The subjects then performed constant workload exercise tests ( 6-min duration ) at a workload equal to approximately 50 % of metabolic reserve with the pacemaker randomly programmed to each of three patterns of chronotropic response : ( 1 ) DDD ( lower rate 60 beats/min ) ; ( 2 ) Fast ( lower rate abruptly programmed to the expected value at 50 % metabolic reserve ) ; and ( 3 ) Overpaced ( lower rate at least 80 % of the age predicted maximum ) .
