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The interactions between systolic arterial pressure (SAP) and R-R interval (RR) fluctuations after acute myocardial infarction (AMI) were investigated by measures of synchronization separating the feedback from the feedforward control and capturing both linear and nonlinear contributions. The causal synchronization, evaluating the ability of RR to predict SAP ([X.sub.s/t]) or vice versa ([X.sub.t/s]), and the global synchronization (X) were estimated at rest and after head-up tilt in 35 post-AMI patients, 20 young and 12 old. Significance and nonlinearity of the coupling were assessed by surrogate data analysis. Tilting increased the number of young subjects in which RR-SAP link was significant (from 17 to 19) and linear (from 11 to 18). In AMI, both significance and linearity of the coupling were low at rest (26 significant and 24 nonlinear) and further reduced after tilt (17 significant and 16 nonlinear). Old subjects showed a partial recovery of linearity after tilt (rest: 1 linear of 7 significant; tilt: 5 linear of 8 significant). In young subjects, the causal synchronization indexes were balanced and increased from rest ([X.sub.t/s] = 0.072 [+ or -] 0.037 and [X.sub.s/t] = 0.054 [+ ot -] 0.028) to tilt ([X.sub.t/s] = 0.125 [+ or -] 0.071 and [X.sub.s/t] = 0.108 [+ or -] 0.053). On the contrary, in old subjects and AMI patients, the feedforward was prevalent to the feedback coupling at rest (old: [X.sub.t/s] = 0.041 [+ or -] 0.023 and [X.sub.s/t] = 0.069 [+ or -] 0.042; AMI: [X.sub.t/s] = 0.050 [+ or -] 0.030 and [X.sub.s/t] = 0.089 [+ or -] 0.053). Tilting blunted the unbalance in old subjects ([X.sub.t/s] [+ or -] 0.065 + 0.052 and [X.sub.s/t] = 0.069 [+ or -] 0.044) but not in AMI patients ([X.sub.t/s] = 0.040 [+ or -] 0.019 and [X.sub.s/t] = 0.060 [+ or -] 0.040). Thus, after AMI, nonlinear mechanisms are elicited in RR-SAP interactions. Furthermore, the neural regulation of the cardiovascular system resulted in imbalance as a consequence of impaired feedback and enhanced feedforward control mechanisms. causal analysis; nonlinear coupling; synchronization; baroreflex regulation

Power-band and spectral analyses of the spontaneous beat-to-beat changes of the atrioventricular (AV) interval reveal two oscillations of low-frequency (LF) and high-frequency (HF) that are synchronous with atrial cycle length changes. Tilt maneuver introduces vagal withdrawal and sympathetic activation to reduce the LF and HF oscillations of AV conduction spectra. Reduction in the HF and LF oscillations of AV interval indicate that the autonomic nervous system affects AV conduction through parasympathetic withdrawal.

Nollo, Giandomenico, Alberto Porta, Luca Faes, Maurizio Del Greco, Marcello Disertori, and Flavia Ravelli. Causal linear parametric model for baroreflex gain assessment in patients with recent myocardial infarction. Am J Physiol Heart Circ Physiol 280: H1830-H1839, 2001.--Spectral and cross-spectral analysis of R-R interval and systolic arterial pressure (SAP) spontaneous fluctuations have been proposed for noninvasive evaluation of baroreflex sensitivity (BRS). However, results are not in good agreement with clinical measurements. In this study, a bivariate parametric autoregressive model with exogenous input (ARXAR model), able to divide the R-R variability into SAP-related and -unrelated parts, was used to quantify the gain ([[Alpha].sub.ARXAR]) of the baroreflex regulatory mechanism. For performance assessing, two traditional noninvasive methods based on frequency domain analysis [spectral, baroreflex gain by autogressive model ([[Alpha].sub.AR]); cross-spectral, baroreflex gain by bivariate autoregressive model ([[Alpha].sub.2AR])] and one based on the time domain [baroreflex gain by sequence analysis ([[Alpha].sub.SEQ])] were considered and compared with the baroreflex gain by phenylephrine test ([[Alpha].sub.PHE]). The BRS evaluation was performed on 30 patients (61 [+ or -] 10 yr) with recent (10 [+ or -] 3 days) myocardial infarction. The ARXAR model allowed dividing the R-R variability (950 [+ or -] 1,099 [ms.sup.2]) into SAP-related (256 [+ or -] 418 [ms.sup.2]) and SAP-unrelated (694 [+ or -] 728 [ms.sup.2]) parts. [[Alpha].sub.AR] (12.2 [+ or ] 6.1 ms/mmHg) and [[Alpha].sub.2AR] (8.9 [+ or -] 5.6 ms/mmHg) as well as [[Alpha].sub.SEQ] (12.6 [+ or -] 7.1. ms/mmHg) overestimated BRS assessed by [[Alpha].sub.PHE] (6.4 [+ or -] 4.7 ms/mmHg), whereas the ARXAR index gave a comparable value ([[Alpha].sub.ARXAR] = 5.4 [+ or -] 3.3 ms/ mmHg). All noninvasive methods were significantly correlated to [[Alpha].sub.PHE] ([[Alpha].sub.ARXAR] and [[Alpha].sub.SEQ] were more correlated than the other indexes). Thus the baroreflex gain obtained describing the causal dependence of R-R interval on SAP showed a good agreement with [[Alpha].sub.PHE] and may provide additional information regarding the gain estimation in the frequency domain. baroreflex sensitivity; spectral analysis; phenylephrine; autoregressive models; R-R-SAP transfer function