## 4 Discussion
### Behaviour of PRV Indices with Haemodynamic Changes  
The behaviour of PRV indices extracted from PPG signals acquired from both phantoms is worth analysing. In general, it can be seen that indices extracted from the DPA phantom tended to have larger variabilities, probably due to the lower quality of the signal obtained from this location, which made it harder to accurately determine cardiac cycles.  
Specifically from time-domain indices, it can be observed that the measurement of AVNN is generally stable except for the cases in which the amplitude of the signal is too small, such as when SR is 150 and 180 bpm and TF is lower than 2 L/min. SDNN and RMSSD had very similar behaviour, with relatively lower values as TF increases. A similar although not as clear behaviour can be observed from NN50 and pNN50.  
In absolute-power frequency-domain indices, the results obtained when SR was 150 and 180 bpm had an important outlier when TF was 1 L/min. Again, this is likely due to the lower quality of the signals at these stroke volumes and care should be taken when analysing the obtained results at these levels. LF, HF, and TP showed similar behaviour,with increased values as TF increased to 3 L/min and then decreased values as TF increased to 5 L/min. Interestingly, it can be observed that there is an important difference in magnitude between VLF, LF and HF, with larger magnitudes in the HF band. This is in line with the results reported by [PERSON] et al. [8], who concluded that this frequency band was more affected by cardiovascular factors such as PTT, even when HRV was dramatically reduced. This can also be observed from nLF and nHF, with most nHF values close to 0.9. However, the trend of increasing and decreasing values as TF increases is not visible in these two indices. Also, as SR increased, LF/HF and nLF tended to decrease with increases in TF, while nHF showed the opposite behaviour. SpEn did not show a clear trend with changes in TF but did show a continuous increase as SR surged.  
Centroid-related indices showed similar results when \(y\)-coordinates were analysed. Again, there is an important outlier as stroke rate increased and target flow was kept low, but the inverted parabolic behaviour observed when SR is lower is also present in these indices. However, \(x\)-coordinates were less affected by changes, meaning that the power of the band tends to be around the same frequencies regardless of stroke volume changes, while the amplitude of the band does vary with haemodynamic changes.  
For Poincare plot indices, and as has been explained in the literature [23], SD1 is expected to behave as RMSSD. This can be seen in the results obtained from Poincare plot indices, where the behaviour was exactly the same. S, SD1/SD2 and COM also have the same trend with varying magnitudes. And SD2 has a similar behaviour as AVNN, which is also expected [23].  
Finally, from the remaining non-linear indices, it can be seen that magnitudes tended to remain relatively stable regardless of changes in stroke rate. A trend is hard to determine from DFA-related indices, while from entropy-related indices, the most visible trend can be observed from SSE, where it increased with rising TF. BSE, ApEn, SampEn and MSE did not show such clear patterns of behaviour.