The oscillating pulse arrival time as a physiological explanation regarding the difference between ECG- and Photoplethysmogram-derived heart rate variability parameters

https://doi.org/10.1016/j.bspc.2022.104033Get rights and content
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Highlights

  • The difference in current and preceding PAT explains the discrepancy of RRI and PPI.

  • The PAT oscillates and is likely coupled to respiration.

  • HRV and PRV measures inherently differ, due to differences between two consecutive PATs.

  • The RP% of PAT oscillation is minimal at ½ amplitude of the PPG and maximal at base point.

  • Momentary PAT and the steepness of PPG change inversely, resulting in compensation.

Abstract

Background and Objective

ECG-based heart rate variability (HRV), and more recently, optical pulse rate variability (PRV) analysis used in wearables are non-invasive tools of neurocardiac investigation. Pulse arrival time (PAT) is derived from their simultaneous recording. We examined the beat-to-beat PAT in greater detail, while analyzed the PRV to HRV relationship.

Methods

We acquired 300sec ECG, photoplethysmogram (PPG), and respiratory-signals from 35 young, healthy volunteers while in a supine position including different breathing patterns. PAT at eight reference points on the PPG-side was assessed by the mean, relative precision (RP%) and spectral analysis. PRV and HRV parameters were compared by relative accuracy error (RAE) and the Bland-Altman-Ratio (BAR).

Results

PAT showed the minimum RP% at the 1/2-amplitude point whereas RP% reached the maximum at the base point; its observed fine oscillation was associated to breathing as confirmed through spectral analysis. The instantaneous slope of PPG rise is inversely proportional to the corresponding PAT. RAE and BAR showed excellent agreement in 15 of 16 analysis at time-domain, mostly excellent or moderate on frequency-domain and nonlinear analysis. The inherent difference between HRV and PRV is formally due to the difference among two consecutive PATs.

Conclusion and Significance

The present study supported the interchangeability of HRV- and PRV-analysis. Our formalistic explanation linking successive PAT-pairs and the inborn difference between HRV and PRV may bear further implications in selecting the appropriate fiducial points, approximating PRV to HRV measures, and in creating innovative neurocardiac parameters or complex PPG models/simulators.

Keywords

Photoplethysmography
Heart rate variability
Pulse rate variability
Pulse arrival time

Abbreviations

AL
acceptance limit
ApEn
approximate entropy
BAR
Bland-Altman ratio
BP
blood pressure
CV%
coefficient of variation
ECG
electrocardiography/electrocardiogram
FFT
fast Fourier transformation
GSM
Global System for Mobile Communications
HF
high frequency power
HRV
heart rate variability
IIR
infinite impulse response (filter)
LoA
limits of agreement
LF
low frequency power
MeanNN
mean of the normal-to-normal intervals
n.u.
normalized unit
PAT
pulse arrival time
PEP
pre-ejection period
pNN50
percentage of the number of successive RRIs with greater difference than 50ms
PPG
photoplethysmography/photoplethysmogram
PPI
PP-interval on the photoplethysmogram
PRV
pulse rate variability
PTT
pulse transit time
RAE
relative accuracy error
RMSSD
root mean square of successive RR-differences
RP%
relative precision
RRI
RR-interval on the ECG
SD
standard deviation
SD1
SD2, standard deviation along the short or long axis of Poincaré-polt
SDNN
standard deviation of normal-to-normal intervals
SampEn
sample entropy

Data availability

The authors do not have permission to share data.

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