Baroreflex function is usually assessed from spontaneous oscillations of blood pressure (BP) and cardiac RR interval assuming a unidirectional influence from BP to RR. However, the interaction of BP and RR is bidirectional-RR also influences BP. Novel methods based on the concept of Granger causality were recently developed for separate analysis of feedback (baroreflex) and feedforward (mechanical) interactions between RR and BP. We aimed at assessing the proportion of the two causal directions of the interactions between RR and systolic BP (SBP) oscillations during various conditions, and at comparing causality measures from SBP to RR with baroreflex gain indexes. Arterial BP and ECG signals were noninvasively recorded in 16 young healthy volunteers during supine rest, mental arithmetics, and head-up tilt test, as well as during the combined administration of these stressors. The causal interactions between beat-to-beat RR and SBP signals were analyzed in time, frequency, and information domains. The baroreflex gain was assessed in the frequency domain using non-causal and causal measures of the transfer function from SBP to RR. We found a consistent increase in the baroreflex coupling strength from SBP to RR during head-up tilt, an insensitivity of the coupling strength along the non-baroreflex direction to both stressors, and no significant effect of mental arithmetics on the feedback coupling strength. It indicates that the proportion of causal interactions between SBP and RR significantly varies during different conditions. The increase in the coupling from SBP to RR with tilt was not accompanied by concomitant variations of the transfer function gain, suggesting that causality and gain analyses are complementary and assess different aspects of the baroreflex regulation of heart rate.
Keywords: Baroreflex; Granger causality; Head-up tilt; Information domain; Mental arithmetics.