###### Abstract
To overcome the common drawback of both methods, in particular their limited performance under non-stationary conditions such as stress tests, modified methods based on first differences analysis have been suggested [6]. Combined with the advances of non-invasive blood pressure measurement techniques, these methods made BRS one of the routinely measured parameters both in clinical and ambulatory settings [7]. BRS has been reported as a highly informative prognostic marker widely applicable in both ambulatory and clinical investigations. In particular, BRS appeared highly predictive of cardiac mortality in post-infarction patients with both reduced ejection fraction [8] as well as preserved left ventricular function [9] including those receiving \(\beta\)-blocker treatment [10] as well as in patients with life-threatening arrhythmias [11], see also [12]. BRS impairment has been also reported as an early indicator of autonomic dysfunction and autonomic failure [13]. In earlier studies, BRS has been shown to exhibit significant changes under exercise as well as postural and other physical stresses [14; 15; 16; 17], while more recent data indicate that these effects are temporary and after a certain adaptation period the baroreflex exhibits a resetting around new absolute blood pressure and pulse interval values [18; 19; 20]. However, successful baroreflex resetting has notable exceptions with one such observed recently in diabetes patients with certain complications, particularly with obesity who exhibited significant baroreflex impairment [21]. While the BRS quantifies explicitly the response of the heart rate to the changes in blood pressure, it does not contain any information whether there was such a response to every significant variation of blood pressure. Time-domain methods simply disregard time intervals without significant changes of heart rate irrelevant to the fact whether there were blood pressure variations, while spectral methods provide characteristics that are averaged over a given frequency band within the entire analysis window. In turn, another alternative method [22] measures the average heart rate acceleration or deceleration, while not considering whether it occurred in response to significant blood pressure variations or not. Thus, neither of them can guarantee that all blood pressure variations were adequately responded. However, a timely response to the changes in blood pressure is essential for homeostasis, since missing or delayed responses lead to the increased blood pressure variability. In contrast, there is recent evidence that baroreflex activation therapy using implantable devices that stimulates carotid baroreceptors significantly improved the blood pressure control efficacy [23]. Accordingly, additionally to the measurement of the BRS itself, it is also important to quantify the activation of the feedback mechanism in response to blood pressure variations. In this paper, we suggest a series of complementary indicators of the blood pressure--heart rate feedback regulation based on their mutual synchronization patterns. Investigation of the physiological signals' mutual synchronization is widely used in chronobiological studies appearing essential for a deeper understanding of the mechanisms related to the influence of various external factors such as geomagnetic field variations, solar cycles, jetlag or night shifts [24; 25; 26]. Recent examples include the synchronization analysis of heart rate and respiration during different sleep phases [27; 28; 29]. While in most cases relation between rhythms with certain quasi-periodic structure is studied, such as breathing cycles modulating heartbeat cycles, we go beyond that and, while following conceptually similar methodology, modify the synchronization analysis methodology to suite the blood pressure--pulse interval analysis that both exhibit rather stochastic behavior, as indicated below.