EFFECTS OF INTRAVENOUS PROPRANOLOL ON HUMAN VENTRICULAR STRENGTH INTERVAL CURVES

Background: Prior studies on the effects of propranolol ore human vent ricular refractoriness have yielded occasionally inconsistent results, Most prior studies have examined refractory periods at a single stimu lus intensity and without using continuous pacing; thus, the effects o f propranolol on repolarization may have not been completely defined, The purpose of the present study was to reevaluate the effects of beta blockade on the human ventricular effective refractory period. Method s: Strength-interval curves were performed in duplicate in a group of 10 patients to demonstrate their reproducibility. Strength-interval cu rves were performed before and after intravenous propranolol administr ation in a second group of 10 patients who had no evidence of structur al heart disease. Results: Propranolol increased the absolute refracto ry period from 208 +/- 9 milliseconds to 212 +/- 10 milliseconds (p = 0.01), However, propranolol decreased the coupling interval at which t he strength-interval curve began to show an increase in the stimulus i ntensity required for capture from 236 +/- 8 milliseconds to 232 +/- 9 milliseconds. This resulted in a decrease of the width of the strengt h-interval curve from 28.0 +/- 5.1 millisecomds to 20.4 +/- 5.5 millis econds (p < 0.005). Propranolol also significantly increased the slope of a logarithmic fit of the strength-interval curves. Conclusion: Pro pranolol exerts complex effects on human ventricular refractoriness. P ropranolol decreases the width and increases the slope of human streng th-interval curve, rather than increasing or decreasing the refractory period, These results have potential implications for antiarrhythmic effects of propranolol.