BARIUM DECREASES DEFIBRILLATION ENERGY-REQUIREMENTS

Certain antiarrhythmic drugs that inhibit myocardial repolarizing curr ents decrease defibrillation energy, but the effect of blocking partic ular currents on defibrillation is not well understood. We therefore i nvestigated the effect of barium, a relatively selective blocker of in wardly rectifying potassium current (Ik(1)) on voltage and energy requ irements for defibrillation in an open-chest dog model. Defibrillation energy and voltage requirements were assessed by delivering monophasi c shocks through epicardial electrode patches at varying voltages to c onstruct a dose-response curve of energy and voltage versus success in defibrillation. The energy and voltage for 50% success in defibrillat ion (E(50) and V-50, respectively) were determined by logistic regress ion. Monophasic action potential duration at 90% repolarization (MAPD( 90)) was measured with a contact electrode, and ventricular refractory period (VERP) was measured. After baseline measurements were obtained of E(50), V-50, MAPD(90), and VERP, saline (control) (n = 6) or bariu m (1.1 mg/kg/min for 5 min followed by 0.25 mg/kg/min) (n = 11) was ad ministered. Defibrillation voltage and energy requirements and electro physiologic measures were repeated after 30 and 120 min of barium or s aline infusion. In control animals, there was no significant change wi th time in V-50 (2.0 +/- 12.4 and -0.2 +/- 16.0% at 30 and 120 min, re spectively), VERP(+3 +/- 5 and -2 +/- 3% at 30 and 120 min, respective ly) or MAPD(90) (+1 +/- 4 and -2 +/- 6, at 30 and 120 min, respectivel y). In contrast, after barium infusion, there was a significant decrea se in V-50 both at 30 and 120 min (-21.8 +/- 19.2, -36.2 +/- 15.8%, p < 0.001), in E(50) (decrease of -34.2 +/- 38 and -59.9 +/- 23% at 30 a nd 120 min, respectively, p < 0.001), and increases in VERP (+20 +/- 4 and +22 +/- 4% at 30 and 120 min, p < 0.03) and in MAPD(90) (+15 +/- 6 and +15 +/- 4% at 30 and 120 min, p < 0.03). We conclude that relati vely specific inhibition of inwardly rectifying potassium current mark edly decreases defibrillation voltage and energy requirements at a dos e that produces slight increases in ventricular refractoriness and MAP D.