Validity and variability of xBRS: instantaneous cardiac baroreflex sensitivity

Abstract

Spontaneous oscillations of blood pressure (BP) and interbeat interval (IBI) may reveal important information on the underlying baroreflex control and regulation of BP. We evaluated the method of continuously measured instantaneous baroreflex sensitivity by cross correlation (xBRS) validating its mean value against the gold standard of phenylephrine (Phe) and nitroprusside (SNP) bolus injections, and focusing on its spontaneous changes quantified as variability around the mean. For this purpose, we analyzed data from an earlier study of eight healthy males (aged 25–46 years) who had received Phe and SNP in conditions of baseline and autonomic blocking agents: atropine, propranolol, and clonidine. Average xBRS corresponds well to Phe/SNP‐BRS, with xBRS levels ranging from 1.2 (atropine) to 102 msec/mmHg (subject asleep under clonidine). Time shifts from BP‐ to IBI‐signal increased from ≤1 sec (maximum correlations within the current heartbeat) to 3–5 sec (under atropine). Plotted on a logarithmic vertical scale, xBRS values show 40% variability (defined as SD/mean) over the whole range in the various conditions, except twice when the subjects had fallen asleep and it dropped to 20%. The xBRS oscillates at frequencies of 0.1 Hz and lower, dominant between 0.02–0.05 Hz. Although xBRS is the result of IBI/BP‐changes, no linear coherence was found in the cross‐spectra of the xBRS‐signal and IBI or BP. We speculate that the level of variability in the xBRS‐signal may act as a probe into the central nervous condition, as evidenced in the two subjects who fell asleep with high xBRS and only 20% of relative variation.

Associations Between Child Maltreatment, Autonomic Regulation, and Adverse Cardiovascular Outcome in an Urban Population: The HELIUS Study

“…Estimates of BRS were obtained from beat-to-beat changes in SBP and heart rate, using the xBRS method. First, the change in heart rate in response to change in BP was assessed, using a cross-correlation method over a sliding 10-s window with various delay compensations (37,38). Second, for each segment with a significant positive cross-correlation, xBRS was obtained by dividing the standard deviation of the IBI by the standard deviation of the SBP.…”

Instantaneous Cardiac Baroreflex Sensitivity: xBRS Method Quantifies Heart Rate Blood Pressure Variability Ratio at Rest and During Slow Breathing

“…The underlying hypothesis is that there is always some spontaneous variability in blood pressure that should allow an estimate of BRS. In this paper, the BRS estimation method under consideration was the xBRS method (Westerhof et al, 2004;Wesseling et al, 2017).…”

“…The xBRS method was developed by Westerhof et al (2004) and initially it set strict constrains on p-value of the cross correlation (p < 0.01). In the last revision (Wesseling et al, 2017), the threshold for the p-value was increased to 0.05, it doubles the percentage of necessary valid windows. Once sequences are selected, the ratio between the standard deviations of BBI and of SBP is used as a measure of baroreflex sensitivity.…”

“…The reasoning for this decision was that the measure based on ratios of standard deviations does not differ significantly from the slopes of the regressions but is computationally more efficient. In our article we implemented the last revision of the xBRS method; the geometric mean of the instant xBRS values was used as BRS estimate per study period reflecting the approximately log-normal distribution of xBRS values (Wesseling et al, 2017).…”

“…Recently (Wessel et al, 2020) we were able to show that the spontaneous sensitivity of the arterial baroreflex (BRS) under resting conditions cannot be estimated by the sequence method (SME), which only quantifies the quotient of heart rate and systolic blood pressure variability. In this paper we test whether the xBRS method (Westerhof et al, 2004;Wesseling et al, 2017) is suitable to quantify the baroreflex sensitivity from non-invasive, non-interventional measurements under resting conditions. Therefore, two surrogate analyses were performed in which, due to the design, no causal relationships between blood pressure and heart rate signal can be present.…”