Sources of Inaccuracy in Photoplethysmography for Continuous Cardiovascular Monitoring

In the paper

“The parameters listed in Table 3 attributed to Ahn et al. yielded a statistically significant, albeit poor, correlation with age [83].”

What the sources say

• Supported

  The Results sections on APG and PPG indices report several statistically significant but weak correlations with age, including the age index (r = 0.296), c_peak (r = −0.288), and A1_time (r = −0.247). This supports the characterization of those individual parameters, though the cited paper describes its combined IPAD index as having a stronger correlation. (LLM, conf=0.86)

  cites [83] Ahn, 2017, New aging index using signal features of both photoplethysmograms and acceleration plethysmograms, Healthc. Inform. Res. source

In the paper

“Using k-nearest neighbor and support vector machines, Ferdinando et al. were able to identify five classes of obesity from PPG waveforms using 17 parameters derived from the decomposition of the PPG waveform into five lognormal functions.”

What the sources say

• ?Could not verify

  The Background and Summary and Methods sections describe a PPG dataset for cardiovascular and blood-pressure research, but do not report obesity classification. Searches for Ferdinando, five obesity classes, 17 parameters, lognormal decomposition, and the named classifiers found no passage addressing the claimed result. The supplied passages have no page numbers. (LLM, conf=0.77)

  cites [75] Liang, 2018, A new, short-recorded photoplethysmogram dataset for blood pressure monitoring in China, Sci. Data sourceExperimental — accuracy not yet measured
• Not checked yetcites [76] Ferdinando, H., Huotari, M., and Myllylä, T. (2019, January 23–27). Photoplethysmography signal analysis to assess obesity, age group and hypertension. Proceedings of the 2019 41st Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC), Berlin, Germany. source

In the paper

“| c/a | Decrease | Takazawa et al., 1998 Elgendi et al., 2012 | [29,59] |”

What the sources say

• Not checked yetcites [29] Takazawa, 1998, Assessment of vasoactive agents and vascular aging by the second derivative of photoplethysmogram waveform, Hypertension source
• Supported

  The “Ratio c/a” subsection states that the c/a ratio decreases with age, reporting findings by Takazawa et al. and Baek et al. This directly supports the trend attributed to the review. (LLM, conf=0.95)

  cites [59] Elgendi, 2012, On the analysis of fingertip photoplethysmogram signals, Curr. Cardiol. Rev. source

In the paper

“RIAV causes significant changes in each peak amplitude.”

What the sources say

• Supported

  The Introduction (section 1.1, page unavailable) describes RIAV as a respiration-driven change in PPG pulse amplitude: amplitude decreases during inspiration and increases during expiration. This supports the citing sentence’s point about changes in peak amplitude; the source does not quantify their size. (LLM, conf=0.87)

  cites [115] Dehkordi, 2018, Extracting instantaneous respiratory rate from multiple photoplethysmogram respiratory-induced variations, Front. Physiol. source
• Supported

  The respiratory-rate algorithm section describes pulse-amplitude modulation: respiration changes cardiac pulse amplitude, with amplitude decreasing during inspiration. Its Fig. 1 caption likewise describes cardiac pulse amplitudes varying over the respiratory cycle. The paper does not quantify the change in each individual peak, so “significant” is supported as a qualitative description, not a statistical result. (LLM, conf=0.88)

  cites [121] Addison, 2012, Developing an algorithm for pulse oximetry derived respiratory rate (RR oxi): A healthy volunteer study, J. Clin. Monit. Comput. source

In the paper

“However, there exists tremendous variation even within a given body site caused by factors such as age, gender, BMI, sun-damage, and experimental methodology, leading to coefficients of variation up to 40% [139,140,141].”

What the sources say

• Partly supported

  The Introduction says skin thickness is related to age and gender, and the Discussion reports variation in thickness at the fingertip (mean 0.57 mm, SD 0.12 mm). The paper does not attribute within-site variation to BMI, sun damage, or experimental methodology, and it does not report a coefficient of variation up to 40%. (LLM, conf=0.85)

  cites [139] Dahlqvist, 2018, Vibrotactile and thermal perception and its relation to finger skin thickness, Clin. Neurophysiol. Pract. sourceExperimental — accuracy not yet measured
• Not checked yetcites [140] Olsen, 1995, High-frequency ultrasound characterization of normal skin. Skin thickness and echographic density of 22 anatomical sites, Skin Res. Technol. source
• Not checked yetcites [141] Martin, 1992, Effects of skin thickness and skinfold compressibility on skinfold thickness measurement, Am. J. Hum. Biol. source

In the paper

“The finger yielded a significant peak point position, meaning a shorter systolic rising edge (p < 0.001 for all sites except under the wrist, which had p = 0.04).”

What the sources say

• Supported

  The Results section, “Effect of Measurement Site on Tp,” reports that finger PPG had a significantly smaller peak-point position (Tp) than every other site under both breathing conditions (p < 0.001, except p = 0.04 for wrist under). The Discussion and Conclusion section relates the smaller Tp at the finger to a shorter pulse rise time. (LLM, conf=0.94)

  cites [95] Hartmann, 2019, Quantitative comparison of photoplethysmographic waveform characteristics: Effect of measurement site, Front. Physiol. source

In the paper

“They also reported that during the high intensity running stage, they saw some of the lowest MAPE from the MA (0.82%), the TT (0.97%), and the BP (3.28%).”

What the sources say

• Partly supported

  The Results report that at the final 9.6 km/h running stage, Mio Alpha and TomTom had MAPE of 0.82% and 0.97%, respectively. The Discussion also reports 3.28% for Basis Peak, but identifies it as the greatest MAPE at that stage, not one of the lowest. (LLM, conf=0.90)

  cites [166] Stahl, 2016, How accurate are the wrist-based heart rate monitors during walking and running activities? Are they accurate enough?, BMJ Open Sport Exerc. Med. sourceExperimental — accuracy not yet measured

In the paper

“PPG-based RCIM devices that are U.S. Food and Drug Administration (FDA) cleared or approved and can accurately and consistently record clinical parameters in a sufficiently diverse population for true health monitoring are scarce.”

In the paper

“While, to the authors’ knowledge, publications that experimentally and explicitly demonstrate the effect of obesity or BMI on the PPG waveform are limited, we will explore works which suggest that there would be a substantial effect [56,59,60,61,62,63,64,65,66,67,68,69,70].”

What the sources say

• Not checked yetcites [56] Gibney, 2010, Skin and subcutaneous adipose layer thickness in adults with diabetes at sites used for insulin injections: Implications for needle length recommendations, Curr. Med. Res. Opin. source
• Not checked yetcites [59] Elgendi, 2012, On the analysis of fingertip photoplethysmogram signals, Curr. Cardiol. Rev. source
• Not checked yetcites [60] Iacopi, 2020, Using Skin Bioengineering to Highlight How Weight and Diabetes Mellitus Modify the Skin in the Lower Limbs of Super-Obese Patients, Diabetes Metab. Syndr. Obes. Targets Ther. source
• Not checked yetcites [61] Chin, 1999, Increased cutaneous blood flow but impaired post-ischemic response of nutritional flow in obese children, Atherosclerosis source
• Not checked yetcites [62] Francischetti, 2011, Skin capillary density and microvascular reactivity in obese subjects with and without metabolic syndrome, Microvasc. Res. source
• Not checked yetcites [63] Czernichow, 2010, Microvascular dysfunction in healthy insulin-sensitive overweight individuals, J. Hypertens. source
• Not checked yetcites [64] Debbabi, 2006, Increased skin capillary density in treated essential hypertensive patients, Am. J. Hypertens. source
• Not checked yetcites [65] Ijzerman, 2004, Impaired microvascular function in obesity: Implications for obesity-associated microangiopathy, hypertension, and insulin resistance, Circulation source
• Not checked yetcites [66] Kapur, 2013, Obesity is associated with a lower resting oxygen saturation in the ambulatory elderly: Results from the cardiovascular health study, Respir. Care source
• Not checked yetcites [67] Petrofsky, 2015, The effect of BMI on oxygen saturation at rest and during mild walking, J. Appl. Med. Sci.

In the paper

“While many sources of noise had documented potential solutions, few had a comprehensive solution and even our understanding of how conditions such as obesity affect the skin and cardiovascular monitoring are still developing.”

In the paper

“A review of the literature shows that the difficulty in creating such a device partially originates from the many sources of noise that researchers encounter.”

In the paper

“As shown in Table 7, we found that sources of noise can be divided primarily into three categories: individual variation, physiological processes, and external perturbation.”

In the paper

“Systems aiming to derive such complex parameters do not always account for realistic sources of noise, as testing is performed within controlled parameter spaces.”

In the paper

“The difficulty in determining features in PPG devices lies in the numerous sources of noise that can impede the output of the PPG.”

In the paper

“The variation in the quasi-DC component is due to many factors: the optical properties of the tissue, average blood volume, respiration, vasomotor activity, vasoconstrictor waves, Traube Hering Meyer waves, and thermoregulation [4,5,6,7,8,9,10,11,12,13,14].”

What the sources say

• Not checked yetcites [4] Burton, 1940, A study of the adjustment of peripheral vascular tone to the requirements of the regulation of body temperature, Am. J. Physiol. Leg. Content source
• Not checked yetcites [5] Burton, 1939, The range and variability of the blood flow in the human fingers and the vasomotor regulation of body temperature, Am. J. Physiol. Leg. Content source
• Not checked yetcites [6] Hertzman, 1942, The vasomotor components in the vascular reactions in the finger to cold, Am. J. Physiol. Leg. Content source
• Not checked yetcites [7] Hertzman, 1942, The reactions of the digital artery and minute pad arteries to local cold, Am. J. Physiol. Leg. Content source
• Not checked yetcites [8] Hertzman, A.B., and Flath, F. (1963). Continuous Simultaneous Registration of Sweating and Blood Flow in a Small Skin Area, Saint Louis University. source
• Not checked yetcites [9] Hertzman, 1940, Applications of photoelectric plethysmography in peripheral vascular disease, Am. Heart J. source
• Not checked yetcites [10] Hertzman, 1942, The absence of vasoconstrictor reflexes in the forehead circulation. Effects of cold, Am. J. Physiol. Leg. Content source
• Not checked yetcites [11] Hyndman, 1971, Spontaneous rhythms in physiological control systems, Nature source
• Not checked yetcites [12] Fiser, 1978, Spectral analysis of resting variability of some circulatory parameters in man, Physiol. Bohemoslov.
• Not checked yetcites [13] Harness, 1989, Low-frequency photoplethysmograph signals, Clin. Phys. Physiol. Meas. source

In the paper

“Additionally, there are many reports of inaccuracy in these devices [33].”

What the sources say

• Not checked yetcites [33] Ramella-Roman, J.C. (2020, January 1–6). Design of wearable devices for diverse populations (Conference Presentation). Proceedings of the Optics and Biophotonics in Low-Resource Settings VI, San Francisco, CA, USA. source

In the paper

“These many sources of noise create limitations in the application of PPG to derive advanced physiological parameters.”

In the paper

“An individual’s skin tone, and thus Fitzpatrick category, is correlated to the amount of eumelanin in their epidermis [38].”

What the sources say

• Not checked yetcites [38] Thody, 1991, Pheomelanin as well as eumelanin is present in human epidermis, J. Investig. Dermatol. source

In the paper

“In adults, while some studies show that at rest there is no significant change in dermal capillary density, the majority of findings find that dermal capillary density does negatively correlate on average with increasing BMI [62,63,64,65].”

What the sources say

• Not checked yetcites [62] Francischetti, 2011, Skin capillary density and microvascular reactivity in obese subjects with and without metabolic syndrome, Microvasc. Res. source
• Not checked yetcites [63] Czernichow, 2010, Microvascular dysfunction in healthy insulin-sensitive overweight individuals, J. Hypertens. source
• Not checked yetcites [64] Debbabi, 2006, Increased skin capillary density in treated essential hypertensive patients, Am. J. Hypertens. source
• Not checked yetcites [65] Ijzerman, 2004, Impaired microvascular function in obesity: Implications for obesity-associated microangiopathy, hypertension, and insulin resistance, Circulation source

In the paper

“For those with a lighter skin tone, this enables a higher signal-to-noise ratio for determining heart rate: the primary parameter derived by PPG.”

In the paper

“By collecting data from 56 patients (34 female, 22 males, 18–54 years old, and at least 8 participants in each Fitzpatrick classification) when they are at rest and exercising (elevating heart rate to 50% of maximum via a treadmill), it was found that there was no statistically significant relationship between measured heart rate from the wearable and an ECG (Bittium Faros 180) reference.”

What the sources say

• Not checked yetcites [46] Bent, 2020, Investigating sources of inaccuracy in wearable optical heart rate sensors, NPJ Digit. Med. source

In the paper

“Mean modulation, often termed the perfusion index, defined as the ratio of AC/DC, was significantly lower in Fitzpatrick V individuals than others for all wavelengths (p < 0.001).”

What the sources say

• Not checked yetcites [52] Fallow, 2013, Influence of skin type and wavelength on light wave reflectance, J. Clin. Monit. Comput. source

In the paper

“The lower absorption of melanin using light sources at higher wavelengths can improve the signal for individuals with a high Fitzpatrick classification.”

In the paper

“In addition to these examples, numerous other studies that stratify results against Fitzpatrick classification identify the same trend; errors in determining heartrate from wearable PPG devices that use primarily green light as their source is increased in individuals with dark skin tones due to the high absorption caused by increased amounts of epidermal melanin [42,43,44,45].”

What the sources say

• Not checked yetcites [42] Wang, 2016, Algorithmic principles of remote PPG, IEEE Trans. Biomed. Eng. source
• Not checked yetcites [43] Jeanne, 2013, Robust pulse rate from chrominance-based rPPG, IEEE Trans. Biomed. Eng. source
• Not checked yetcites [44] Shao, 2016, Simultaneous monitoring of ballistocardiogram and photoplethysmogram using a camera, IEEE Trans. Biomed. Eng. source
• Not checked yetcites [45] Addison, 2018, Video-based heart rate monitoring across a range of skin pigmentations during an acute hypoxic challenge, J. Clin. Monit. Comput. source

In the paper

“The variation of BMI across individuals is thus a potential source of variation for PPG measurements.”

In the paper

“Furthermore, one component of optimizing source/detector separation is the depth of the target vessel and intervening pulsating arterioles, which may vary across individuals and within individuals with body location and due to factors such as level of obesity [56].”

What the sources say

• Not checked yetcites [56] Gibney, 2010, Skin and subcutaneous adipose layer thickness in adults with diabetes at sites used for insulin injections: Implications for needle length recommendations, Curr. Med. Res. Opin. source

In the paper

“This effect of obesity could lead to a decrease in the dominant “DC” component of a PPG waveform due to the increased blood volume of the obese [61,62,63,64].”

What the sources say

• Not checked yetcites [61] Chin, 1999, Increased cutaneous blood flow but impaired post-ischemic response of nutritional flow in obese children, Atherosclerosis source
• Not checked yetcites [62] Francischetti, 2011, Skin capillary density and microvascular reactivity in obese subjects with and without metabolic syndrome, Microvasc. Res. source
• Not checked yetcites [63] Czernichow, 2010, Microvascular dysfunction in healthy insulin-sensitive overweight individuals, J. Hypertens. source
• Not checked yetcites [64] Debbabi, 2006, Increased skin capillary density in treated essential hypertensive patients, Am. J. Hypertens. source

In the paper

“There are many components to a PPG signal and its derivatives that literature records having variation as a function of age [83].”

What the sources say

• Not checked yetcites [83] Ahn, 2017, New aging index using signal features of both photoplethysmograms and acceleration plethysmograms, Healthc. Inform. Res. source

In the paper

“In a study with participants ranging in age from 30 to 60+, Yousef et al. found that there is no significant increase in pulse magnitude measured at the index finger with an increase in age.”

What the sources say

• Not checked yetcites [88] Yousef, 2012, The analysis of PPG morphology: Investigating the effects of aging on arterial compliance, Meas. Sci. Rev. source

In the paper

“Aging leads to various anatomical and physiological changes that impact the ability to use PPG to assess cardiovascular health.”

In the paper

“Overall, the morbidly obese exhibit a reduction in water, measured as an increase in the TEWL compared to normal BMI subjects [69].”

What the sources say

• Not checked yetcites [69] Rodrigues, 2017, Excessive weight favours skin physiology-up to a point: Another expression of the obesity paradox, Skin Pharmacol. Physiol. source

In the paper

“It was found that TTW decreases with age [89].”

What the sources say

• Not checked yetcites [89] Hasan, 2015, A New First-Derivative Related Index to Assess Pulse Wave Transit Time from a Photoplethysmographic Waveform: Age Dependence and Agreement with Normative Data, Int. J. Bioinform. Biomed. Eng.

In the paper

“Corroborating with other literature, an increase in pulse wave velocity with age is related to a diminished dicrotic notch.”

What the sources say

• Not checked yetcites [90] Allen, 2003, Age-related changes in the characteristics of the photoplethysmographic pulse shape at various body sites, Physiol. Meas. source

In the paper

“By analyzing the change in PTT as a function of age within a population of 134 healthy, Caucasian subjects (median age 43, total range: 13–72), it was found that there is a statistically significant decrease at the toes (r2 = 0.48), fingers (r2 = 0.26), and ears (r2 = 0.15) [91,92].”

What the sources say

• Not checked yetcites [91] Allen, 2002, Age-related changes in peripheral pulse timing characteristics at the ears, fingers and toes, J. Hum. Hypertens. source
• Not checked yetcites [92] 2011, Non-invasive estimate of blood glucose and blood pressure from a photoplethysmograph by means of machine learning techniques, Artif. Intell. Med. source

In the paper

“As arteries age, the tunica intima and tunica media layers within the arteries thicken with an increase in the number and density of collagen [78].”

What the sources say

• Not checked yetcites [78] Knight, 2017, Anatomy and physiology of ageing 1: The cardiovascular system, Nurs. Times

In the paper

“PTT is inversely related to PWV—the same authors note in a different work that as age increases, PTT decreases.”

What the sources say

• Not checked yetcites [91] Allen, 2002, Age-related changes in peripheral pulse timing characteristics at the ears, fingers and toes, J. Hum. Hypertens. source
• Not checked yetcites [92] 2011, Non-invasive estimate of blood glucose and blood pressure from a photoplethysmograph by means of machine learning techniques, Artif. Intell. Med. source

In the paper

“We believe that the aforementioned literature provides strong evidence that obesity, when assessed in combination with a subject’s metabolic state, body location, gender and age will likely influence PPG systems and can manifest themselves as significant noise sources.”

In the paper

“Lastly, oxygen saturation of hemoglobin has consistently been shown to be inversely associated with BMI across various populations [66,67,73].”

What the sources say

• Not checked yetcites [66] Kapur, 2013, Obesity is associated with a lower resting oxygen saturation in the ambulatory elderly: Results from the cardiovascular health study, Respir. Care source
• Not checked yetcites [67] Petrofsky, 2015, The effect of BMI on oxygen saturation at rest and during mild walking, J. Appl. Med. Sci.
• Not checked yetcites [73] Littleton, 2017, The effects of obesity on lung volumes and oxygenation, Respir. Med. source

In the paper

“For example, the increase in skin thickness affects the vessel depth, and Boonya-ananta et al. used Monte Carlo simulations that predicted a 40% loss of PPG signal amplitude due to this effect in obese individuals, specifically on the wrist as the simulated radial artery increases in depth from 2.5 to 3.5 mm [74].”

What the sources say

• Not checked yetcites [74] Hirt, 2019, Skin changes in the obese patient, J. Am. Acad. Dermatol. source

In the paper

“Overall, these changes can manifest themselves in variations observed across PPG waveforms either in the shape or amplitude of the waveform.”

In the paper

“One observation regarding the skin of the obese which may serve to increase PPG signal intensity is the increase in trans epidermal water loss (TEWL) as BMI increases [69].”

What the sources say

• Not checked yetcites [69] Rodrigues, 2017, Excessive weight favours skin physiology-up to a point: Another expression of the obesity paradox, Skin Pharmacol. Physiol. source

In the paper

“The cross-linking of these fibers, along with fractured and fatigue elastin, leads to a loss of compliance and increased artery stiffness [78].”

What the sources say

• Not checked yetcites [78] Knight, 2017, Anatomy and physiology of ageing 1: The cardiovascular system, Nurs. Times

In the paper

“Once adolescence is reached, skin is known to thin as age increases [70,80,81,82].”

What the sources say

• Not checked yetcites [70] Derraik, J.G., Rademaker, M., Cutfield, W.S., Pinto, T.E., Tregurtha, S., Faherty, A., Peart, J.M., Drury, P.L., and Hofman, P.L. (2014). Effects of age, gender, BMI, and anatomical site on skin thickness in children and adults with diabetes. PLoS ONE, 9. source
• Not checked yetcites [80] Theeten, 2017, High frequency ultrasound to assess skin thickness in healthy adults, Vaccine source
• Not checked yetcites [81] Laurent, 2007, Echographic measurement of skin thickness in adults by high frequency ultrasound to assess the appropriate microneedle length for intradermal delivery of vaccines, Vaccine source
• Not checked yetcites [82] Firooz, 2017, The influence of gender and age on the thickness and echo-density of skin, Skin Res. Technol. source

In the paper

“Overall, the literature has shown that obesity dramatically affects physiological factors associated with PPG signal intensity and quality, including capillary density, capillary recruitment, blood flow, SpO2, TEWL, and skin thickness.”

In the paper

“| e/a | Decrease | Takazawa et al., 1998 Elgendi et al., 2012 | [29,59] |”

What the sources say

• Not checked yetcites [29] Takazawa, 1998, Assessment of vasoactive agents and vascular aging by the second derivative of photoplethysmogram waveform, Hypertension source
• Not checked yetcites [59] Elgendi, 2012, On the analysis of fingertip photoplethysmogram signals, Curr. Cardiol. Rev. source

In the paper

“| Dicrotic notch shape | Decrease | Allen et al., 2003 | [90] |”

What the sources say

• Not checked yetcites [90] Allen, 2003, Age-related changes in the characteristics of the photoplethysmographic pulse shape at various body sites, Physiol. Meas. source

In the paper

“| PTT | Decrease | Allen et al., 2002 Monte-Moreno, 2011 | [91,92] |”

What the sources say

• Not checked yetcites [91] Allen, 2002, Age-related changes in peripheral pulse timing characteristics at the ears, fingers and toes, J. Hum. Hypertens. source
• Not checked yetcites [92] 2011, Non-invasive estimate of blood glucose and blood pressure from a photoplethysmograph by means of machine learning techniques, Artif. Intell. Med. source

In the paper

“| Inflection point area | Decrease | Ahn et al., 2017 | [83] |”

What the sources say

• Not checked yetcites [83] Ahn, 2017, New aging index using signal features of both photoplethysmograms and acceleration plethysmograms, Healthc. Inform. Res. source

In the paper

“| Magnitude of d | Decrease | Ahn et al., 2017 | [83] |”

What the sources say

• Not checked yetcites [83] Ahn, 2017, New aging index using signal features of both photoplethysmograms and acceleration plethysmograms, Healthc. Inform. Res. source

In the paper

“| TTW | Decrease | Cáceres et al., 2015 | [89] |”

What the sources say

• Not checked yetcites [89] Hasan, 2015, A New First-Derivative Related Index to Assess Pulse Wave Transit Time from a Photoplethysmographic Waveform: Age Dependence and Agreement with Normative Data, Int. J. Bioinform. Biomed. Eng.

In the paper

“| Reflection index | Increase | Ahn et al., 2017 | [83] |”

What the sources say

• Not checked yetcites [83] Ahn, 2017, New aging index using signal features of both photoplethysmograms and acceleration plethysmograms, Healthc. Inform. Res. source

In the paper

“| PWV | Increase | Cáceres et al., 2015 | [89] |”

What the sources say

• Not checked yetcites [89] Hasan, 2015, A New First-Derivative Related Index to Assess Pulse Wave Transit Time from a Photoplethysmographic Waveform: Age Dependence and Agreement with Normative Data, Int. J. Bioinform. Biomed. Eng.

In the paper

“| Artery compliance | Decrease | Knight et al., 2017 Allen et al., 2002 | [78,90] |”

What the sources say

• Not checked yetcites [78] Knight, 2017, Anatomy and physiology of ageing 1: The cardiovascular system, Nurs. Times
• Not checked yetcites [90] Allen, 2003, Age-related changes in the characteristics of the photoplethysmographic pulse shape at various body sites, Physiol. Meas. source

In the paper

“This work suggests that the decrease in vessel compliance will contribute more to variations in a PPG waveform than other effects of aging [88].”

What the sources say

• Not checked yetcites [88] Yousef, 2012, The analysis of PPG morphology: Investigating the effects of aging on arterial compliance, Meas. Sci. Rev. source

In the paper

“Regarding blood pressure, Reckelhoff reported the mean blood pressure was 6–10 mmHg higher in males than in pre-menopausal females.”

What the sources say

• Not checked yetcites [101] Reckelhoff, 2001, Gender differences in the regulation of blood pressure, Hypertension source

In the paper

“| Diastolic peak amplitude | Decrease | Ahn et al., 2017 | [83] |”

What the sources say

• Not checked yetcites [83] Ahn, 2017, New aging index using signal features of both photoplethysmograms and acceleration plethysmograms, Healthc. Inform. Res. source

In the paper

“In 2010, Monte-Moreno utilized machine learning to determine blood pressure from a PPG obtained from subjects ranging from 9–80 years old.”

What the sources say

• Not checked yetcites [92] 2011, Non-invasive estimate of blood glucose and blood pressure from a photoplethysmograph by means of machine learning techniques, Artif. Intell. Med. source

In the paper

“These physiological baseline differences can skew PPG signals along with interpretation algorithms for the determination of cardiovascular health per gender.”

In the paper

“| Slope of line between b and d | Increase | Ahn et al., 2017 | [83] |”

What the sources say

• Not checked yetcites [83] Ahn, 2017, New aging index using signal features of both photoplethysmograms and acceleration plethysmograms, Healthc. Inform. Res. source

In the paper

“| Systolic rising edge slope | Decrease | Allen et al., 2003 | [90] |”

What the sources say

• Not checked yetcites [90] Allen, 2003, Age-related changes in the characteristics of the photoplethysmographic pulse shape at various body sites, Physiol. Meas. source

In the paper

“| Skin thickness | Decrease | Derraik et al., 2014 Van Mulder et al., 2017 Farage et al., 2013 | [70,80,93] |”

What the sources say

• Not checked yetcites [70] Derraik, J.G., Rademaker, M., Cutfield, W.S., Pinto, T.E., Tregurtha, S., Faherty, A., Peart, J.M., Drury, P.L., and Hofman, P.L. (2014). Effects of age, gender, BMI, and anatomical site on skin thickness in children and adults with diabetes. PLoS ONE, 9. source
• Not checked yetcites [80] Theeten, 2017, High frequency ultrasound to assess skin thickness in healthy adults, Vaccine source
• Not checked yetcites [93] Farage, 2013, Characteristics of the aging skin, Adv. Wound Care source

In the paper

“| Systolic time | Decrease | Ahn et al., 2017 | [83] |”

What the sources say

• Not checked yetcites [83] Ahn, 2017, New aging index using signal features of both photoplethysmograms and acceleration plethysmograms, Healthc. Inform. Res. source

In the paper

“| d/a | Decrease | Takazawa et al., 1998 Elgendi et al., 2012 | [29,59] |”

What the sources say

• Not checked yetcites [29] Takazawa, 1998, Assessment of vasoactive agents and vascular aging by the second derivative of photoplethysmogram waveform, Hypertension source
• Not checked yetcites [59] Elgendi, 2012, On the analysis of fingertip photoplethysmogram signals, Curr. Cardiol. Rev. source

In the paper

“| Capillary Recruitment | Decrease | Leveque et al., 1984 | [94]. |”

What the sources say

• Not checked yetcites [94] Leveque, 1984, In vivo studies of the evolution of physical properties of the human skin with age, Int. J. Dermatol. source

In the paper

“While this work utilized a population of individuals with good cardiovascular health, it was found to correlate with both systolic and diastolic blood pressure, with an r2 of approximately 0.90 [92].”

What the sources say

• Not checked yetcites [92] 2011, Non-invasive estimate of blood glucose and blood pressure from a photoplethysmograph by means of machine learning techniques, Artif. Intell. Med. source

In the paper

“| Magnitude of c | Increase | Ahn et al., 2017 | [83] |”

What the sources say

• Not checked yetcites [83] Ahn, 2017, New aging index using signal features of both photoplethysmograms and acceleration plethysmograms, Healthc. Inform. Res. source

In the paper

“| b/a | Increase | Ahn et al., 2017 | [83] |”

What the sources say

• Not checked yetcites [83] Ahn, 2017, New aging index using signal features of both photoplethysmograms and acceleration plethysmograms, Healthc. Inform. Res. source

In the paper

“Shelley et al. demonstrates diastole variability in the PPG signal is related to peripheral venous pulsation [116].”

What the sources say

• Not checked yetcites [116] Shelley, 1993, The detection of peripheral venous pulsation using the pulse oximeter as a plethysmograph, J. Clin. Monit. source

In the paper

“PPG optical signal modulation by respiration is most commonly manifested as baseline and amplitude modulation [117].”

What the sources say

• Not checked yetcites [117] Charlton, 2016, An assessment of algorithms to estimate respiratory rate from the electrocardiogram and photoplethysmogram, Physiol. Meas. source

In the paper

“Nowara et al. report insignificant differences in blood volume pulse SNR in iPPG (imaging PPG) between males and females [112].”

What the sources say

• Not checked yetcites [112] Nowara, E.M., McDuff, D., and Veeraraghavan, A. (2020, January 14–19). A Meta-Analysis of the Impact of Skin Tone and Gender on Non-Contact Photoplethysmography Measurements. Proceedings of the IEEE/CVF Conference on Computer Vision and Pattern Recognition Workshops, Seattle, WA, USA. source

In the paper

“The difference in compliance between the arterial and venous systems, as well as the lower-pressure gradient, translates to the relative amplitude of the AC venous component of the PPG being smaller than the AC arterial component [125].”

What the sources say

• Not checked yetcites [125] Shelley, 2005, The effect of venous pulsation on the forehead pulse oximeter wave form as a possible source of error in Spo2 calculation, Anesth. Analg. source

In the paper

“Physiologically, respiration and cardiac output are inherently linked, as an increase in respiration rate can directly affect the variation of heart rate through the nervous system reduced inhibitory control [118].”

What the sources say

• Not checked yetcites [118] Meredith, 2012, Photoplethysmographic derivation of respiratory rate: A review of relevant physiology, J. Med. Eng. Technol. source

In the paper

“However, the results indicate that respiration rate extraction is more precise when performed on ECG data as opposed to PPG.”

What the sources say

• Not checked yetcites [117] Charlton, 2016, An assessment of algorithms to estimate respiratory rate from the electrocardiogram and photoplethysmogram, Physiol. Meas. source

In the paper

“Interestingly, algorithms operating on the time domain provided more accurate results than frequency domain extraction [117].”

What the sources say

• Not checked yetcites [117] Charlton, 2016, An assessment of algorithms to estimate respiratory rate from the electrocardiogram and photoplethysmogram, Physiol. Meas. source

In the paper

“Modulation of the baseline due to respiration presents itself as a superposition of the PPG signal corresponding to the cardiac cycle and a lower-frequency sinusoidal waveform [117].”

What the sources say

• Not checked yetcites [117] Charlton, 2016, An assessment of algorithms to estimate respiratory rate from the electrocardiogram and photoplethysmogram, Physiol. Meas. source

In the paper

“RIFV induces a phase shift between each cycle by elongating or squeezing each wave.”

What the sources say

• Not checked yetcites [115] Dehkordi, 2018, Extracting instantaneous respiratory rate from multiple photoplethysmogram respiratory-induced variations, Front. Physiol. source
• Not checked yetcites [121] Addison, 2012, Developing an algorithm for pulse oximetry derived respiratory rate (RR oxi): A healthy volunteer study, J. Clin. Monit. Comput. source

In the paper

“From experimentation results, it is concluded that respiration signal amplitude has the strongest influence on respiratory-induced variation of the PPG signal.”

What the sources say

• Not checked yetcites [122] Li, 2010, Comparison of respiratory-induced variations in photoplethysmographic signals, Physiol. Meas. source

In the paper

“Dehkordi et al. and Addison et al. identified the three significant variations to the PPG signal by respiratory rate after analyzing PPG from 139 healthy adults (67–18 years old) and the Capnobase dataset [115,121]. These variations are respiratory-induced intensity variation (RIIV), respiratory-induced amplitude variation (RIAV), and respiratory induced frequency variation (RIFV), as shown in FIG”

What the sources say

• Not checked yetcites [115] Dehkordi, 2018, Extracting instantaneous respiratory rate from multiple photoplethysmogram respiratory-induced variations, Front. Physiol. source
• Not checked yetcites [121] Addison, 2012, Developing an algorithm for pulse oximetry derived respiratory rate (RR oxi): A healthy volunteer study, J. Clin. Monit. Comput. source

In the paper

“The presence of the venous pulsation associated peak in the diastolic phase of the PPG waveform can present errors with identification and quantification of the dicrotic notch and diastolic features.”

In the paper

“Shifts in blood volume in the venous system due to changes in respiratory behavior will cause a corresponding change to the baseline amplitude of the PPG signal as blood volume increases and decreases [120].”

What the sources say

• Not checked yetcites [120] Ysehak Abay, T., Shafqat, K., and Kyriacou, P.A. (2019). Perfusion Changes at the Forehead Measured by Photoplethysmography during a Head-Down Tilt Protocol. Biosensors, 9. source

In the paper

“It appears that changes to diastolic amplitude due to the presence of the venous waveform can increase diastolic phase amplitude up to 40% of the total alternating amplitude of the PPG signal.”

What the sources say

• Not checked yetcites [116] Shelley, 1993, The detection of peripheral venous pulsation using the pulse oximeter as a plethysmograph, J. Clin. Monit. source

In the paper

“When comparing the accuracy of commercial devices as done by Shcherbina et al., a significantly higher device measurement error is seen in males than females for all devices [109].”

What the sources say

• Not checked yetcites [109] Shcherbina, A., Mattsson, C.M., Waggott, D., Salisbury, H., Christle, J.W., Hastie, T., Wheeler, M.T., and Ashley, E.A. (2017). Accuracy in wrist-worn, sensor-based measurements of heart rate and energy expenditure in a diverse cohort. J. Pers. Med., 7. source

In the paper

“RIIV modulates the DC baseline of the PPG curve superimposing the wave on top of a low frequency sinusoid.”

What the sources say

• Not checked yetcites [115] Dehkordi, 2018, Extracting instantaneous respiratory rate from multiple photoplethysmogram respiratory-induced variations, Front. Physiol. source
• Not checked yetcites [121] Addison, 2012, Developing an algorithm for pulse oximetry derived respiratory rate (RR oxi): A healthy volunteer study, J. Clin. Monit. Comput. source

In the paper

“The pulse oximetry values were both lower in cases where CVP contributed to the signal.”

What the sources say

• Not checked yetcites [125] Shelley, 2005, The effect of venous pulsation on the forehead pulse oximeter wave form as a possible source of error in Spo2 calculation, Anesth. Analg. source
• Not checked yetcites [127] Sami, 1991, Central venous pulsations associated with a falsely low oxygen saturation measured by pulse oximetry, J. Clin. Monit. source

In the paper

“It is reported that the lower-frequency wave manifestation in the total PPG signal can be attributed to the venous vascular system [119].”

What the sources say

• Not checked yetcites [119] Alian, 2014, Photoplethysmography, Best Pract. Res. Clin. Anaesthesiol. source

In the paper

“Venous pressure can also add low-frequency noise to the PPG waveform, the same effects seen by respiration, which is caused by venous anatomy [129,130,131]. Often between 0 and 0.5 Hz, this noise contributes to be baseline oscillations.”

What the sources say

• Not checked yetcites [129] Walton, 2010, Measuring venous oxygenation using the photoplethysmograph waveform, J. Clin. Monit. Comput. source
• Not checked yetcites [130] Nilsson, 2007, Combined photoplethysmographic monitoring of respiration rate and pulse: A comparison between different measurement sites in spontaneously breathing subjects, Acta Anaesthesiol. Scand. source
• Not checked yetcites [131] Nilsson, 2003, Macrocirculation is not the sole determinant of respiratory induced variations in the reflection mode photoplethysmographic signal, Physiol. Meas. source

In the paper

“This is one of the most significant sources of error in heart rate measurements using PPG, even though respiratory rate is the most sensitive vital sign, often used as an indicator of clinical deterioration [117].”

What the sources say

• Not checked yetcites [117] Charlton, 2016, An assessment of algorithms to estimate respiratory rate from the electrocardiogram and photoplethysmogram, Physiol. Meas. source

In the paper

“Moreover, as the respiratory rate increases, the respiratory signal decreases, indicating that higher rates of respiration leads to lower fluctuations in respiratory influence changes to PPG [122].”

What the sources say

• Not checked yetcites [122] Li, 2010, Comparison of respiratory-induced variations in photoplethysmographic signals, Physiol. Meas. source

In the paper

“For example, Iacopi et al. found the skin of the foot of obese patients to be about 7 degrees Celsius higher than the foot of nonobese patients, and thus caution should be noted if PPG is measured at the foot [60].”

What the sources say

• Not checked yetcites [60] Iacopi, 2020, Using Skin Bioengineering to Highlight How Weight and Diabetes Mellitus Modify the Skin in the Lower Limbs of Super-Obese Patients, Diabetes Metab. Syndr. Obes. Targets Ther. source

In the paper

“However, many of these studies suggested that external temperature has a much more significant effect on the PPG waveform.”

In the paper

“Askaraian et al. reported that while measuring PPG with a submerged finger of 20 healthy volunteers aged 18 to 80 years old, they found that a drop in 40 degrees Celsius contributed to a 12 dB drop in PPG signal amplitude [157].”

What the sources say

• Not checked yetcites [157] Askarian, B., Jung, K., and Chong, J.W. (2019). Monitoring of Heart Rate from Photoplethysmographic Signals Using a Samsung Galaxy Note8 in Underwater Environments. Sensors, 19. source

In the paper

“Anatomical variations in parameters such as skin thickness and blood basal perfusion will lead to changes in the AC and DC amplitude, and the duration of a PPG waveform.”

In the paper

“Overall, it has been found that locations on the head and finger provide the greatest PPG amplitude [95,145].”

What the sources say

• Not checked yetcites [95] Hartmann, 2019, Quantitative comparison of photoplethysmographic waveform characteristics: Effect of measurement site, Front. Physiol. source
• Not checked yetcites [145] Lee, Y., Han, H., and Kim, J. (2008, January 14–17). Influence of motion artifacts on photoplethysmographic signals for measuring pulse rates. Proceedings of the 2008 International Conference on Control, Automation and Systems, Seoul, Korea.

In the paper

“Khan et al. investigated the effects of finger temperature on PPG signal on 20 healthy adult volunteers (24.5 ± 4.1 years of age), reporting a reduction in PPG Root Mean Square values as ambient temperatures went from warm to cold temperatures [154].”

What the sources say

• Not checked yetcites [154] Khan, 2015, Investigating the effects of temperature on photoplethysmography, IFAC Pap. Online source

In the paper

“Jeong et al. found that local skin surface temperature changes affect PPG components of 16 healthy subjects aged 23–30 years (26 ± 2.1) and BMI ranging between 20 and 26 (23 ± 4.8), hence recommending that temperature be monitored in order to reliably evaluate cardiovascular parameters [158].”

What the sources say

• Not checked yetcites [158] Jeong, 2014, Effects of skin surface temperature on photoplethysmograph, J. Healthc. Eng. source

In the paper

“For example, many of the photodetectors are subject to creating artifacts from sensor-tissue motion and sensor deformation changes [160].”

What the sources say

• Not checked yetcites [160] Wijshoff, 2012, Reducing motion artifacts in photoplethysmograms by using relative sensor motion: Phantom study, J. Biomed. Opt. source

In the paper

“Lindberg et al. found that PPG amplitude showed a direct response to skin temperature elevation, especially in the finger skin of 10 Caucasian young adults (aged 22–30) using three different arrangements of PPG probes with different source/detector separations [153].”

What the sources say

• Not checked yetcites [153] Lindberg, 1991, Photoplethysmography, Med. Biol. Eng. Comput. source

In the paper

“They report significant reductions in PPG pulse wave amplitudes (p < 0.0001) between the groups both at rest and after cooling the finger to 16 degrees Celsius [156].”

What the sources say

• Not checked yetcites [156] Hahn, 1999, Local cold exposure test with a new arterial photoplethysmographic sensor in healthy controls and patients with secondary Raynaud’s phenomenon, Microvasc. Res. source

In the paper

“Thus, beyond variation found within individuals and changing physiologies, external factors can also significantly impact RCIM PPG.”

In the paper

“During sedentary moments, respiration rate, thermoregulation, and sympathetic nervous system activity cause the DC baseline to wander [162].”

What the sources say

• Not checked yetcites [162] Maeda, 2011, Relationship between measurement site and motion artifacts in wearable reflected photoplethysmography, J. Med. Syst. source

In the paper

“Overall, the literature has demonstrated that different anatomies demonstrate PPGs with varying waveforms.”

In the paper

“Due to the presence of a distinct dicrotic notch and low skin thickness, the finger is likely to be a successful anatomy that can be used with consistency.”

In the paper

“Heart rate tracking devices will pick up on the cyclical motion and mistake it for the cardiac cycle, causing false readings as well [46].”

What the sources say

• Not checked yetcites [46] Bent, 2020, Investigating sources of inaccuracy in wearable optical heart rate sensors, NPJ Digit. Med. source