Table 3 Methods used to derive BRS in the selected papers

BRS sequence method [34, 36, 42, 45, 47, 49]

Identifying sequences of at least three consecutive beats, where both SBP and RRI either increase or decrease, and then calculating the average slope of the identified sequences within a defined time frame

BRS spectral method [38,39,40,41, 43, 45, 49]

Calculating the modulus or gain of the transfer function at a frequency of 0.1 Hz using the formula:

\(\mathrm{BRS }[{\text{ms}}/{\text{mmHg}}]= \frac{Gxy (f)}{Gxx (f)}\)

Gxy(f): cross-spectral density between SBP and RRI; Gxx(f): power spectral density of SBP

BRSf [39, 40, 45]

Using the same formula as BRS spectral method, calculating the modulus at 0.1 Hz using the instantaneous values of the heart rate (in Hz) and SBP

\(\mathrm{BRS }[{\text{Hz}}/{\text{mmHg}}]= \frac{Gxy (f)}{Gxx (f)}\)

Gxy(f): cross-spectral density between HR and RRI; Gxx(f): power spectral density of SBP

BRS alpha-index [36, 44]

Calculating the square root of the ratio of the spectral powers of RRI and SBP within a band of a particular frequency. In [36], alpha-index for LF band (0.04–0.15 Hz) was considered. In [44], both LF and HF band (0.20–0.35 Hz) were considered and combined alpha-index was calculated: 0.5 × [LF alpha-index + HF alpha-index]