Thay đổi thông cảm hoặc phó giao cảm trong ANS mà dẫn đến thay đổi trong nhịp tim và điện tâm đồ hình thái bất đối xứng. Đó là, thay đổi năng động xảy ra như làm tăng nhịp tim, không phù hợp (một cách đối xứng thời gian) khi nhịp tim giảm và có một độ trễ (một số beat) trong phản ứng giữa sự thay đổi khoảng thời gian R | Nonstationarities in the ECG 65 timing and ECG morphology as nonstationary they can actually be well represented by nonlinear models see Section 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. Heart Rate Hysteresis 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 QT interval correction factors6 are a gross simplification of the relationship and that a more dynamic model is required. Furthermore it has been shown that the relationship between the QT and RR interval is highly individual-specific 20 perhaps because of the dynamic nature of the system. In the QT-RR phase plane the trajectory is therefore not confined to a single line and hysteresis is observed. That is changes in RR interval do not cause immediate changes in the QT interval and ellipsoid-like trajectories manifest in the QT-RR plane. Figure illustrates this point with each of the central contours indicating a response of either tachycardia RT and bradycardia RB or normal resting. From the top right of each contour moving counterclockwise or anticlockwise as the heart rate increases the RR interval drops the QT interval .
