## 3 Results
Table 2 outlines the targets of the studies described in the included documents, while Table 3 reports an overview of which ECG-derived parameters used as cardiac risk indices were normalized in the included documents, of how normalization was performed, and of the reason why normalization was applied. Among the included documents, 21 documents analyzed the ECG spectrum, with 19 (49% of all included documents) presenting normalized HRV indices derived from the frequency analysis of the tachogram. The remaining 18 documents analyzed the ECG in the time domain. Of these, 7 documents presented normalized measures for the analysis of QT interval variability, while 2 documents applied normalization to TWA measures. \begin{table}
\begin{tabular}{c c} \hline \hline
**Ref** & **Target of the Study** \\ \hline
[19] & Evaluate the influence of diurnal changes on cardiac repolarization \\ \hline
[20] & Evaluate if exaggerated sympathetic activity characterizes active ulcerative colitis \\ \hline
[21] & Evaluate the effect of gutkha chewing on cardiac autonomic modulation \\ \hline
[22] & Evaluate cardiovascular autonomic function in insomnia patients \\ \hline
[23] & Evaluate the effect of age and physical activity on cardiac autonomic control \\ \hline
[24] & Evaluate the role of autonomic nervous system in a blunted fall in nocturnal blood pressure \\ \hline
[25] & Evaluate the role of autonomic nervous system in the pathogenesis of blood pressure abnormalities \\ \hline
[26] & Evaluate the association between T-wave morphology and cardiac autonomic modulation \\ \hline
[27] & Evaluate the effect of a 24 h medical on-call duty on physiological functions \\ \hline
[28] & Evaluate the cardiac autonomic modulation during slow-wave sleep in sleepwalkers \\ \hline
[29] & Evaluate objective psychiatric indicators in geriatric patients with emotional distress \\ \hline
[30] & Propose an index for cardiac risk stratification \\ \hline
[31] & Evaluate the U-wave morphology and its relation to the T wave \\ \hline
[32] & Propose a method to enhance and improve acquisition, analysis, and display of ECG \\ \hline
[33] & Propose a new index (f99) to characterize repolarization \\ \hline
[34] & Evaluate the predictive power of f99 index for the occurrence of ventricular arrhythmias \\ \hline
[35] & Evaluate the beneficial effect of moderate and intensive physical exercise \\ \hline
[36] & Evaluate the role of autonomic nervous system in hypertrophic cardiomyopathy \\ \hline
[37] & Evaluate early changes in autonomic nervous system tone in newborns with complex congenital heart disease \\ \hline
[38] & Evaluate the effect of betamethasone on autonomic modulation of fetal heart rate \\ \hline
[39] & Evaluate the role of autonomic regulatory mechanisms in transient myocardial ischemia \\ \hline
[40] & Evaluate the effect of betamethasone on autonomic modulation of fetal heart rate \\ \hline
[41] & Evaluate autonomic dysfunctions in patients with idiopathic sleep behavior disorders \\ \hline
[42] & Evaluate autonomic residual effects of COVID-19 infection \\ \hline
[43] & Evaluate the relation between repolarization lability and ventricular arrhythmias \\ \hline
[44] & Evaluate the effect of sympathetic stimulation on the autonomic control of the heart \\ \hline
[45] & Evaluate the relationship between indices of cardiac repolarization \\ \hline
[46] & Evaluate if patent ductus arteriosus status can be assessed from standard neonatal intensive care monitoring \\ \hline
[47] & Evaluate R-wave sensing for implantable cardiac monitor \\ \hline
[48] & Propose an automatic heartbeat classification system \\ \hline
[49] & Evaluate if head-down (\(-6^{\circ}\)) bed rest increases TWA \\ \hline
[50] & Evaluate the role of QT parameters as predictors of sudden cardiac death \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Targets of the studies conducted in the included documents.