A tachyarrhythmia detection and classification system classifies tachyarrhythmias based on an analysis of morphological features of a cardiac signal enhanced by using one or more physiological parameters indicative of hemodynamic stability and/or activity level. The tachyarrhythmia detection and classification system computes a measure of similarity between an arrhythmic waveform of the cardiac signal a template waveform for that cardiac signal, such as a correlation coefficient representative of the correlation between morphological features of the arrhythmic waveform and morphological features of the template waveform. A detected tachyarrhythmia episode is classified by comparing the measure of similarity to a threshold that is dynamically adjusted using the one or more physiological parameters.