New criteria for estimating baroreflex sensitivity using the transfer function method

Computer simulations were carried out to appraise three new criteria for the estimation of baroreflex sensitivity (BRS) using the transfer function method. The major goal was to identify a computation procedure able to overcome the intrinsic limitations of the classical coherence criterion. Four representative shapes of the gain function and three different average gains (2, 5 and 8 ms(mmHg)(-1) in the low-frequency (LF) band (0.04-0.15Hz) were considered. The signal-to-noise ratio was made to vary so that the peak coherence in the LF band changed from 0.15 to 0.9. All simulation parameters were derived from previous observations in healthy subjects and heart disease patients. The error of the estimated gain function was obtained from its confidence interval. BRS was computed as average gain in the LF band: (a) including in the average only those points having error < or = threshold (criterion 1, C1); (b) calculating the mean error in the band and accepting BRS measurements only when this error was < or = threshold (criterion 2, C2); (c) including in the average all points, regardless of the error (criterion 3, C3). The three criteria were compared in terms of measurability (percentage of measured BRS) and accuracy (bias and SD of BRS). Using C1 and C2, measurability dropped to 10% when the peak coherence in the LF band decreased, respectively, to 0.18-0.41 and to 0.26-0.53, depending on the shape and strength of the gain. In this condition (lower bound of measurability), worst bias and SD (average gain: 8 ms(mmHg)(-1)) were, respectively, 0.8 ms(mmHg)(-1) and 3.3ms(mmHg)(-1) (C1), and 0.1 ms(mmHg)(-1) and 1.0 ms(mmHg)(-1) (C2). C3, by definition, always ensured 100% measurability and showed bias and SD comparable with, or even lower than, C1 and C2, within the common range of measurable BRS. In the extreme condition of 0.15 coherence, bias and SD were, respectively, 1.7 ms(mmHg)(-1) and 2.3ms(mmHg)(-1) (average gain: 8ms(mmHg)(-1)). Hence, error checking (C1 and C2) dramatically reduced measurability and did not improve accuracy of BRS measurements compared with performing no error check (C3). In conditions of low signal-to-noise ratio and/or impaired baroreflex gain, leading to markedly reduced coherence, the simple average of the gain function in the LF band allows BRS to be estimated with accuracy adequate for clinical purposes.