A rate responsive pacemaker includes a detector for detecting the anaerobic threshold of the patient. The threshold can be logged or can be used to manipulate the pacing regime of the pacemaker. The detector may include two metabolic demand sensors: one which is sensitive to the anaerobic threshold and one that is not. In this manner, the anaerobic threshold can be detected by monitoring the correlation between the two parameters. The parameter sensitive to the anaerobic threshold may be a parameter dependent on the patient's breathing cycle, such as minute volume, while the other parameter is preferably a threshold dependent on the sympathetic control of the body such as the QT interval.