Letter to the Editor
Frequency analysis of photoplethysmogram and its derivatives

https://doi.org/10.1016/j.cmpb.2015.09.021Get rights and content

Abstract

There are a limited number of studies on heat stress dynamics during exercise using the photoplethysmogram (PPG). We investigate the PPG signal and its derivatives for heat stress assessment using Welch (non-parametric) and autoregressive (parametric) spectral estimation methods. The preliminary results of this study indicate that applying the first and second derivatives to PPG waveforms is useful for determining heat stress level using 20-s recordings. Interestingly, Welch's and Yule–Walker's methods in agreement that the second derivative is an improved detector for heat stress. In fact, both spectral estimation methods showed a clear separation in the frequency domain between measurements before and after simulated heat-stress induction when the second derivative is applied. Moreover, the results demonstrate superior performance of the Welch's method over the Yule–Walker's method in separating before and after the three simulated heat-stress inductions.

Introduction

Body core temperature (BCT) is the gold standard criterion for assessment of an individual's heat stress response. The invasive nature of core body temperature assessment and the nuances of measurement render its application inappropriate for use in selected settings. While ingestible temperature sensors have vastly improved the individual thermal assessment in field settings, the requirement to obtain measurements immediately post consumption limit the application of this technology to non-acute emergency responses. Non-invasive alternatives to core body temperature assessment in such environments are warranted, with surrogate measures of core body temperature assessed in tropical field settings. In a previous study, heart rate (beats per minute), demonstrated a greater relationship to core body temperature than other commonly assessed physiological variables, inclusive of mean arterial pressure and tympanic temperature [1]. The relationship between body temperature and frequency of cardiac cycles is well known [2], [3]. Furthermore, analysis of the arterial pulse wave has been shown to provide valuable information on aortic stiffness and elasticity [4], [5], [6]. It has been widely used to evaluate the vascular effects of aging, hypertension, and atherosclerosis [7], [8], [9], [10].

Photoelectric plethysmography is the most commonly used method for pulse-wave analysis, which has been referred to as photoplethysmography (PTG/PPG), blood volume pulse (BVP), and digital volume pulse (DVP) analysis; however, the acronym PPG will be used exclusively in this study, according to the recommendations in Elgendi et al. [11]. Fingertip PPG is a non-invasive measurement that mainly reflects the pulsatile volume changes in the finger arterioles, as shown in Fig. 1(a) and (b). Analyzing the PPG wave contour is difficult; therefore, researchers have applied the derivative to emphasize and easily quantify the delicate changes in the PPG contour [12], as shown in Fig. 1(c)–(f).

Applying derivatives to PPG signals detect faster physiological changes (bigger gradient) in the PPG signal. For example, the first derivative of the PPG signal represents the velocity of blood and the second derivative represents the acceleration of the blood flow inside the fingertip. Therefore, it is expected that applying derivatives will magnify the differences between PPG signals measured before and after heat stress induction, especially in the frequency domain. Throughout this paper, we will use the VPG (velocity of PPG) acronym to refer to the first derivative of the PPG. The acronym of the second derivative of the PPG is usually SDPPG or APG; however, APG will be used exclusively within this study, according to the recommendations in Elgendi et al. [11].

Despite the application of PPG to cardiac variables and that clinical significance of PPG measurement has been well investigated, there is still a lack of studies that focus on heat stress assessment using PPG signals. Matsuyama [13] assessed the most suitable index for heat stress assessment in the APG signals, determining that the typical time-domain indicators of APG analysis, however, Matsuyama's investigation showed that the time-domain indices are unsuitable for heat stress assessment. Therefore, our investigation is focused on examining the frequency domain of PPG and its derivatives in detecting subjects measured after simulated heat-stress induction.

Section snippets

Data collection

The heat stress PPG data for this study were collected as part of a National Critical Care and Trauma Response Centre (NCCTRC) project to assess the physiological and perceptual responses of emergency responders to a simulated chemical, biological, and radiological (CBR) incidents in tropical environmental conditions, to compare the efficacy of various cooling methods. The background of the NCCTRC's thermal research can be found in Brearley [14]. Forty healthy, heat acclimatized emergency

Results and discussion

The core temperature data confirms that overall, candidates had experienced substantial body heat storage. We would have seen a different HR response if we heated the candidates without the use of exercise (sitting still in a sauna), however that model has limited application to the assessment of heat stress in real world settings. Fitness plays a role in HR recovery; fitter candidates have finer regulation of blood flow and require fewer cardiac cycles per minute to restore the body to normal

Conclusion

The findings of this preliminary study indicate that heat stress can be assessed using derivatives of PPG signals. Results of this study indicate that PPG can be a potential modality for heat stress analysis and identification of individuals at risk. Our preliminary study demonstrates indicative results, and now motivates the need for a larger study that validates the derivative analysis of PPG signals against traditional heat stress tolerance indices.

Conflict of interest

None declared.

Funding

None declared.

Informed consent

All participants provided written informed consent before participation, which was approved by the Human Research Ethics Committee of the Northern Territory Department of Health and the Menzies School of Health Research.

Acknowledgements

Mohamed Elgendi would like to gratefully acknowledge the NT Fire and Rescue Service and the NCCTRC for assistance with the research project. He also appreciates the support of Prof. Friso De Boer.

References (25)

  • K. Takazawa et al.

    Assessment of vasoactive agents and vascular aging by the second derivative of photoplethysmogram waveform

    Hypertension

    (1998)
  • A. Bortolotto et al.

    Assessment of vascular aging and atherosclerosis in hypertensive subjects: second derivative of photoplethysmogram versus pulse wave velocity

    Am. J. Hypertens.

    (2000)
  • Cited by (21)

    • Redundant Gaussian dictionary in compressed sensing for ambulatory photoplethysmography monitoring

      2021, Biomedical Signal Processing and Control
      Citation Excerpt :

      Almost all of the testing records in the IEEE SPC 2015 database contain strong noise that leads to high PRD value. Considering an anti-aliasing filter is always applied before acquisition or signal processing in practice [30], a four-order Butterworth low-pass zero-phase filter was added before the CS sensing step, the cut-off frequency of the filter was 20 Hz [42,43]. The new outcomes of the filtered signals are presented in Table 3.

    • Photoplethysmography signal processing and synthesis

      2021, Photoplethysmography: Technology, Signal Analysis and Applications
    • Blind, Cuff-less, Calibration-Free and Continuous Blood Pressure Estimation using Optimized Inductive Group Method of Data Handling

      2020, Biomedical Signal Processing and Control
      Citation Excerpt :

      As a result, for having enough details in APG and VPG signal, frequency 100 Hz is enough to record basis points. Furthermore, the 10 bit resolution which is used in this research is a minimum resolution, comparing with Choi and Shin [64] and Elgandi et al. [65] who used 16 bit resolution. Nevertheless, the proposed algorithm try to extract some robust features which change owing to non-stationarity.

    • Adaptive Band Limit Estimation based PPG data compression for portable home monitors

      2019, Measurement: Journal of the International Measurement Confederation
      Citation Excerpt :

      The 0.2 Hz margin is kept to account for the heart rate variability within the data for long term monitoring (considering a maximum variation of 12 bpm). The higher frequency components of the PPG signal correspond to the harmonics of the basic component and a maximum of 3rd to 4th harmonic is present [29]. The high frequency content of the data is largely affected by a lot of factors like, ageing and arterial stiffness [5].

    • Camera-Based Blood Pressure Estimation via Windkessel Model and Waveform Features

      2023, IEEE Transactions on Instrumentation and Measurement
    • A Facial-Image-Based Blood Pressure Measurement System Without Calibration

      2022, IEEE Transactions on Instrumentation and Measurement
    View all citing articles on Scopus
    View full text