# A Heart Rate Variability-Based Paroxysmal Atrial Fibrillation Prediction System
## 4 Discussion
### Medical Implications from the Features  
Recall that we generated four feature matrices for the input of CNN by discretizing four extended Poincare plots including two inter-person features and two intra-person features. We computed the average counts of each element in the four matrices for PAF patients and for non-PAF subjects. Then, the four averaged matrices of non-PAF subjects were subtracted accordingly from those of PAF patients, as shown in Figure 5. Consequently, the elements with larger differences (dark-red color in the image) are highly associated with the PAF patients and can be regarded as risk factors for AF. According to the inter-person feature matrices, PAF patients seem to have either a lower heart rate of around 60 beats/min or higher heart rate of around 100 beats/min, and they tend to exhibit more off-diagonal pixels (yellow pixels along the off-diagonal lines), indicating interchanges of fast-slow or slow-fast heart rates in the neighbor beats. In addition, from the intra-person feature matrices, PAF patients often have smaller heart rate variations \(\leq\)5 beats/min in the lag-4 and lag-5 sequences, but they may occasionally exhibit large heart rate changes of \(\geq\)20 beats/min (brighter pixels along the vertical and horizontal axes). Such phenomena coincided with our understanding since PAF patients tend to exhibit more irregular beat-to-beat intervals than non-PAF patients, as shown in Figure 6. For example, premature atrial beats are frequently observed in PAF patients [8], resulting in a short R-R interval followed by a prolonged R-R interval, which leads to a fast-slow heart rate change in an HRV sequence.  
On the other hand, the pixels in dark-blue colors in Figure 5 are predominant in non-PAF subjects and they can be considered as protective factors from AF. From the inter-person feature matrices, non-PAF subjects tend to have a heart rate range of around 82-94 beats/min. Moreover, from the intra-person features, non-PAF subjects often exhibit medium heart rate changes of around 10 beats/min. However, such changes tend to increase from the average heart rate, rather than decrease. Other phenomena in Figure 5 also drew our attention. Nevertheless, their medical implications warrant further investigations.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Literature** & **Databases** & **Data Length** & **Features** & **Cross-** & & **Results (\%)** & \\  & & **(min)** & **Features** & **Validation** & **SEN** & **SPE** & **ACC** \\ \hline [PERSON] and [PERSON] [44] & AFPDB & 5 & HRV power spectral density and PACs & 5-fold & 51.0 & 79.0 & 68.0 \\ [PERSON] and [PERSON] [45] & AFPDB & 5 & P-wave power spectral density & 5-fold & 81.0 & 69.0 & 75.6 \\  & & & Combination of 9 HRV & & & & \\ [PERSON] et al. [9] & AFPDB & 5 & features in time and frequency domains & 10-fold & 86.8 & 88.7 & 87.7 \\  & & & Combination of 26 HRV & & & & \\ [PERSON] et al. [10] & AFPDB & 5 & features in time and frequency domains & 10-fold & 92.0 & 88.0 & 90.0 \\  & & & frequency domains & & & & \\ [PERSON] et al. (This study) & AFPDB * & \(\sim\)5 & 32 \(\times\) 32 \(\times\) 4 images from extended and discretized Poincaré plot & Single-fold & 80.4 & 89.0 & 87.2 \\ \hline  & & & & & & \\ \end{tabular}  
* AFPDB, NSRDB, and AFDB are abbreviations for the PAF Prediction Challenge Database, MIT-BIH Normal Sinus Rhythm Database, and MIT-BIH Atrial Fibrillation Database, respectively.  
\end{table}
Table 2: Similar studies in the literature.