###### Abstract  
Six different metrics of mutual coupling of simultaneously registered signals representing blood pressure and pulse interval dynamics have been considered. Stress test responses represented by the reaction of the recorded signals to the external input by tilting the body into the upright position have been studied. Additionally, to the conventional metrics like the joint signal coherence Coher and the sensitivity of the pulse intervals response to the blood pressure changes baroreflex sensitivity (BRS), also alternative indicators like the synchronization coefficient Sync and the time delay stability estimate TDS representing the temporal fractions of the analyzed signal records exhibiting rather synchronous dynamics have been determined. In contrast to BRS, that characterizes the intensity of the pulse intervals response to the blood pressure changes during observed feedback responses, both Sync and TDS likely indicate how often such responses are being activated in the first place. The results indicate that in most cases BRS is typically reciprocal to both Sync and TDS suggesting that low intensity of the feedback responses characterized by low BRS is rather compensated by their more frequent activation indicated by higher Sync and TDS. The proposed additional indicators could be complementary for the differential diagnostics of blood pressure regulation efficacy and also lead to a deeper insight into the involved concomitant factors this way also aiming at the improvement of the mathematical models representing the underlying feedback control mechanisms. feedback regulation; digital signal analysis; control efficacy 202010 10 209990  
in the body position, reactions to drugs, changes in the subject's mental conditions and so on. The increase of blood pressure is sensed by baroreceptors located in blood vessels that in turn invoke a response by the autonomous nervous system. The response is generally twofold, including the decrease of heart rate and the reduction of the vascular resistance, both leading to the consequent drop of blood pressure. Thus the efficacy of this feedback mechanism largely determines timely and adequate responses to the changes in blood pressure. Over a long time quantitative assessment of the short-term blood pressure--heart rate feedback regulation efficacy has been limited to the analysis of the baroreflex sensitivity (BRS) defined as the measure of the relative change of the pulse interval (in ms) in response to the change in the systolic blood pressure (in mmHg), altogether measured in (ms/mmHg). Historically, BRS was first measured as the increase in the pulse interval to the pharmacologically induced increase in blood pressure that guaranteed the feedback mechanism activation for a certain time fragment, with the BRS being quantified by the linear regression coefficient of pulse intervals on blood pressure over this time fragment. In the last three decades, several methods to measure BRS from simultaneously recorded spontaneous fluctuations of both pulse intervals and blood pressure without applying any external stimuli have been suggested. The most common time-domain approach, often termed as the sequence method, simply focuses on finding time fragments where both pulse intervals and blood pressure either increase or decrease consecutively and monotonously over several heartbeat cycles, sometimes referred to as baroreflex sequences. Next for each time fragment a linear regression coefficient of pulse intervals on blood pressure is calculated, representing the local BRS estimate [1; 2]. To improve the accuracy, averaging over several baroreflex sequences is usually performed, at the cost of lower temporal resolution. In contrast, spectral-based methods do not require selection of certain time fragments, instead focusing on the blood pressure--pulse intervals transfer function analysis in a certain frequency band where the coherence between them exceeds a certain threshold [3; 4; 5].