Adaptive rules for a more accurate sinus tachycardia detection mechanism for implantable device (IMD) and other devices. In an embodiment, the rule is incorporated into a prioritized, rule-based arrhythmia classification system in an IMD. If the rule is met, anti-tachyarrhythmia therapy is withheld even if a lower priority rule determines presence of a treatable tachyarrhythmia. The rule includes adaptive onset criterion, where an expected range of R—R is calculated based on a trimmed mean and a metric of R—R variability derived over a preceding series of R—R intervals. Pattern codes indicative of gradual rate changes and normal sinus rhythm will be within a grammar defined by a continuous recognition machine and will if they persist, result in the ST rule being met. All other codes inconsistent with sinus tachycardia will not adhere to the associatd grammar and will result in the ST rule not being met.