## 3 Results  
### Feature Selection  
During SCA, the mean resting heart rate was observed to continuously fluctuate from 3 h prior to the onset of SCA to 1 min before the onset of SCA. However, a sudden decrease in heart rate between 1 min prior to SCA and during the onset of SCA was observed (See Figure 4). Compared to the normal patients, SCA patients exhibited a higher mean resting heart rate, with an 18.7 \(\pm\) 3.5 bpm difference observed between the two cohorts (See Table A1, Appendix A). Overall there was no statistically significant difference in mean resting heart rate between normal and SCA patients with the exemption of 3 h prior to SCA, F(1,18) = 4.70, \(p\) = 0.04 and 6 min prior to SCA, F(1,18) = 4.23, \(p\) = 0.05 (See Table A2, Appendix B). Post-hoc analyses using Tukey Honestly Significant Difference (Tukey HSD) indicated that there was a significant difference in mean resting heart rate observed between both patient cohorts at 3 h prior to SCA (\(p\) = 0.04), but however found no significant difference in mean resting heart rate observed between the two patient cohorts at 6 min prior to SCA onset (\(p\) = 0.06) (See Table A4, Appendix B). Similarly, there was no statistically significant difference in heart rate between each time-frame and the onset of SCA (See Table A3, Appendix B). Similarly, the heart rate of patients in the SCA cohort exhibited a higher mean R-R interval duration compared to the normal patients (See Table A1, Appendix A). The mean R-R duration was observed to fluctuate in a similar fashion to heart rate prior to the onset of SCA before exhibiting a sudden increase between 1 min prior to SCA and during the onset of SCA (See Figure 5). There was a statistically significant difference in the mean R-R interval duration at 3 h, F(1,18) = 4.57, \(p\) = 0.05, 2 h F(1,18) = 2.72, \(p\) = 0.03, 1 h, F(1,18) = 5.34, \(p\) = 0.03, 50 min, F(1,18) = 6.88, \(p\) = 0.02, 40 min, F(1,18) = 8.50, \(p\) = 0.01, 30 min, F(1,18) = 7.09, \(p\) = 0.02, 20 min, F(1,18) = 5.41, \(p\) = 0.03, 10 min, F(1,18) = 5.89, \(p\) = 0.03 and 9 min, F(1,18) = 5.79, \(p\) = 0.03 prior to SCA onset (See Table A2, Appendix B). Post-hoc analyses using [PERSON]'s HSD indicated that mean R-R interval duration was significantly higher in the SCA patient cohort compared to the normal patient cohort (See Table A4, Appendix B). Figure 4: Time series plot of the change in heart rate prior to the onset of sudden cardiac arrest (\(n\) = 10). ECG recordings were taken from patients in the SCA cohort and subjected to processing via MATLAB. The results were recorded as mean heart rate (\(\pm\)standard deviation) denoted by the error bars. The asterisk denotes the statistical significance by repeated measures ANOVA where * \(p\) < 0.05. Figure 5: Time series plot of the change in R-R Interval duration prior to the onset of sudden cardiac arrest (n = 10). ECG recordings were taken from patients in the SCA cohort and subjected to processing via MATLAB. The results were recorded as mean R-R interval duration (\(\pm\)standard deviation) denoted by the error bars. Asterisks denote statistical significance by repeated measures ANOVA where * \(p\) < 0.05 and * \(p\) < 0.01. For the HRV features, both the RMSSD and SDSD obtained from the SCA cohort were larger compared to the normal cohort (See Table 1, Appendix A). There was gradual increase in RMSSD observed which peaked at 50 min prior to SCA before fluctuating in a similar fashion to heart rate and R-R interval duration. Similarly, SDSD was observed to fluctuate prior to the onset of SCA, peaking at 3 min prior to SCA. Both RMSSD and SDSD, then exhibited a sudden increase between 1-min prior and during the onset of SCA (See Figure 6). There was a statistically significant difference in RMSSD observed at 3 min prior to SCA between normal and SCA patients, F(1,18) = 4.28, p = 0.05 (See Table 2, Appendix B).