Standing revised: assessing baroreflex sensitivity by the modified transfer function method

Baroreflex sensitivity (BRS) can be estimated noninvasively by computing the transfer function modulus (TF) between spontaneous oscillations of systolic arterial pressure (SAP) and RR interval within the LF band. In 21 healthy volunteers we tested the hypothesis that during standing the reliability of BRS-TF increases as a result of increased SAP power. We also assessed whether standing causes any change in BRS-TF. BRS-TF was computed averaging the TF over the whole LF band and only within -3 db points. We found that during standing the error of TF estimates markedly decreased compared to the supine posture. As this change was also accompanied by a decrease of BRS-TF, the reduction of the relative error was modest. Hence, standing is capable of reducing the uncertainty of TF estimates but this beneficial effect is partly offset by a simultaneous reduction of TF values. Of the two approaches for measuring BTS-TF, the average between -3 db points showed the highest reliability.