Are inter-beat intervals from blood pressure a valid alternative to R-R intervals for the multiscale entropy analysis of heart rate variability?

When an ECG recording is not available to derive the R-R interval (RRI) series, the heart rate variability (HRV) may be evaluated considering the inter-beat intervals (IBI) from other cardiovascular signals, like the arterial blood pressure (ABP). However, it is unknown how the ABP-derived IBI series quantify the multiscale entropy (MSE) of HRV. Thus our aim is to describe differences between RRI and the ABP-derived IBI in MSE estimates. We recorded the ECG and the finger ABP in 40 volunteers. We derived the RRI series from the ECG (reference IBI) and the series of systolic-systolic intervals (SSI), diastolic-diastolic intervals (DDI), and intervals between maxima of the first- (d'PI) and second- (d"PI) derivative of ABP. For each IBI series, we estimated the MSE at the scale s=1 beat (SampEn), at high-frequency scales (MSEHF, for 2≤s≤7 beats) and low-frequency scales (MSELF, for 7