## 4 Discussion  
In the present study, we observed a significant increase in the average PAT values on dual-paced versus single-paced breathing patterns at each reference point except T0 LM; however, the tendency was the same. On the contrary, the PI and GI significantly increased on 1:1 dual-paced breathing when compared to single-paced and 1:2 dual-paced respiration. The increased symmetry of breathing results in higher heart rate symmetry based on the HRA parameters. In considering 50.0 as perfect symmetry by PI and GI, the significantly increased parameters on 1:1 breathing versus 1:0 and 1:2 indicate the higher-level of symmetry (PI = 48.98 versus 46.91 and 46.50, respectively; GI = 49.42 versus 46.86 and 46.74, respectively). In support of our previous observations [15], the HRA changes due to respiration patterns reflect a proportional relation between breathing and heart rate control. At a 1:2 breathing ratio, there is relatively less time to inhale compared to 1:1 breathing. Thus, due to the \"shorter\" inhalation time, relatively fewer beats belong in the acceleration cloud. To preserve the mean heart rate in steady state conditions, the acceleration requires larger steps in \"fewer RR intervals\" in a 1:2 breathing. The short-term HRA due to breathing-related oscillations of RRIs is mediated by the parasympathetictic tone rather than by the sympathetic activity, since the parasympathetic effects on the heart occur almost immediately (0.5 sec), while there is a delay of approximately 5 s prior to the sympathetic effects becoming noticeable [38]. Previous studies have also demonstrated how changes in posture affect the HRA parameters: [PERSON] et al. [39] investigated the effect of the head-up tilt (75\({}^{\circ}\), HUTT) through the changing in HRA (PI, GI) and HRV parameters among healthy individuals. Their results showed that the GI significantly increased during the HUTT, indicating the sensitivity of HRA to orthostatic stress. This phenomenon was explained by the shift in the sympathovagal-balance from the supine position-related vagal saturation by vagal withdrawal, assuring a greater play for the RRI variability associated to respiratory changes. These results are in accordance with the previous study, in which [PERSON] et al. observed the asymmetry in the RRIs is proportionally more prominent during increased head-up tilt (75\({}^{\circ}\) and 90\({}^{\circ}\)) when compared to the supine position among healthy adults [17]. The close relation between the breathing and beat-to-beat PP interval variation has been observed by spectral evaluation of the PPG signal [7]. In the present research, FFT analysis was performed on a volunteer's 5 min-long ECG recordings at each breathing pattern and the corresponding beat-to-beat PAT values at T0 EP, in which the most prominent respiratory oscillation can be detected [5] (Figure 6.). The peak at 0.22 Hz, associated with the 4500 ms respiratory cycle of paced breathing, was noticeable both at RRI and at PATs as well. There is a second harmonic at 0.44 Hz, due to the asymmetry of 1:2 i/e ratio, in contrast to 1:1 breathing, in which the second harmonic nearly disappeared. In addition to the direct connections between the respiratory and circulatory centers, short-term intrathoracic pressure changes affect the heart rate and blood pressure due to respiration through hemodynamic actions on preload and subsequently stroke volume, thus influencing the PEP and PTT, and consequently PAT [40]. [PERSON] et al. [41] investigated the relationship between PAT and BP during physiologically induced BP changes when the pathway contains resistance vessels (PPG) or merely the conducting arteries (radial artery tonometry) to investigate the BP-independent neurological effect on the peripheral vascular resistance. They found no BP-independent autonomic influence on PTT and PAT during rest, cold pressor test, cycling and isometric handgrip exercises.