Trong những tình huống nhất định, chẳng hạn như trong thời gian thiếu máu cục bộ, QRS hình thái hiện vật biến đổi lâu dài không liên quan đến hô hấp. Điều này thúc đẩy một cập nhật liên tục của vòng tham khảo để tránh việc lập dự toán của các góc quay được tạo ra bởi các biến thể như vậy chứ không phải là hô hấp | EDR Algorithms Based on Beat Morphology 225 where the matrices UT and VT contain the left and right singular vectors from the SVD of ZT YRjTY. The estimate of Y is then obtained by 35 tr Yr t Yr Y tr YrTJTY Qt The parameters QT and YT are calculated for all values of T with Q resulting from that T which yields the minimal error e. Finally the rotation angles are estimated from Q using the structure in 22 ộ Y arcsin q13 ộX arcsin q2ị 8-9 ộZ arcsin q1ộ where the estimate qkl denotes the k l entry of Q. In certain situations such as during ischemia QRS morphology exhibits longterm variations unrelated to respiration. This motivates a continuous update of the reference loop in order to avoid the estimation of rotation angles generated by such variations rather than by respiration 28 . The reference loop is exponentially updated as Yr ì 1 aYR i 1 - a Y i 1 where i denotes the beat index at time instant ti . YR ti YR i and Y ti Y i . The parameter a is chosen such that long-term morphologic variations are tracked while adaptation to noise and short-term respiratory variations is avoided. The initial reference loop Yr 1 can be defined as the average of the first loops in order to obtain a reliable reference. Figure displays lead X of YR at the beginning and peak exercise of a stress test and illustrates the extent by which QRS morphology may change during exercise. An example of the method s performance is presented in Figure where the estimated rotation angle series are displayed as well as the VCG leads and the related respiratory signal. Unreliable angle estimates may be observed at poor SNRs or in the presence of ectopic beats calling for an approach which makes the algorithm robust against outlier estimates 28 . Such estimates are detected when the absolute value of the angle estimates exceed a lead-dependent threshold nj ti j X Y Z . The threshold nj t is defined as the running standard deviation SD of the N most recent angle .
