To the Editor: We read with great interest the article by Kudenchuk et al.' in the May 1997 issue of the Journal. The authors prospectively evaluated the bipolar pacing threshold of the right ventricle before and after biphasic waveform ventricular deflbrillation shocks in 67 patients. They noted that serial pacing thresholds following successful defibrillation were completely unchanged after 141 of 177 (80%) ventricular fibrillation inductions. They concluded that there was no need to increase the bradycardia pacing energy above three times the baseline pacing threshold in patients with implantable cardioverter defibrillators (ICDs). The study was based on the premise that monophasic waveform ventricular defibrillation shocks would increase the ventricular pacing threshold.2"̂ However, none of the referenced studies using monophasic wavefomi deflbrillation shocks were performed in patients with ICDs. We previously conducted a prospective study on the right ventricular pacing threshold in 18 patients with ICDs that incorporated only monophasic waveform deflbrillation shocks.' We were able to demonstrate that the right ventricular effective refractory period actually decreased from 251 ±24 msec to 222 ± 30 msec (P < 0.01) with a corresponding reduction of the monophasic action potential duration (APDJQ and APD(x,) (210 ± 16 msec -<• 179 ± 23 msec and 274 ± 24 msec -^ 240 ± 26 msec, respectively, both P < 0.01). Of note, overall, the pacing threshold was not significantly altered (0.26 ±0.14 mA at 2.0 msec -^ 0.23 ± 0.12 mA at 2.0 msec, P = NS), and serum levels of epinephrine and norepinephine were elevated (51 ± 34 pg/mL ->• 114 ± 108 pg/mL and 403 ± 130 pg/mL ^ 534 ± 208 pg/mL, respectively, both P < 0.05). We speculated that postdefibrillation increases in levels of endogenous catecholamines (and sympathetic tone) may prevent or reverse the otherwise expected decline in postshock ventricular excitability. We are not surprised to learn that Kudenchuk et al. made similar observations with regard to the bradycardia pacing energy requirement in patients who received biphasic wavefonn defibrillation shocks from ICD.
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