Causal Linear Parametric Model for Baroreflex Gain Assessment in Patients with Recent Myocardial Infarction

Spectral and cross-spectral analysis of RR interval and systolic arterial pressure (SAP) spontaneous fluctuations have been proposed for noninvasive evaluation of the baroreflex sensitivity (BRS). However, results are not in good agreement with the standard phenylephrine measurement (PheBRS). In this study a bivariate ARXAR model able to divide the RR variability into SAP-related and SAP-unrelated parts was introduced to quantify the gain (ARXAR) of the baroreflex regulatory mechanism. Two other methods (spectral: aAR; cross-spectral: 2AR) for noninvasive BRS evaluation were considered for comparison. Phenylephrine test was used as a reference standard. The BRS evaluation was performed in 30 patients (61±10 years) with recent (10±3 days) myocardial infarction (MI). Spectral and cross-spectral gain indexes (AR:12.2±6.1, 2AR:8.9±5.6) overestimated BRS assessed by PheBRS (6.4±4.7), whereas the ARXAR index gave a comparable value (ARXAR:5.4±3.3). All noninvasive methods showed a significant correlation with PheBRS. However, regression analysis selected aARXAR first in correlation with PheBRS (r=0.76, y=0.86+0.86x; p<0.001). Thus, baroreflex gain obtained from a dynamic parametric model describing the causal dependence of RR interval on SAP showed better agreement with PheBRS than the gain obtained from spectral and cross-spectral methods unable to take causality into account.