac volume conductor alter LPG distribution and local epicardial SI. These differences can produce significant variation in DFT and measured potential gradients; they must be considered in interpreting defibrillation studies