Comparison of Logdose and Bracket Protocols for Determination of Epinephrine Arrhythmia Thresholds in Dogs Anesthetized with Thiopental-Halothane

Abstract

Previous studies in dogs of anesthetic-epinephrine arrhythmias have used logdose or bracketed epinephrine infusion protocols to determine the arrhythmic dose of epinephrine (ADE) or plasma level of epinephrine at arrhythmias (PCE). Reported logdose ADE values for halothane preceded by thiopental induction (thiopental-halothane) are twice those with the bracket protocol. There are no reported PCE data for the bracket protocol, and neither protocol has been directly compared in the same dogs. Therefore, direct comparisons were made of thiopental-halothane ADE and PCE in seven dogs (group 1). Dogs were induced with thiopental (20 mg/kg), followed by halothane inhalation at end-tidal concentrations equivalent to MAC 1.25. Epinephrine infusion protocols were compared on two weekly test occasions, with the sequence and order of protocol testing randomized. Logdose ADE for four or more ventricular beats within 15 s was 3.92 +/- 0.60 micrograms/kg (mean +/- standard error), higher than the bracket ADE (2.54 +/- 0.34 micrograms/ml) (P less than 0.05). PCE at ADE were similar for both protocols, but six separate infusions of epinephrine were required to establish ADE with the logdose compared to four with the bracket protocol (P less than 0.05). These findings suggested enhanced epinephrine clearance with the logdose protocol. Therefore, five additional but similarly anesthetized dogs (group 2) were tested to determine if physiologic or hemodynamic conditions prior to epinephrine infusions ("initial conditions") were equivalent for both protocols. Protocols were modified to avoid provocation of ventricular arrhythmias.(ABSTRACT TRUNCATED AT 250 WORDS)