Thiopental and epinephrine-induced dysrhythmias in dogs anesthetized with enflurane or isoflurane.

Epinephrine-induced dysrhythmias were studied in 19 dogs anesthetized with 1.25 MAC enflurane or isoflurane, or the same preceded by thiopental (20 mg/kg). In 11 (group 1) dogs, thiopental reduced the dose of epinephrine required for production of ventricular ectopy, bigeminy and tachycardia with enflurane, and only ventricular tachycardia with isoflurane (P less than 0.05). Thiopental potentiation of epinephrine-induced dysrhythmias with enflurane lasted 4 hr after induction. In eight (group 2) dogs, the arrhythmic dose (ADE in microgram/ml) and plasma level of epinephrine (PLE in ng/ml) for four or more ventricular extrasystoles in 15 sec were determined in the same animal under each of the four test conditions. ADE and PLE values (X +/- SEM) were, respectively, enflurane, 9.1 +/- 1.0 and 141 +/- 24 (8/8 dogs); enflurane-thiopental, 5.0 +/- 0.6 and 63 +/- 16 (8/8 dogs); isoflurane, 28.3 and 330 (1/7 dogs); and isoflurane-thiopental, 15.2 +/- 2.8 and 265 +/- 59 (5/7 dogs). In addition, thiopental had no effect on plasma epinephrine levels reached during epinephrine infusions with 1.0 (enflurane only), 2.0 (enflurane, isoflurane) and 4.0 micrograms X kg-1 X min-1 (isoflurane only). Nor were epinephrine levels reached during enflurane or enflurane-thiopental different from those reached during isoflurane or isoflurane-thiopental. It is concluded that thiopental potentiates several types of epinephrine-induced ventricular dysrhythmias with enflurane, but only ventricular tachycardia with isoflurane. Furthermore, isoflurane or isoflurane-thiopental were less sensitizing than enflurane or enflurane-thiopental. Finally, neither thiopental nor the anesthetic agents affected plasma epinephrine levels reached during epinephrine infusions lasting 3 min.