A medical device capable of delivering automatic rate-adjusting pacing therapies is provided having an adjustable upper rate limit responsive to an indication of myocardial irritability. The device, which may be embodied as a pacemaker, a pacemaker/cardioverter/defibrillator, or the like, responds to the detection of an arrhythmia as an indicator of myocardial irritability by adjusting an upper rate limit. The adjusted upper rate limit is applied as the maximum allowable pacing rate during delivery of any pacing therapies previously defined as “long-term” pacing therapies.