## 3 Results  
In order to evaluate the CNN-BiLSTM model performance on the MIMIC II dataset, this section gives the experimental results of the proposed model not including subject information. The completely subject-independent model gives experimental results that do not include subject information. Figure 3 shows the model input PPG signal. Here, the PPG signal of a 3 s segment input to the model was given. Figure 4 shows a comparison between the reference ECG and reconstructed ECG signals. In Figure 4, the blue line represents the actual ECG signal (reference ECG), and the red line represents the reconstructed ECG signal. Figure 4a shows the ECG signal reconstructed by the proposed model and the reference ECG signal. Figure 4b shows the reconstructed ECG signal and the reference ECG signal after using cross-correlation alignment for Figure 4a. When cross-correlation alignment is not used for the reference ECG signal and the reconstructed ECG signal, the \(r\), RMSE, and PRD between them are 0.996, 0.027, and 1.745, respectively. After using cross-correlation alignment, the \(r\) value between the reference ECG signal and the reconstructed ECG signal becomes 0.995, and the other values remain unchanged. As can be seen from Figure 4, the ECG reconstructed by the proposed model is very similar to the reference ECG. However, using cross-correlation alignment for the reconstructed ECG signal and the reference ECG signal has almost no change in the model performance. Figure 3: Model input: PPG signal. In order to verify the performance of the proposed CNN and BiLSTM hybrid model, this study compares the experimental results of the CNN model with the CNN-BiLSTM model. Figure 5 shows the changes in \(r\), RMSE and PRD values between the reconstructed ECG and reference ECG signals by the CNN model and the CNN-BiLSTM model, and the changes in \(r\), RMSE and PRD values after the cross-correlation alignment of the reconstructed ECG signals and the reference ECG signals by the CNN model and the CNN-BiLSTM model are given. This study compares these four different experiments. In this study, the test set contains 120 ECG signals of 8 min. Figure 5 shows the performance evaluation changes of 120 records in four experiments. In Figure 5, the blue solid line shows the performance evaluation changes of the CNN model; the red solid line shows the performance evaluation changes of the proposed CNN and BiLSTM hybrid model; the purple dotted line shows the performance evaluation changes of the reconstructed ECG signal of the CNN model and the reference ECG after the cross-correlation alignment; the green dotted line shows the performance evaluation changes of the reconstructed ECG signal of the proposed CNN and BiLSTM hybrid model and the reference ECG after the cross-correlation alignment. Figure 5 shows the change in \(r\) value between 120 ECG signals in the test set and the ECG signals reconstructed by the model. From Figure 5, we can find that the \(r\) value of the CNN and BiLSTM hybrid model proposed in some records is higher than that of the CNN model, the \(r\) value of the CNN and BiLSTM hybrid model proposed in some records is lower than that of the CNN model, and the \(r\) value of the CNN and BiLSTM hybrid model proposed in some records is the same as that of the CNN model. The comprehensive comparison shows that the \(r\) value of the proposed CNN and BiLSTM hybrid model is  
Figure 4: ECG signal reconstructed from PPG signal experimental results. The abbreviations \(r\), RMSE, and PRD are the Pearson’s correlation coefficient, root mean square error, and percentage root mean square difference, respectively. (**a**) Comparison of the reconstructed ECG signal and reference ECG signal. (**b**) Comparison of the reconstructed ECG signal and reference ECG signal with cross-correlation alignment. higher than that of the CNN model. However, the values using cross-correlation alignment do not vary much. Figure 5b shows the change in RMSE value between 120 ECG signals in the test set and the ECG signals reconstructed by the model.