[Effect of beta-receptor block on QT/QS2 behavior during isometric and dynamic exercise].

Our study is aimed to evaluate the change of QT/QS2 ratio in normal subjects during both isometric and dynamic exercise before and after propranolol administration. We studied 10 young volunteers healthy subjects who performed an isometric exercise by squeezing a grip dynamometer at 70% of their maximal voluntary contraction as long as possible. They also performed a dynamic exercise undergoing a submaximal bicycle stress test. Both tests were performed before and after administration of propranolol (0.15 mg/Kg e.v.) QT and QS2 intervals were measured at rest, during exercise and in the recovery period. Heart rate and blood pressure were also determined. Isometric exercise induces a significant shortening of both intervals although minor for QT so that the ratio significantly increases in comparison to baseline (p less than .001). At rest propranolol induces a significant decrease of heart rate and only a slight lengthening of QT and QS2 so that the ratio is unchanged. During exercise propranolol does not influence the increase of heart rate and blood pressure and the shortening of QT interval but prevent exercise-induced QS2 shortening so that the ratio after beta-blockade is significantly reduced at the peak of exercise (p less than .005). During dynamic exercise QT and QS2 behaviour is similar to that of isometric exercise; in fact both intervals are shortened and QS2 decrease is major than QT so that the ratio increases (p less than .001). These results confirm that QT/QS2 ratio can monitor the effects of adrenergic stimulation on the heart during physiological manoeuvres enhancing sympathetic discharge like occurs during both isometric and dynamic exercise.(ABSTRACT TRUNCATED AT 250 WORDS)