Adrenergically mediated variations in the energy required to defibrillate the heart: observations in closed-chest, nonanesthetized dogs.

The day-to-day variations in epicardial defibrillation threshold (DFT) were examined in closed-chest, unanesthetized dogs. In 11 animals, DFT decreased from 15.8 +/- 2.1 J (mean +/- SE) at the beginning of the study (day 1), to 7.4 +/- 1.7 J on day 2 (p less than .0001). DFT measured daily for 5 consecutive days in seven dogs decreased from 22.1 +/- 3.1 J on day 1 to 9.3 +/- 2.3 J on day 2 (p less than .01) and remained stable from day 2 to day 5. Transcardiac impedance, measured in six dogs, decreased from 112 +/- 6 omega on day 1 to 100 +/- 6 omega on day 2 (p = NS). Propranolol given on day 2 in 14 dogs increased DFT from 12.0 +/- 2.2 to 18.0 +/- 3.1 J (p less than .05). The effects on DFT of sequential administration of isoproterenol and propranolol were examined in 10 dogs. Isoproterenol decreased DFT from 10.0 +/- 1.9 to 5.5 +/- 1.5 J when given before propranolol (p less than .001, n = 10), and from 11.7 +/- 3.0 to 9.7 +/- 3.1 J when given after propranolol (p less than .05, n = 9). Propranolol increased DFT from 10.6 +/- 3.0 to 14.6 +/- 3.9 J when given before isoproterenol (p less than .02, n = 9), and from 10.7 +/- 1.4 to 14.4 +/- 1.5 J when given after isoproterenol (p less than .01, n = 10). These experiments demonstrate a sustained cardiac effect of epicardial defibrillation reflected by a decrease in DFT that is partially reversible by propranolol.(ABSTRACT TRUNCATED AT 250 WORDS)