Potentiation by thiopental of halothane--epinephrine-induced arrhythmias in dogs.

Epinephrine-induced arrhythmias were studied in 14 dogs (Group 1) anesthetized with halothane alone (1.09% end-tidal), and on another occasion, at the same halothane concentration following intravenous thiopental (20 mg/kg). Surface (Lead II), catheter His bundle and high right atrial electrocardiograms, and airway and femoral arterial pressures were recorded. Graded doses of epinephrine (EPI-least dose 0.25 microgram . kg-1 . min-1) were infused over five minutes, but terminated sooner if ventricular tachycardia occurred (maximal sensitization). Sensitizing EPI doses (microgram . kg-1) were calculated (dose X time to arrhythmia) for: Shift in or wandering atrial pacemaker (SAP-WAP), atrial ectopy (At Ect), A-V dissociation (AVD), and ventricular ectopy, bigeminy, or tachycardia (V Ect, V Bigem, V Tach). With halothane alone, SAP-WAP occurred at the least dose of EPI followed by At Ect, AVD, V Ect, V Bigem, and V Tach in order of increasing EPI dose. Following thiopental, EPI doses for AVD, V Ect, V Bigem, and V Tach were reduced, as well as EPI dose differences for At Ect, AVD, V Ect, and V Bigem. In an additional seven dogs (Group 2), anesthesia was induced with thiopental (20 mg/kg) followed by halothane (1.09% end-tidal). These animals were observed for arrhythmias during graded EPI infusions at 1-2 h and 3-4 h following thiopental. Sensitizing EPI doses for SAP-WAP and V Tach were similar at each time period. The authors concluded that with halothane and increasing EPI dose, sensitization constitutes a spectrum of arrhythmias, beginning with atrial and progressing to severe ventricular arrhythmias. Thiopental reduces the EPI dose needed for AVD and ventricular, but not atrial, arrhythmias. It also reduces the EPI dose discrepancies for atrial and ventricular arrhythmias.