Letter to the EditorFrequency analysis of photoplethysmogram and its derivatives
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.
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