## 3 Results
### Relationship between PRV and Haemodynamic Changes  
Factorial analyses were performed to understand the relationship between each of the extracted indices and SR and TF changes. Figures 7 and 8 summarise the effect estimates and significance for each of the factors and their combination when PRV indices were extracted from the radial and DPA phantoms, respectively. All interactions showed statistically significant differences, meaning that changing SR and TF had a statistically significant effect on all PRV indices. When measured from the radial phantom, some of the indices were clearly more affected by SR, such as AVNN, NN50 and SD2, in which the estimate for the effect of SR was clearly larger than the estimate for the effect of TF and the interaction. Similarly, some indices had larger estimates for TF, such as SDNN, RMSSD, COM, absolute frequency domain indices, and \(y\)-coordinate centroid-related indices. 1 is the only index measured from this location that had a larger effect estimate for the interaction between the two factors. In the case of indices extracted from the DPA phantom, this behaviour changes. SD2, AVNN and NN50 continued to have a remarkable larger effect estimate with changes in SR, while the interactions became stronger in this case, with most indices showing larger effects for these than for the independent factors. Figure 6: Example of photoplethysmographic (PPG) signals acquired from the constructed rig under different stroke rates and target flows. Figures 9-12 show the post hoc analyses for each of the independent factors. Target flow changes induced statistically significant differences, especially on the time domain, Poincare plot, DFA and entropy-related indices, both for radial and DPA measured PRV, while relative and centroid-related frequency-domain indices were more affected by TF changes when extracted from signals obtained from the radial phantom. The effects of changing SR showed a very similar behaviour regardless of the point of measurement, with  
Figure 8: Effect estimates for stroke rate (SR), target flow (TF) and their interaction on each of the indices extracted from pulse rate variability (PRV) measured from the deep palmar arch phantom. Red stars on top of the bars indicate statistically significant effects. Figure 7: Effect estimates for stroke rate (SR), target flow (TF) and their interaction on each of the indices extracted from pulse rate variability (PRV) measured from the radial phantom. Red stars on top of the bars indicate statistically significant effects. significant differences observed among most levels, especially again on time-domain and non-linear indices. Figure 10: Post-hoc comparisons for target flow (TF) levels when pulse rate variability indices are measured from the deep palmar arch phantom. The appearance of a circle indicates a significant difference between the corresponding levels for each of the indices. Figure 9: Post-hoc comparisons for target flow (TF) levels when pulse rate variability indices are measured from the radial phantom. The appearance of a circle indicates a significant difference between the corresponding levels for each of the indices. Figures 13 and 14 show the results for the post-hoc analyses of the interactions. Both for radial and DPA PRV indices the results followed the same trend. Time-domain, centroid-related frequency-domain and non-linear indices were largely affected by the changes in target flow and stroke rate, while absolute and relative frequency-domain indices showed less significant differences among possible combinations. There were more significant differences as SR and TF increased. Figure 11: Post-hoc comparisons for stroke rate (SR) levels when pulse rate variability indices are measured from the radial phantom. The appearance of a circle indicates a significant difference between the corresponding levels for each of the indices. Figure 12: Post-hoc comparisons for stroke rate (SR) levels when pulse rate variability indices are measured from the deep palmar arch phantom.