## 4 Discussion
As a result, there is a full compensatory pause before the next successive heartbeat following a PVC which, in turn, may contribute to the late decrease in heart rate [27]. It has been widely reported across various large cohort studies, such as the Framingham heart study, that a higher mean resting heart rate is associated with an increase in cardiovascular mortality. Studies have exhibited that an increase in SCA risk was observed in heart rates exceeding 75 bpm [14; 22; 27]. However, the mechanisms underlying the relationship between elevated resting heart rate and SCA risk are still poorly understood and remain to be elucidated. **Mean R-R Interval Duration. ** In a similar fashion to resting heart rate, the mean R-R interval duration was longer in SCA patients compared to normal patients. This observation aligns with early studies conducted on ambulatory ECGs of SCA patients, where long R-R interval cycles ranging between 640-1110 ms were observed in approximately 90% of patients prior to SCA [14; 28]. These studies have suggested that this prolongation in the R-R interval observed during SCA occurs as a result of PVCs. As stated in the preceding section, PVCs are followed by a full compensatory pause as the depolarizations from the sinoatrial node fail to reach the ventricles [14]. This, consequently, results in disturbances in the cardiac cycle, characterized by the shortening followed by the lengthening of the R-R cycle intervals [29]. These early studies have also exhibited that the frequency of PVCs increase significantly from 1-h prior to SCA until the onset of SCA, which may explain the statistically significant differences in the mean R-R interval duration observed between a single time-frame prior to SCA and SCA onset observed in 50% of the SCA cohort [14; 28]. It is also important to consider that external factors, such as the patient's underlying etiology and medication, may also contribute to R-R interval prolongation. **Heart Rate Variability (RMSSD and SDSD). ** The final two features investigated in this study were two time-domain measures of heart rate variability (HRV); the square root of the mean of differences between adjacent R-R intervals (RMSSD) and the standard deviation of the differences between adjacent R-R intervals (SDSD). Studies have shown that beat-to-beat changes in the R-R interval may accurately reflect any variability in sinoatrial node activity and features that utilize the difference between adjacent R-R intervals are ideal for long term measurements of HRV [30]. Both the RMSSD and the SDSD of the SCA cohort obtained in this study were higher compared to the normal cohort, although not to a statistically significant extent. Since these features utilize the R-R interval, the difference observed between the SCA and normal patient cohort may be attributed to the mechanisms related to the increase in PVC frequency, as previously discussed in the preceding section. However, the values obtained in this study do not correlate with literature findings which found lower HRV indices in SCA patients [19]. Other literature studies have associated low HRV values as a marker of cardiac dysfunction, while high HRV on the other hand is associated with efficient cardiac autonomic mechanisms [31; 32]. Thus, a high HRV index while the patient is at rest is deemed favorable as it indicates that the body is either able to tolerate stress or is strongly recovering from prior accumulated stress [32]. However, it is important to note that a majority of patients in the SCA cohort possess underlying cardiovascular conditions and, as the patients in this study are older than the normal cohort, their hearts have been exposed to stress for a period of time. As a result, their heart may have already acclimatized to these stressful conditions, thus resulting in the high HRV values. **Statistical Classification and Machine Learning. ** The data was then subjected to statistical classification with the use of various classifiers.