## 4 Discussion
### Relationship between PRV and Cardiac Output Changes  
The main aim of this study was to assess if PRV indices were affected by stroke rate and target flow changes, in the absence of HRV and autonomic regulation. From the factorial analyses, it was observed that all indices measured from both phantoms had statistically significant effects due to the interaction of factors and the factors individually.  
From the radial phantom, and similarly to what was observed from the behaviour of PRV indices, AVNN and SD2 had an important effect due to SR. MSE showed also larger estimates due to the effect of SR on its magnitude. In addition to these indices, NN50, S, SD1/SD2, A2 and BSE also showed larger estimates due to the effect of SR than TF or the interaction when measured from the DPA phantom. This might be due to the increased number of outliers and the lower quality of the signal. Also, in concordance with the qualitative observations of the behaviour of the indices, SDNN, RMSSD, pNN50, absolute-power and \(y\)-coordinates of centroid-related frequency domain indices, S, SD1, SD1/SD2 and COM showed that TF had the larger effect estimate when measured from the radial phantom. When obtained from the DPA phantom, and again most likely due to the quality of the signal, the greater effect estimates were found on the interaction of both factors.  
Regardless of the differences between phantoms and signal qualities obtained, it is remarkable that all indices showed a statistically significant effect due to these haemodynamic changes, meaning that even in the absence of HRV and autonomic regulation, changing stroke rate and target flow affect PRV indices. Given the greater susceptibility of DPA-related indices to noise, the remaining discussion is focused solely on data obtained from the radial phantom.  
Post hoc analyses were performed to evaluate the origin of these differences. Only focusing on target flow changes, most differences occurred when comparing lower flows to TF values greater than 2.5 L/min and for time-domain, centroid-related frequency-domain, and non-linear indices. For absolute and relative power indices, the majority of the differences were observed when comparing \(\text{TF}=1\) L/min to the rest of the conditions, but this behaviour might be related to the lower quality of the signal at this level and the presence of outliers on these indices (see Figures 15-16). Changes in stroke rate had a significant effect on most indices, especially when SR was increased to 180 bpm. The indices that showed the lower amount of significant comparisons among levels were VLF, LF and COM. Moreover, the interactions showed significant differences in most possible combinations of factors, showing that both SR and TF play an important role in PRV and its regulation even in the absence of HRV.  
These results corroborate those reported by [PERSON] et al. [8] and [PERSON] et al. [9], who concluded that PRV is modulated by other aspects different to HRV. Although from these results it is evident that in this in-vitro setup PRV was present and affected by stroke volume changes, further studies should be performed to better understand the origins of the changes in PRV and to try to replicate these results in an in-vivo model. Moreover, these results alongside the development of the in-vitro model could allow for a better understanding of PRV in a controlled manner and its relationship with cardiovascular diseases.