## 3 Results
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref** & **Dom** & **Feature** & \multicolumn{2}{c}{**Normalization**} \\ \cline{4-5}  & & & **HOW** & **WHY** \\ \hline
[19] & T & & QT interval & \(\frac{\text{QT}}{\text{mean}\left(\text{QT}\right)}\) &
\begin{tabular}{c} To overcome bias engendered by the longer measured QT intervals of paced complexes \\ \hline
[20-29] & F & \(\bullet\) & Low-frequency (LF) HRV & &
\begin{tabular}{c} To minimize the effect of changes in total power on LF and HF, \\ \end{tabular} \\ \cline{4-5}  & \(\bullet\) & High-frequency (HF) HRV & \(\frac{\text{LF}}{\text{TF}-\text{VLF}}\) \(\cdot\) 100 & emphasizing the cardiac modulation \\ \hline
[30] & T & & QT interval & \
\begin{tabular}{c} number (ECG with unstable QT) \\ number (ECG with PA) \\ \end{tabular} &
\begin{tabular}{c} To quantify the interdependence of the instability in QT dynamics and the occurrence of PA \\ \end{tabular} \\ \hline
[31] & T & \(\bullet\) & U-wave integral (Uwi) & \
\begin{tabular}{c} Uwi \\ mean (Uwi) \\ \end{tabular} \(\cdot\)
\begin{tabular}{c} To assess the effectiveness of parameters in discriminating myocardial infarction patients \\ \end{tabular} \\ \hline
[32] & T & & Voltage spread (VS) & \
\begin{tabular}{c} VS at ST interval \\ VS at IP interval \\ VS at IP interval \\ VS at IP interval \\ VS at IP interval \\ \end{tabular} &
\begin{tabular}{c} To account for the electrical variance \\ among patients \\ \end{tabular} \\ \hline
[33,34] & F & \(\bullet\) & Repolarization signal (RPS) & \\ \cline{4-5}  & F & power & RPS power spectrum & To account for the signal energy \\ \cline{4-5}  & \(\bullet\) & RPS energy & & amplitude \\ \hline
[35-42] & F & \(\bullet\) & Low-frequency (LF) HRV &
\begin{tabular}{c} To minimize the effect of changes in total power on LF and HF, \\ emphasizing the cardiac modulation \\ of the autonomic nervous system \\ \end{tabular} \\ \hline
[43-45] & T & & QT variability index &
\begin{tabular}{c} variance (QT)/mean (QT)\({}^{2}\) \\ variance (HR)/mean (HR)\({}^{2}\) \\ \end{tabular} & To compensate for the effect of HRV \\ \hline
[46] & T & \(\bullet\) & Blood pressure fall time (BPFt) & \begin{tabular}{c} BPF time (BPRt) \\ \end{tabular} & \begin{tabular}{c} BPRt \\ \end{tabular} & \
\begin{tabular}{c} PIT \\ \end{tabular} &
\begin{tabular}{c} To compensate for the effect of HRV \\ \end{tabular} \\ \cline{4-5}  & T & \(\bullet\) & Pulse transit time (PTT) & \\ \hline
[47] & T & & R-wave amplitude (Ramp) & \
\begin{tabular}{c} Ramp \\ electrode space difference \\ \end{tabular} &
\begin{tabular}{c} To consider correlation between Ramp \\ and electrode distance \\ \end{tabular} \\ \hline
[48] & T & \(\bullet\) & Pre-RR interval (PrRR) & \begin{tabular}{c} PrRR \\ \end{tabular} & \begin{tabular}{c} PrRR \\ \end{tabular} & \begin{tabular}{c} PrRR \\ \end{tabular} & \begin{tabular}{c} PrRR \\ \end{tabular} & \begin{tabular}{c} PrRR \\ \end{tabular} & \begin{tabular}{c} PrRR \\ \end{tabular} \\ \cline{4-5}  & T & \(\bullet\) & Post-RR interval (PoRR) & \begin{tabular}{c} mean(RR) \\ \end{tabular} & \begin{tabular}{c} To see if normalized features can \\ improve the accuracy of classification \\ \end{tabular} \\ \cline{4-5}  & T & \(\bullet\) & 1-min RR interval (1 mRR) & \begin{tabular}{c} 1 mRR \\ \end{tabular} & \
\begin{tabular}{c} 20 mRR \\ mean(RR) \\ \end{tabular} &
\begin{tabular}{c} To take into account possible changes \\ of T-wave morphology \\ \end{tabular} \\ \hline
[49] & T & & TWA & \
\begin{tabular}{c} TWA \\ \end{tabular} &
\begin{tabular}{c} To take into account possible changes \\ of T-wave morphology \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Overview of how and why the included documents apply normalization.