Impulse response analysis of baroreceptor sensitivity

The impulse response function (IRF) can express the dynamic relationship between systolic arterial pressure (SAP) and pulse interval (PI) and, consequently, represents an alternative method to assess baroreceptor sensitivity (BRS) in humans. Five normotensive and 13 hypertensive subjects (age 68 +/- 5 yr, range 60-74 yr) were studied at rest in the supine position during baseline conditions and after injections of phenylephrine and sodium nitroprusside. SAP and PI signals were derived from multiple 5-min noninvasive recordings of arterial blood pressure (Finapres) and electrocardiogram. Standard estimates of BRS were obtained by the slopes of transient changes in SAP and PI after the injection of phenylephrine and sodium nitroprusside (BRS(PE) and BRS(SNP)) and by spectral analysis (alpha-index). Impulse responses were obtained by the inverse Fourier transform of the transfer function between PI and SAP. The temporal pattern of the IRF was characterized by a main peak at t = 0, preceded by a "trough" at t = -1 s. A mathematical model of the baroreflex suggests that the peak value of IRF is linearly related to the BRS. The peak value and its smoothed version were shown to be significantly correlated to alpha, BRS(PE), and BRS(SNP) and significantly reduced in the hypertensive group during the three stages of the protocol. We suggest that IRF might be the ideal method to assess BRS because it does not require any subjective preselection of data segments or spectral bands.