Effects of Intravenous Propranolol on Human Ventricular Strength Interval Curves

BACKGROUND: Prior studies on the effects of propranolol on human ventricular refractoriness have yielded occasionally inconsistent results. Most prior studies have examined refractory periods at a single stimulus intensity and without using continuous pacing; thus, the effects of 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. METHODS: Strength-interval curves were performed in duplicate in a group of 10 patients to demonstrate their reproducibility. Strength-interval curves were performed before and after intravenous propranolol administration in a second group of 10 patients who had no evidence of structural heart disease. RESULTS: Propranolol increased the absolute refractory period from 208 +/- 9 milliseconds to 212 +/- 10 milliseconds (p = 0.01). However, propranolol decreased the coupling interval at which the strength-interval curve began to show an increase in the stimulus intensity required for capture from 236 +/- 8 milliseconds to 232 +/- 9 milliseconds. This resulted in a decrease of the width of the strength-interval curve from 28.0 +/- 5.1 milliseconds to 20.4 +/- 5.5 milliseconds (p < 0.005). Propranolol also significantly increased the slope of a logarithmic fit of the strength-interval curves. CONCLUSION: Propranolol exerts complex effects on human ventricular refractoriness. Propranolol decreases the width and increases the slope of human strength-interval curve, rather than increasing or decreasing the refractory period. These results have potential implications for antiarrhythmic effects of propranolol.