A standardized isoproterenol sensitivity test. The effects of sinus arrhythmia, atropine, and propranolol

Isoproterenol hydrochloride sensitivity was measured from dose/response curves to rapid intravenous injections as the dose required to increase the heart rate by 25 beats per minute (chronotropic dose₂₅or CD₂₅). The most accurate assessment of CD₂₅was achieved by using the three shortest R-R intervals of the electrocardiogram to calculate heart rate before and after administration of isoproterenol to eliminate the effect of sinus arrhythmia. In ten subjects the CD₂₅appeared to increase with age between 21 and 35. Neither the chronotropic nor hypotensive effects of the CD₂₅for isoproterenol were affected by atropine sulfate, suggesting that vagal withdrawal does not contribute to the chronotropic effect of this dose. Propranolol hydrochloride increased the CD₂₅by acting as a competitive antagonist. The test procedure was safe, accurate, reproducible, and convenient for measuring both β-receptor responsiveness and β-adrenergic blockade in man.