Causal linear parametric model for baroreflex gain assessment in patients with recent myocardial infarction

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 (α_ARXAR) of the baroreflex regulatory mechanism. For performance assessing, two traditional noninvasive methods based on frequency domain analysis [spectral, baroreflex gain by autogressive model (α_AR); cross-spectral, baroreflex gain by bivariate autoregressive model (α_2AR)] and one based on the time domain [baroreflex gain by sequence analysis (α_SEQ)] were considered and compared with the baroreflex gain by phenylephrine test (α_PHE). The BRS evaluation was performed on 30 patients (61 ± 10 yr) with recent (10 ± 3 days) myocardial infarction. The ARXAR model allowed dividing the R-R variability (950 ± 1,099 ms²) into SAP-related (256 ± 418 ms²) and SAP-unrelated (694 ± 728 ms²) parts. α_AR (12.2 ± 6.1 ms/mmHg) and α_2AR (8.9 ± 5.6 ms/mmHg) as well as α_SEQ (12.6 ± 7.1 ms/mmHg) overestimated BRS assessed by α_PHE (6.4 ± 4.7 ms/mmHg), whereas the ARXAR index gave a comparable value (α_ARXAR = 5.4 ± 3.3 ms/mmHg). All noninvasive methods were significantly correlated to α_PHE (α_ARXAR and α_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 α_PHE and may provide additional information regarding the gain estimation in the frequency domain.