TABLE OF CONTENTS Nonstationarities in the ECG manifest both in an interbeat basis (as RR interval timing changes) and on an intrabeat basis (as morphological changes). Although the former changes are often thought of as rhythm disturbances and the latter as beat abnormalities, the etiology of the changes are often intricately connected. To be clear, although we could categorize the beat-to-beat changes in the RR interval timing and ECG morphology as nonstationary, they can actually be well represented by nonlinear models (see Section 3.7 and Chapter 4). This chapter therefore refers to these changes as stationary (but nonlinear). The transitions between rhythms is a nonstationary process (although some nonlinear models exist for limited changes). In this chapter, abnormal changes in beat morphology or rhythm that suggest a rapid change in the underlying physiology are referred to as nonstationary.
So far we have not considered the dynamic effects of heart rate on the ECG morphology. Sympathetic or parasympathetic changes in the ANS which lead to changes in the heart rate and ECG morphology are asymmetric. That is, the dynamic changes that occur as the heart rate increases, are not matched (in a time symmetric manner) when the heart rate reduces and there is a (several beat) lag in the response between the RR interval change and the subsequent morphology change. One well-known form of heart rate-related hysteresis is that of QT hysteresis. In the context of QT interval changes, this means that the standard...
