We thank Dr Haines for his interest in our article. As my mentor and a pioneer in radiofrequency catheter ablation, his insights carry much weight. At issue is the use of tetrazolium staining to define the acute radiofrequency lesion boundary. Specifically, the consideration is raised that for acute radiofrequency lesions, the tetrazolium staining method represents the isotherm for heat denaturation of cellular dehydrogenase enzymes rather than the boundary of cell death. If true, tetrazolium

more »

... aining performed late after lesion formation would define a larger lesion size because of timedependent dissipation of enzymatic activity in a perimeter of devitalized tissue. The definition of the lethal isotherm for radiofrequency ablation is an important issue for evolving temperature monitoring technologies, and all possible sources of error should be examined. We believe, however, that latent enzymatic activity in otherwise devitalized tissue does not significantly distort the determination of the lethal isotherm for the following reasons. First, the issue of latent tetrazolium staining in thermally devitalized tissue has been addressed by direct experimentation. 1 The dimensions of tissue necrosis as determined by tetrazolium staining are the same when samples are studied 10 minutes, 24 hours, or 48 hours after laser-induced thermal injury. 1 Second, using glass microelectrode recordings, our laboratory has shown that electric activity is recorded 0.1 mm from the visible edge of an acute radiofrequency lesion. 2 This finding is consistent with the absence of a significant zone of stain-reactive but devitalized tissue about the lesion. Third, the studies that provide the lethal isotherm estimates of 50°C to 56°C also used the tetrazolium staining method to define acute lesion size. 3,4 These studies found the lethal isotherm to be less than the thermal inactivation temperature of dehydrogenase enzymes (62°C for porcine isoforms), suggesting that thermal enzymatic inactivation is not the limiting factor in determining lesion boundaries. 3, 4 The discrepancy between these estimates and our work is likely due to the direct measurement of the temperature at the edge of the lesion in our study and estimation of that temperature by extrapolation in the prior studies. Disclosures None. References 1. Neumann RA, Knobler RM, Pieczkowski F, Gebhart W. Enzyme histochemical analysis of cell viability after argon laser-induced coagulation necrosis of the skin. J Am Acad Dermatol. 1991;25:991-998. 2. Wood MA, Fuller IA. Acute and chronic electrophysiologic changes surrounding radiofrequency lesions. J Cardiovasc Electrophysiol. 2002;13: 56 -61. 3. Whayne JG, Nath S, Haines DE. Microwave catheter ablation of myocardium in vitro. Assessment of the characteristics of tissue heating and injury. Circulation. 1994;89:2390 -2395. 4. Haines DE, Watson DD. Tissue heating during radiofrequency catheter ablation: a thermodynamic model and observations in isolated perfused and superfused canine right ventricular free wall. Pacing Clin Electrophysiol. 1989;12:962-976. (Circ Arrhythm Electrophysiol. 2011;4:e68.)