## 4 Heart Rate Variability (HRV) Study
### Results
#### 4.3.1 Time Domain Parameters  
Figure 8a shows the results of the evaluated parameters in the time domain, as a mean over all subjects. They are further listed in Table 1. SDNN, RMSSD, SDSD, pNN50 and meanRR showed an increase in the HRV during immersion compared to the positions at the poolside.  
Figure 6: Study procedure for each subject.  
Figure 7: Positions of the subjects during the study. (**a**) Sitting position outside water. (**b**) Supine position outside water. (**c**) Sitting position inside water. (**d**) Supine position inside water.  
In the sitting position during immersion, the SDNN significantly increased (\(p=0.012\)) compared to outside the water. In contrast, SDNN only showed a slight increase in the supine position.  
The RMSSD significantly increased (\(p=0.012\)) in the sitting position from outside the water (36.03 ms) to immersed in water (76.33 ms). In the supine position during immersion, this measure only slightly increased with no significant change.  
The SDSD measure was significantly lower (\(p=0.0022\)) sitting outside the water (36.08 ms) compared to sitting immersed in water (76.46 ms). In the supine position, it did not differ significantly.  
In the sitting position outside the water, the pNN50 showed a significant decrease (\(p=0.0022\)) compared to the immersed condition. Outside the water, the pNN50 showed a value of 6.43% and during immersion 18.24%. In the supine position, the two conditions showed no significant difference (immersed 18.24%, outside the water 16.43%, respectively).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Sitting** & **Sitting** & **Supine** & **Supine** \\  & **Outside Water** & **Inside Water** & **Outside Water** & **Inside Water** \\ \hline SDNN [ms] & 67.63 & 80.74 * & 71.65 & 73.80 \\ RMSSD [ms] & 36.03 & 76.33 * & 69.13 & 69.81 \\ SDSD [ms] & 36.08 & 76.46 ** & 69.24 & 69.92 \\ pNN50 [\%] & 6.43 & 18.24 ** & 16.43 & 18.24 \\ meanRR [ms] & 801 & 969 ** & 977 & 1017 \\ \hline \hline \end{tabular}
**RMSSD** root mean square of successive differences. **SDSD** = standard deviation of successive differences. **pNN50** = percentage of the number of adjacent RR intervals which differ more than 50 ms. **meanRR** = arithmetic mean value of all RR intervals. * \(p<0.05\) and ** \(p<0.01\) compared to outside water condition.  
\end{table}
Table 1: Time domain HRV parameters as a mean over all subjects.  
Figure 8: Time domain HRV parameters as a mean over all subjects. Standard deviation of NN intervals (SDNN), standard deviation of successive differences (SDSD) and the root mean square of successive differences (RMSSD) are visualized in (**a**), on the left side for the sitting and on the right side for the supine position. Bright colors are outside the water and dark colors from the immersed condition. In (**b**) the changes in the percentage of the number of adjacent RR intervals which differ more than 50 ms (pNN50) is given and in (**c**) the differences of the arithmetic mean value of all RR intervals.  
The meanRR in the sitting position was significantly higher (\(p=0.0022\)) in the immersed condition with 969 ms (61.9 ppm) compared to outside the water with 801 ms (74.9 ppm). In the supine position, the meanRR showed also only a slight difference in the two conditions. outside the water 977 ms (61.4 ppm), during water immersion 1017 ms (59.0 ppm).  
Overall, all time parameters showed a significant increase in the sitting position during water immersion compared to outside the water. In contrast, the values did not differ significantly in the supine position. While immersed, the measures showed only a slight increase.