The electrical parameter used to define defibrillation strength is energy. Peak current, however, may more accurately reflect the field quantities (i.e., electric field strength and current density) that mediate defibrillation and therefore should be a better clinical descriptor of threshold than energy. Though transthoracic impedance is a major determinant of energy-based threshold and is sensitive to operator-dependent changes in impedance (electrode-subject interface), an ideal threshold descriptor should be invariant with respect to these changes in impedance. We therefore compared the relative invariance of energy- and current-based thresholds when transthoracic impedance was altered by one of two methods: (a) change in electrode size (protocol A) or (b) change in electrode force (protocol B). In protocol A, impedance was altered in each dog by a mean of 95%. Energy thresholds determined at both low and high impedance were 44 +/- 21 J (mean +/- SD) and 105 +/- 35 J, respectively, P less than 0.0001. In contrast, peak current (A) thresholds were independent of transthoracic impedance, 22 +/- 5 A (low impedance) vs. 24 +/- 6 A (high impedance), P = NS. Energy and current thresholds showed a similar relationship for animals tested in protocol B. Therefore, current-based thresholds, in contrast to energy thresholds are independent of operator-dependent variables of transthoracic impedance and are invariant for a given animal. These results suggest that redefining defibrillation threshold in terms of peak current rather than energy provides a superior method of defibrillation.
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