Table 6.

Descriptions of the handcrafted features used for the Mutual Information analyses.

Feature	Description
Heart Rate (HR)	Measurement of the number of pulsations of the heart in a minute. Calculated as the inverse of median time between each heart beat. The PPG signal was used for MIMIC (because time alignment with the ECG signal was not precise), the ECG signal was used for Aurora-BP.
Heart Rate Variability (HRV)	Measurement of the variation in time between each heart beat. Calculated as the mean of standard deviations of normal-normal (NN) intervals (SDNN).
Quality	Measurement of the quality of the PPG signal. A heuristic based algorithm that takes the signal-to-noise ratio, artifacts, consistency between the pulses in a window into consideration and computes a normalized score between 0 and 1.
$\frac{\mathit{dp}}{\mathit{dt}}$	Measurement of the mean systolic rise times normalized with respect to the duration of each beat in the PPG signal.
rPAT	Measurement of the delay between the R-peak in the ECG signal and systolic peak of the PPG signal. This can only be computed for Aurora-BP due to imprecise synchronization in MIMIC.
Inv. PAT	1/rPAT.
$\mathrm{\Delta }$ Feature	Measured as the difference between the calculated value and baseline value. A baseline value of each feature for all patients is computed in Aurora-BP (not available for MIMIC).
std.Feature	Measures the fluctuation of a feature across a fixed time period.