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# Can Wearable Devices Accurately Measure Heart Rate Variability? A Systematic Review

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Background: A growing number of wearable devices claim to provide accurate, cheap and easily applicable heart rate variability (HRV) indices. This is mainly accomplished by using wearable photoplethysmography (PPG) and/or electrocardiography (ECG), through simple and non-invasive techniques, as a substitute of the gold standard RR interval estimation through electrocardiogram. Although the agreement between pulse rate variability (PRV) and HRV has been evaluated in the literature, the reported results are still inconclusive especially when using wearable devices. Aim: The purpose of this systematic review is to investigate if wearable devices provide a reliable and precise measurement of classic HRV parameters in rest as well as during exercise. Materials and methods: A search strategy was implemented to retrieve relevant articles from MEDLINE and SCOPUS databases, as well as, through internet search. The 308 articles retrieved were reviewed for further evaluation according to the predetermined inclusion/exclusion criteria. Results: Eighteen studies were included. Sixteen of them integrated ECG - HRV technology and two of them PPG - PRV technology. All of them examined wearable devices accuracy in RV detection during rest, while only eight of them during exercise. The correlation between classic ECG derived HRV and the wearable RV ranged from very good to excellent during rest, yet it declined progressively as exercise level increased. Conclusions: Wearable devices may provide a promising alternative solution for measuring RV. However, more robust studies in non-stationary conditions are needed using appropriate methodology in terms of number of subjects involved, acquisition and analysis techniques implied.
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Folia Medica I 2018 I Vol. 60 I No. 1
DOI: 10.2478/folmed-2018-0012
REVIEW
Can Wearable Devices Accurately Measure Heart Rate Variability?
A Systematic Review
Konstantinos Georgiou1, Andreas V. Larentzakis2, Nehal N. Khamis3, Ghadah I. Alsuhaibani3,
Yasser A. Alaska3, Elias J. Giallafos4
1 Department of Biological Chemistry, Faculty of Medicine, National and Kapodistrian University of Athens, Athens, Greece
2 1st Propaedeutic Surgical Clinic, Medical School, National and Kapodistrian University of Athens, Athens, Greece
3 Clinical Skills & Simulation Center, Department of Medical Education, College of Medicine, King Saud University, Riyadh, KSA
4 Medical Physics Simulation Center, Medical School, National and Kapodistrian University of Athens, Athens, Greece
Correspondence:
Konstantinos Georgiou, Depart-
ment of Biological Chemistry,
Faculty of Medicine, National and
Kapodistrian University of Athens,
75 Mikras Asias Str., Goudi, 11527
Athens, Greece
E-mail: kongeorgiou@med.uoa.gr
Tel: +306942066216
Accepted: 19 Jan 2018
Published Online: 31 Jan 2018
Published: 30 March 2018
Key words: heart rate, heart rate
variability, wearable, photople-
thysmography
Citation: Georgiou K, Larentzakis
AV, Khamis NN, Alsuhaibani GI,
Alaska YA, Giallafos EJ. Can wear-
able devices accurately measure
heart rate variability? A system-
atic review. Folia Med (Plovdiv)
2018;60(1):7-20.
doi: 10.2478/folmed-2018-0012
Background: A growing number of wearable devices claim to provide accurate,
cheap and easily applicable heart rate variability (HRV) indices. This is mainly ac-
complished by using wearable photoplethysmography (PPG) and/or electrocar-
diography (ECG), through simple and non-invasive techniques, as a substitute of
the gold standard RR interval estimation through electrocardiogram. Although
the agreement between pulse rate variability (PRV) and HRV has been evaluated
in the literature, the reported results are still inconclusive especially when using
wearable devices.
Aim: The purpose of this systematic review is to investigate if wearable devices
provide a reliable and precise measurement of classic HRV parameters in rest as
well as during exercise.
Materials and methods: A search strategy was implemented to retrieve relevant
articles from MEDLINE and SCOPUS databases, as well as, through internet search.
The 308 articles retrieved were reviewed for further evaluation according to the
predetermined inclusion/exclusion criteria.
Results: Eighteen studies were included. Sixteen of them integrated ECG - HRV
technology and two of them PPG - PRV technology. All of them examined wear-
able devices accuracy in RV detection during rest, while only eight of them during
exercise. The correlation between classic ECG derived HRV and the wearable RV
ranged from very good to excellent during rest, yet it declined progressively as
exercise level increased.
Conclusions: Wearable devices may provide a promising alternative solution for
measuring RV. However, more robust studies in non-stationary conditions are
needed using appropriate methodology in terms of number of subjects involved,
acquisition and analysis techniques implied.
BACKGROUND
Monitoring and analysis of heart rate (HR) pro-
vide valuable information regarding health status
and have been extensively investigated in various
activities of healthy subjects as well as in patients
suffering from various diseases.1,2 Heart rate vari-
ability (HRV) has emerged as a non-invasive tool
to estimate the vagal activity in several conditions,
including monitoring of athletic responses to training.
Decreased HRV has been reported as a predictive
factor for adverse outcomes in disease states and
has been found to be associated with fatigue, stress,
and even burnout during athletic performance.3-5
High HRV is an indication of a better general health
status as it allows better adjustment to external and
internal stimuli.6
Due to the fact that traditional HRV recording
methods, such as using electrocardiography (ECG)
and specialized software, often involve expensive
equipment, which is primarily found in research
laboratories, alternative methods have been used,
yet, with variable results. Photoplethysmography
(PPG) is a simple and low-cost method used to
detect volumetric changes in the peripheral blood
circulation at the skin surface.7
In recent years, several wearable pulse rate moni-
8
K. Georgiou et al
Folia Medica I 2018 I Vol. 60 I No. 1
tors using PPG technology have been developed and
have become widely available. The general concept
of operation of these small, robust and user-friendly
devices is that they contain sensors which reliably
monitor minor changes in the intensity of light from
high intensity light emitting diodes (LEDs) that is
transmitted through or re ected from the human
tissues. Although they have obvious advantages
over the classical ambulatory ECG recording, the
fact that they use PPG, i.e. a different detecting
approach, raises the question of how much accurate
and reliable are their results when compared to the
gold standard ECG method?
The purpose of this systematic review is to
present the available literature comparing the ECG
derived RR and HRV with that of the wearable
commercially available devices in terms of accuracy
and reliability as well as to reveal their strengths
and limitations in the everyday clinical practice.
MATERIALS AND METHODS
SEARCH STRATEGY
This systematic review was conducted by search-
ing medical literature in MEDLINE and SCOPUS.
The search was guided by the Preferred Reporting
Items for Systematic Reviews and Meta-Analyses
(PRISMA) statement in conjunction with the PRIS-
MA explanation and elaboration document.8,9 The
last search was conducted in April 2017. All the
retrieved article titles and abstracts were screened for
relevant manuscripts. Full texts were then retrieved
for the relevant articles and these were thoroughly
read to judge if they meet the inclusion criteria of
the current systematic review.
Medical Subject Heading (MeSH) terms and text
words were used based on the following search
strategy:
Group A terms: (HRV) OR heart rate variability
Group B terms: ((((smart) OR Smartphone$) OR wearable$) OR phone$OR plethysmography OR photoplethysmography OR impedance) Group C terms: (((((holter) OR continuous ECG) OR continuous electrocardiogram) OR con- tinuous electrocardiography) OR ambulatory ECG) OR ambulatory electrocardiogram OR ambulatory electrocardiography. Group A, group B, and group C were combined and humans’ studies and English language limits were applied. We also searched the reference lists of retrieved full text review manuscripts for the relevant articles (regardless of their publishing date) and included these in our systematic review. Well de ned inclusion/exclusion criteria were applied to lter the retrieved literature. Of the ar- ticles retrieved through the above described search strategy only those that met the following criteria were considered for this systematic review: Inclusion criteria: Studies related to human subjects. Studies reported in full text English language. Studies related to HRV. Studies on HRV detec- tion and/or analysis and/or interpretation and/or ltering were included. Studies that compare ECG/Holter recordings of HRV with any other wearable HRV detection and capture method(s) were included. Original papers were only included; however, reference lists from other kind of manuscripts were used to retrieve any relevant original studies. Exclusion criteria: Studies that did not include HRV were excluded. St udies that did not use commercially available wearable hardware were excluded. In addition, we performed Google search for wearable devices which claimed that they can measure HRV via plethysmography HR and are available on the market. Our search spanned the last 5 years. The devices list (Table 1) was used as an additional retrograde search tool for any relevant studies through manufacturers’ commercial websites. Two of the authors (KEG and AL) independently applied the above described search strategy to re- trieve and screen the articles. Any disagreements were resolved by a third author (AJG) and a nal decision was made accordingly. RESULTS Applying the search strategy described in the methods section, we retrieved 57 articles from MEDLINE and 269 from SCOPUS. Exclusion of duplicates yielded 308 articles. Of these articles, 272 were excluded after title and abstract screening according to the prede ned criteria. Thirty-six articles were selected for full text review. Full text reading resulted in exclusion of further 30 articles, which were found to be either irrelevant (n=28) or non-comparison papers (n=2). The remaining six articles were included in the study. Additionally, 12 relevant peer-reviewed articles were identi ed from the reference list of the reviews and from manufacturers’ commercial websites and added to the study. So, 18 articles were nally included in this systematic review. The ow diagram of the selection process is shown in Fig. 1. Wearable Devices Accuracy of HRV Estimation 9Folia Medica I 2018 I Vol. 60 I No. 1 Table 1. Capable to measure HRV wearable models: sensor location, release year and connectivity type Model Sensor site Company Release Year Connectivity 1 4IIII VIIIIVA chest 4IIII 2013 B & A 2 60beat HR Monitor chest 60beat 2016 B & A 3 HRM Blue chest BlueLeza 2016 B & A 4 Dash Earphones ear Bragi 2014 B 5 TP3 chest Cardiosport 2015 B & A 6 Hexoskin torso Carre Technologies 2014 B & USB 7 Empatica E4 Wristband wrist Empatica 2016 B 8 R2 Smart Fitness HRM Wristband chest Cositea 2016 B & A 9 EQ02 LifeMonitor chest Equivital 2012 B, P & USB 10 Forerunner 935 wrist Garmin 2016 B & A 11 HRM Tri wrist Garmin 2016 B & A 12 Premium Heart Rate Monitor chest Garmin 2016 B & A 13 Vívoactive HR+ wrist Garmin 2015 B & A 14 910XT wrist Garmin 2016 B, P & A 15 920XT wrist Garmin 2016 B, P & A 16 HeartMath emWave Pro ear HeartMath 2015 B & A 17 Athos torso, legs & thighs Mad Apparel 2015 B 18 HxM Smart HR chest Medronic 2017 B & Gateway 19 Alpha 2 wrist Mio 2015 B & A 20 Oxstren hand Oxstren 2015 B 21 H7 Heart Rate Monitor chest Polar 2012 B & WiFi 22 H10 chest Polar 2016 B & WiFi 23 QardioCore chest Qardio 2017 B 24 SmartBand 2 wrist Sony 2016 B 25 Smart Sensor chest Suunto 2017 B & A 26 Spartan Sport wrist Suunto 2017 B & A 27 DGYAO® Mobile HR Monitor ear Top Yao 2016 B 28 Tickr Heart Rate Monitor chest Wahoo Fitness 2015 B & A 29 Tickr X Workout Tracker chest Wahoo Fitness 2017 B & A 30 WHOOP Strap 2.0 wrist Whoop 2016 B 31 Tinké Fitness & Wellness Tracker ngers Zensorium 2016 B & A B: Bluetooth, A: ANT+, P: proprietary 10 K. Georgiou et al Folia Medica I 2018 I Vol. 60 I No. 1 Figure 1. Flow diagram. Table 2 summarizes the quantitative and quali- tative methodology used and the categorization (measurement, position, location, sample number, technology used) as well as the main ndings of the included studies. The total number of subjects involved in the included studies was 686, with one study hav- ing 339 subjects.10 All 18 articles have examined healthy subjects except one: Vasconcellos et al. (2015) who studied obese adolescents.11 Also, one study examined children.12 In most of the articles (n=16), a chest device was used,10-25 while a n- ger device was used in two studies26,27. Sixteen of the studies utilized a similar to ECG - HRV technology10-25 and the remaining two integrated PPG - PRV technologies26,27. REST All studies examined the HRV/PRV at rest as baseline. Overall, agreement between the several indices of HRV and PPG as measured by Holter and wearable devices, respectively, was very good to excellent (ranging from 0.85 to 0.99). The RR interval correlation ranged from 0.91 to 0.999. Moreover, in two studies,12,15 the error rate in detection of R waves was evaluated and found to range between 0.28 and 0.4%. This was estimated as an accepted ratio. Regarding time domain indi- ces of HRV, correlations ranged from 0.98 to 0.99, while in the frequency domain the correlation was Wearable Devices Accuracy of HRV Estimation 11Folia Medica I 2018 I Vol. 60 I No. 1 Table 2. Results # Study Rest Supine Upright / Sitting Walking / Running / Exercise Wearable device Location n Wearable device Technology Comparison Details 1 Akintola et al., 2016 Y Y Y Y Chest (Belt) 18 ECG Correlation depends on artifacts Average artifact % 19%. AMV (r: 0.967), SDNN (0.393), RMSSD (0.285), SDANN (0.680), pnn50 (0.982). 2 Esco & Flatt 2017 Y Y Y N Finger (silicone sheath) 30 PPG Good agreement LOA Ln RMSSD constant error ± SD: -0.13 ± 2.83 for the supine values, -0.94 ± 3.47 for the seated values, -1.37 ± 3.56 for the standing values. (r values from 0.98 to 0.99). 3 Flatt & Esco, 2013 Y Y N N Chest (Belt) 25 ECG Total agreement No signi cant difference, correlation nearly perfect for RMSSD (r: 0.99). 4 Gamelin et al, 2006 Y Y Y N Chest (Belt) 20 ECG Good agreement Supine vs. standing: differences for uncorrected & corrected RR coef cient correlation: 0.88 & 0.91 for supine & standing. No differences except RMS- SD, SD1 in standing. Detection Error Rate of R waves: 0.4%. 5 Gamelin et al, 2008 Y Y N N Chest (Belt) 12 ECG No difference Correlation between ECG & Polar RR intervals (corrected & uncorrected) was 0.80. No signi cant differences for Time Domain, FFT & Poincare plot except for SD2. R waves detection er- ror rate: 0.28%. 12 K. Georgiou et al Folia Medica I 2018 I Vol. 60 I No. 1 6 Giles et al, 2016 Y Y Y N Chest (Belt) 18 ECG Accurate. Good agreement No signi cant differences for SDNN, RMSSD, pNN50, VLF, LF, HF, nnLF 7 Heathers, 2013 Y N Y Y Finger (silicone sheath) 20 PPG Good agreement at rest. Mildly reduced agreement during exercise. Experiment 1: Close agreement – small overall bias & acceptable limits of agreement. RR & PP correlation coef- cient (0.988 – 0.999). All HRV from SPRV > than ECG. Experiment 2: Close agreement be- tween RR & PP (highest at rest, r: 0.993 – 0.997, slightly attenuated in exercise (0.965 – 0.998). 8Hernando et al, 2016 Y N Y Y Chest (Belt) 23 ECG As exercise increases, correla- tion decreases 19/23 had high correlation at rest (r> 0.8). Discrepancy increased from 1.67% (at rest) 4.8% at the exercise peak. As exercise increases, reliability & agree- ment indices drop below 0.5. 9Hong et al, 2009 N N N Y Chest (Bioshirt) 18 ECG As exercise increases, cor- relation decreases (artefacts) Coef cient correlation of HRV (r2: 0.965 or higher), HF, LF 10 Kingsley et al, 2005 Y N Y Y Chest (Belt) 8 ECG High agreement higher at rest. Reduced agree- ment at exercise. Short relationship between RR internals during exercise. Similar results obtained for the RR internal but signi cant differ- ences occurred for HRV indices. 11 Nunan et al, 2008 Y Y N N Chest (Belt) 33 ECG Accurate / Good agreement No signi cant differences for HRV indi- ces – LOA for mean RR, LFnn, HFnn, LF/HF 12 Nunan et al, 2009 Y Y N N Chest (Belt) 33 ECG Good agreement Not for all indi- cators Correlation coef cient: 0.99, 0.86, 0.85 for mean RR, LFnn, HFnn respectfully. Near perfect correlation for SDNN, RMSSD (0.99, 0.37). Good correlation for LF (0.92), HF (0.94), LF/HF (0.87). All measures of HRV ranged from 0.85- 0.99. Wearable Devices Accuracy of HRV Estimation 13Folia Medica I 2018 I Vol. 60 I No. 1 LOA: level of agreement 13 Plews et al, 2017 Y N Chest (Belt) 29 ECG Acceptable agree- ment Almost perfect correlation all HRV indi- ces (RMSSD) 14 Romagnoli et al, 2014 Y N Y Y Chest (Vest) 12 ECG Good agreement for RR / unequal results for HRV. Mean RR, SDNN & SD2, excellent LOA. RMSSD, HF, LF/HF, HF & SD1 worst LOA. 15 Vanderlei et al, 2008 Y N Y Y Chest (Belt) 15 ECG As exercise increases, correla- tion decreases. HRV indices assessed: PNN50, RMSSD, LFnu, HFnu, LF/HF. 16 Vasconcellos et al, 2015 Y Y N Chest (Belt) 14 ECG Very good cor- relation Moderate to strong agreement (0.68-0.98) for HR, RR, pNN50, rMSSD, LF, HF, LF/HF. In general moderate agreement for frequencies domain measures. 17 Wallen et al, 2012 Y Y N N Chest (Belt) 339 ECG Good to excellent agreement 328 subjects. Gender & age dependent: females especially > 60 yrs old present reduced correlation). ICC coef cients in women than men more pronounced in SDNN.HRV from Polar Gender + Age dependent. ICC > 0.8 on all HRV param- eters among men. Agreement moderate on all HRV among men. 18 Weippert et al, 2010 Y Y N Y Chest (Belt) 19 ECG Limited agree- ment Good correlation (narrow LOA & small bias). ICC for HRV frequencies were high, however in most cases LOA showed unacceptable discrepancies. 14 K. Georgiou et al Folia Medica I 2018 I Vol. 60 I No. 1 found to range from 0.85 to 0.94. Finally, in the non-linear multivariate analysis, the correlation was found to be > 0.9. EXERCISE Eight out of the 18 examined studies used an exer- cise protocol, with a total of 131 participants. All of them reported that although there was an excellent correlation between HRV and PPG as measured by Holter and wearable devices at rest, this seemed to decrease up to 0.85 as the level of exercise and/or motion increased. Overall, RR agreement was moder- ate to excellent ranging from 0.786 to 1. Regarding time domain, HRV parameters correlation was found to range from 0.786 (at the exercise peak) to 1 (rest and exercise 1st phase). Similar pattern occurred in the frequency domain HRV parameters where the correlation was found to range from 0.8 to 1. Also, RR to pulse pressure (PP) wave correlation was ranging from 0.8 to 0.998. DISCUSSION In recent years, technology advances have been used to capture HRV through wearable devices during daily activities. The accuracy of these devices versus classical methods like ECG is still under evaluation. This systematic review aims to present the avail- able relevant literature and discuss their ndings, their limitations as well as to provide possible explanations for these ndings. It is important to realize that the basic difference between PPG and ECG is the captured signal per se: the electrical activity of the heart is depicted by ECG, whereas the PPG is a mechanical signal measuring the propagation of the peripheral pulse wave. Therefore, the time of propagation of the PP wave from the heart to the distal arterioles is called pulse transit time (PTT). It is a measure of the time that elapsed between the R-wave of QRS complex in the ECG and the arrival point to PPG device.28 Several studies have shown that PTT seems to be a surrogate marker of ANS in parallel to HRV29 and that PTT is dependent on the properties of the pulse wave velocity, the vascular path from the heart to the location of the detector and is negatively correlated with blood pressure, arterial stiffness and age.7 Therefore, we will discuss HRV and PPG fun- damentals as well as the factors affecting them in order to elaborate PPG versus ECG for HRV measurement. HRV FUNDAMENTALS HRV analysis is a widely available and accurate non-invasive technique used as a quantitative assess- ment tool of the autonomic nervous system (ANS) function.30 Studies have shown that reduced HRV indices, as assessed by the RR interval analysis, is associated with increased cardiovascular morbidity and mortality in patients with various diseases as well as in the general population.31 In addition, heart rate and HRV analysis have been used for estimation of mental stress and athlete’s tness levels, fatigue and overload.5 The quanti cation of ANS function is feasible by calculating several HRV parameters according to time-domain, frequency-domain and nonlinear analysis of consecutive RR intervals.1 These indices represent different components of the sympathetic and/or parasympathetic system of the ANS. For instance, the high frequency (HF) component de- rived by the frequency domain analysis represents the parasympathetic activity, while the LF/HF ratio represents the balance of sympathetic to parasym- pathetic activity.32 However, it is important to realize that these HRV indices depends on the recording quality, the subject’s activity during the recording, the exclu- sion of artifacts, the detection of arrhythmic beats and the recording duration ranging from seconds (short term HRV) to even days (long term). Some of these indices, like the root mean square of standard deviation (RMSSD) of RR interval can be calculated from a 10-second session while others need more than one-hour recording time.33 Despite being accepted as gold standard methods for RR interval monitoring and analysis, both the classical ECG and the ambulatory Holter monitoring have several drawbacks regarding the proper and accurate detection of RR intervals. For example, patients with tremor or elderly patients with fragile skin have bad quality of recordings with a lot of noise and artifacts.34 Similarly, other factors such as surface electromyography, increased electrode impedance, respiration induced baseline drift, and electrode contact movement can cause noise and motion artifacts.7 Additionally, morphological varia- tions in the ECG waveform and heterogeneity in the QRS complex can often make dif cult to identify the RR interval.35 Another limitation can be the need for the presence of a specialized technician/ doctor, thus increasing the cost and accordingly decreasing the wide applicability. Finally, a reported drawback in ECG wearable devices that do not record standard ECG derivations is their inability Wearable Devices Accuracy of HRV Estimation 15Folia Medica I 2018 I Vol. 60 I No. 1 to distinguish some arrhythmias and ectopic beats.10 PPG FUNDAMENTALS Photoplethysmography (PPG), a cheap, simple and widespread technology has been used as an alternative approach to obtain HRV indices.7 The PPG based devices have a sensor that uses infrared emitter and a detector. This emitter is integrated to a probe which is comfortable to wear in stable places of the body that are rich in microcirculation. Thus, the blood volume changes in the microvascu- lar bed which are synchronous to the heartbeat can be traced without the inconveniences of electrode installation or the need to undress the examinee.1 The simplicity of the technique, cost-effectiveness, easy signal acquisition and remote monitoring are the main and obvious advantages of the PPG versus the gold standard ECG. Therefore, PPG is often used in conditions of measurements where mobil- ity, simplicity, time ef ciency, exibility and low cost are of paramount importance, e.g., in exercise monitoring, every day motion, monitoring of the elderly, or with disabled patients, etc.7,36 In the relevant literature of the wearable PPG devices the terms ‘heart rate’ and ‘pulse rate’ are frequently used interchangeably. Also, the term ‘pulse rate variability’ (PRV), which is derived from PPG, has been suggested as a potential ana- log of HRV.37,38 As PRV is further processed identically to HRV, the derived parameters can be extracted from both methods such as the standard deviation of normal to normal (SDNN) R–R intervals (NN), root mean square of successive differences between adjacent NN intervals (RMSSD), proportion of NN50 in total NN intervals (pNN50), low frequency (LF) power, high frequency (HF) power and LF/ HF ratio (LF/HF). Recently, latest technology smart phones ap- plications with wearable devices26,27 use PPG for assessing HRV as an alternative to ECG monitoring in clinical research. More information is available in Table 2. WAVELENGTH USED PPG uses low intensity infrared or green light, which are more strongly absorbed by the blood than the surrounding tissues. It has been shown that 530 nm light (green) PPG showed higher accuracy of pulse rate detection than the 645 (red) and 470 (blue) nm light for monitoring HR.39 MEASUREMENT SITE A at skin surface with rich microvasculature is required to rmly attach the PPG sensor to obtain an accurate measurement. As such, the usual mea- surement sites for wearable PPGs are the wrist and the chest. Most of the wearable devices are placed on the wrist and considerably fewer of them on the chest. However, it is worth noting that there is also a bunch of quasi-wearable devices attached to either the ear lobe or the nger which compared HRV to PRV parameters. The ear is chosen as a measuring site because it is a natural anchoring point, and it is discrete since the device can be partially hidden by hair. Weinschenk et al.28 compared PPG to ECG HRV measurements in deep breathing test in 343 female subjects in resting conditions and using appropri- ate parameters they found an excellent correlation. Finger tips have also been used as a measuring site but only in stationary conditions: it has been shown that HRV derived from ngertip PPG had an excellent correlation to ECG in stationary con- ditions.38 However, a comparative study of nger derived PRV and HRV in healthy subjects40 using a stationary equipment found a poor correlation and suggested that nger derived PRV might not be suitable to substitute ECG derived HRV, as has been mentioned by other authors, too41. The use of a smart phone camera as PPG sensor can provide an acceptable agreement for some HRV indices when compared with ECG,22 but there are increasing differences in HRV and HR detection in the setting of movements or exercise42. This is because the application algorithm is cancelled in the ltering process of inaccurate signals as well as due to the dif culty of stabilizing the ngertip on the camera while exercising. PARAMETERS INVOLVED IN PPG MEASUREMENTS Several parameters have to be considered when interpreting PPG measurements. These include: 1. Motion artifact: Special attention must be ex- ercised during data PPG acquisition to eliminate motion-induced artifacts.7 The contact force between the site and the sensor should be considered as PPG is vulnerable to such type of artifacts. However, despite the importance of this factor, we found only one study in 16 male ischemic patients measuring the accuracy of a smart phone derived pulse rate versus ECG, where an excellent correlation was found at rest, which was slightly deteriorated during exercise.42 2. Respiration: Since respiration alters the intra- thoracic pressure and causes blood ow variations 16 K. Georgiou et al Folia Medica I 2018 I Vol. 60 I No. 1 in both the veins the DC component of the PPG waveform shows minor changes with respiration.43 Thus, it has been shown that the short-term variabil- ity (RMSSD, SD1, and HF) and LF/HF agreement between PRV and HRV deteriorates as a result of the vulnerability to respiratory changes.44 3. Age, gender and environmental factors: Normal HRV values for various age and gender groups are still not available in the literature. However, it is well known that the elderly have increased arterial stiffness which leads to faster pulse transmission in the periphery and thus pulse transit time (PTT) dif- ferences observed between HRV and PRV could be attributed to aging.45 The reviewed articles, except one,23 involved young population (mean range from 20.9 to 39.2 years). Regarding gender in uence, in our review, just one study showed that measuring HRV at rest was age and gender dependent, the correlation with ECG being lower in women than men and further decreasing in older women.10 As there is no strong evidence provided so far that age and/or gender can play a role in the studied correlations, further studies are needed to investigate these two variables in different populations while using ap- propriate sampling and prospective study design with a longitudinal follow-up.28 Environmental factor effects such as temperature was investigated in one of the studies. This study concluded that ambient temperature could induce a difference in the short-term variables that re ect the parasympathetic activity between HRV and PRV.38 4. Software analysis: Some proprietary software systems for collection and analysis of RV data exist like the PPT5 or the IthleteTM software ap- plication14,26,27 or freely available software may be used e.g. Kubios (http://www.kubios.com)46. According to guidelines2, manual editing should be preferred instead of automated data analysis as automatic lters are known to be unreliable and may potentially introduce errors. In our review only six studies used automated analysis only whether the rest used both manual and automated analysis. 5. Statistical analysis: The Bland-Altman plot must be used to compare the agreement among a new measurement technique with a gold standard, as even a gold standard does not imply to be without error. This plot allows the identi cation of any systematic difference between the measurements.47 In our review only 17 studies used this technique while four studies did not apply the Bland-Altman analysis and therefore only the correlation, but not agreement between the two methods, could be determined from these publications. 6. Sampling rate: The sampling rate is a matter of difference between the two approaches. Sampling rate of PPG is usually 20 Hz much less than that of ECG which is 125 to 250 Hz. This obviously implies weaker ability of the PPG devices for events detection.36 COMPARISON OF PPG VS. ECG FOR HRV MEASUREMENT There are several studies examining the correlation between HRV and PRV with inconclusive results.40 This may be due to different experimental settings or to the absence of standardization of the meth- ods of analysis used.44 It is worth noting that the disagreement between the two methods does not apply to the same extent to all HRV parameters. Additionally, PPG is susceptible to motion artifacts. As such, the accuracy of PRV as obtained from PPG should be interpreted with caution.36 REST Our search revealed that the comparison studies performed in stationary conditions have generally revealed that PRV is a good surrogate of HRV. This is in line with other studies not involving wearable devices, which also found an acceptable agreement between HRV and PRV at sitting and resting positions.48 PRV becomes stronger at a standing than at a supine position, as it re ects the mechanical cou- pling between respiration and thoracic vasculature tone. Therefore, when a subject changes his/her position from supine to upright, even in resting conditions, a PRV divergence from HRV becomes apparent. Additionally, HRV indices derived from PPG data are very sensitive to different factors including noise, artifacts, stature, atherosclerosis, location of sensor and sampling rate. It is probably due to these reasons that some studies comparing the two methods found differences among normal healthy subjects7,36,45 as well as in patients40. EXERCISE There are many non-stationary situations where autonomic balance signi cantly changes like in stress, during motion or exercise. Unfortunately, in such situations where PRV would be more useful as a surrogate measurement of HRV, its clinical value is questionable: a moderate agreement was observed in some studies about the factors affecting the measurements when the subject is exercising or having mental stress i.e. increased noise produc- Wearable Devices Accuracy of HRV Estimation 17Folia Medica I 2018 I Vol. 60 I No. 1 tion, contraction of muscles which are in contact with the sensors, sweating, increased intrathoracic pressure altering the venous return, increased pe- ripheral vasoconstriction and the respiratory effort during exercise. RR interval variables of Bioshirt ECG were compared to those from conventional ECG and found that R-peak detecting capabilities of these two devices were largely similar. However, as the level of exercise was increasing, the correla- tion was decreasing due to artifact production. It must also be noted that a disadvantage of chest band wearable devices during intense exercise is the discomfort that a subject senses as the chest expands with deep breathing.18 Hernando et al. (2016)17 observed that although an agreement between the detected R-peaks and the RR intervals from the Polar wearable and ECG existed, as the exercise intensity level was increased, the discrepancy of the RR pairs Bland-Altman plot also increased. They noticed a good correlation in some but not in all of the HRV indices, due to the disagreement of the relative error of the Polar derived high frequency with that of the ECG, as the level of the exercise increases.17 Akintola et al. (2016) used a chest wearable device detecting ECG and HRV and reported enor- mous amount of artifacts during daily activities in 18 healthy subjects and they concluded that this is a limitation of the wearable device used.13 In agreement with all of the above, another study showed that the limits of agreement were deteriorating as the exercise was intensi ed, implying an in uence of adrenergic input, respiratory effort and unreliable algorithm detection and recording RR ability.19 In contrast to the many negative results reported during intense exercise, there are other research- ers who reported an overall stronger agreement.49 Also, some other research groups reported that PPG yielded higher HRV values.36 However, all these studies involved only a sample size of few subjects. Unfortunately, most of the ndings from our review showed that the correlation was fading out as the level of exercise and/ or motion increases. Furthermore, the data from the reviewed studies are not able to support an in-depth quantitative analysis due to the differences in their methodology. FUTURE DIRECTIONS As wearable healthcare technology and the research of light propagation in human tissues are progressing, it is expected that PPG applications will expand. For instance, there is a growing interest to remotely depict PPG through imaging such as contactless video-photoplethysmography (vPPG)50 or imaging PPG (IPPG)51. It is essential to develop advanced wearable devices with higher accuracy, to minimize motion artifacts as well as improved algorithms to better detect and identify errors that may occur during exercise and higher intensity motion. Additionally, as the availability of wearable devices is expanding, more research is obviously warranted to establish age- and sex-dependent normal PRV values as well as to standardize both acquisition protocols and analytical methods in order to get reliable and accurate results, thus permitting these methods to become a valid sur- rogate for HRV parameters. CONCLUSION Our systematic review revealed that wearable de- vices, especially those using PPG, may provide a promising alternative solution for measuring HRV. However, it is evident that more robust studies in non-stationary conditions are needed with appropriate methodology in terms of number of subjects involved, acquisition and analysis techniques implied, before being able to recommend any of the commercially available devices. Therefore, so far wearable devices can only be used as a surrogate for HRV at resting or mild exercise conditions, as their accuracy fades out with increasing exercise load. REFERENCES 1. 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Exp Ther Med 2015;10(6):2311-8. 45. Gil E, Orini M, Bailón R, et al. Photoplethysmog- raphy pulse rate variability as a surrogate measure- ment of heart rate variability during non-stationary conditions. Physiol Meas 2010;31(9):1271-90. 46. Tarvainen MP, Niskanen JP, Lipponen JA, et al. Kubios HRV–heart rate variability analysis software. Comput Methods Programs Biomed 2014;113(1):210-20. 47. Bland JM, Altman DG. Statistical methods for as- sessing agreement between two methods of clinical measurement. Lancet 1986;1:307-10. 48. Qiu Y, Cai Y, Zhu Y, et al. Poincaré plot analysis for pulse interval extracted from non-contact photople- thysmography. Conf Proc IEEE Eng Med Biol Soc 2005;2:1964-7. 49. Chreiteh S, Belhage B, Hoppe K, et al. Sternal pulse rate variability compared with heart rate variability on healthy subjects. Conf Proc IEEE Eng Med Biol Soc 2014;2014:3394-7. 50. Blackford EB, Estep JR. Effects of frame rate and image resolution on pulse rate measured using multiple camera imaging photoplethysmography. In: B. Gimi, & R. C. Molthen, (Eds.). Proceedings of SPIE 9417, Medical imaging 2015: Biomedical applications in molecular structural and functional imaging. 2015. p. 1-14. 51. Iozzia L, Cerina L, Mainardi L. Relationships be- tween heart-rate variability and pulse-rate variability obtained from video-PPG signal using ZCA. Physiol Meas 2016;37(11):1934-44. 20 K. Georgiou et al Folia Medica I 2018 I Vol. 60 I No. 1 Возможности переносных приборов для точного измерения вари- абельности сердечного ритма. Систематическое обозрение Константинос Георгиу1, Андреас В. Ларенцакис2, Нехал Н. Камис3, Гада И. Алсунхаибани3, Ясер А. Аласка3, Елиас Дж. Джиалафос4 1 Кафедра биохимии, Медицинский факультет, Национальный Афинский университет им. Каподистрия, Афины, Греция 2 Первая клиника хирургической пропедевтики, Медицинский факультет, Национальный Афинский университет им. Каподистрия, Афины, Греция 3 Центр клинических умений и симуляции, Медицинский факультет, Медицинский колледж, Университет короля Сауда, Эр-Рияд, Саудовская Аравия 4 Симуляционный центр медицинской физики, Медицинский факультет, Национальный Афинский университет им. Капо- дистрия, Афины, Греция Адрес для корреспонденции: Константинос Георгиу, Кафедра биохимии, Медицинский фа- культет, Национальный Афин- ский университет им. Капо- дистрия, ул. „Микрас Асиас” 75, Гоуди, 11527 Афины, Греция E-mail: kongeorgiou@med.uoa.gr Тел: +306942066216 Дата получения: 27 декабря 2017 Дата приемки: 19 января 2018 Дата онлайн публикации: 31 января 2018 Дата публикации: 30 марта 2018 Ключевые слова: сердечный ритм, вариабельность сер- дечного ритма, переносные, фотоплетизмография Образец цитирования: Georgiou K, Larentzakis AV, Khamis NN, Alsuhaibani GI, Alaska YA, Giallafos EJ. Can wearable devices accurately measure heart rate variability? A systematic review. Folia Med (Plovdiv) 2018;60(1):7-20 doi: 10.2478/folmed-2018-0012 Введение: Утверждается, что всё большее количество переносных приборов обеспечивает установление точных, недорогих и легко применимых показате- лей вариабельности сердечного ритма (ВСР). В основном это обеспечивается приборами переносной фотоплетизмографии (ФПГ) и/или электрокардиогра- фии (ЭКГ) с помощью простых и неинвазивных методов в качестве заместите- лей золотого стандарта оценки интервала RR с помощью электрокардиограм- мы. Хотя соответствие между вариабельностью частоты пульса (ВЧП) и ВСР было исследовано в литературе, полученные результаты по-прежнему неубе- дительны, особенно в отношении использования переносных устройств. Цель: Целью настоящего систематического обозрения является установление возможностей переносных приборов обеспечивать надёжное и точное изме- рение классических параметров ВСР как в состоянии покоя, так и во время физической нагрузки. Материалы и методы: Была использована стратегия сбора данных для на- хождения соответствующих статей в базах данных MEDLINE и SCOPUS, а также в Интернете. Найденные 308 статей были рассмотрены для дальнейшей оцен- ки в соответствии с заранее определёнными критериями включения / исклю- чения. Результаты: Восемнадцать исследований были включены. В шестнадцати из них применялась технология ЭКГ – ВСР, а в двух из них - технология ФПГ – ВЧП. Все они исследовали точность переносных приборов при измерении СР во время отдыха, а только восемь из них во время физической нагрузки. Корреляция между классической ВСР, полученной от ЭКГ, и переносной ВСР варьировалась от очень хорошей до превосходной во время отдыха, но она постепенно снижалась по мере увеличения нагрузки. Заключение: Переносные приборы могут обеспечить альтернативное реше- ние для измерения ВСР. Тем не менее, необходимы более надёжные исследо- вания в нестационарных условиях с использованием соответствующей ме- тодологии в отношении количества участвующих в исследовании субъектов, использованных методов измерения и анализа. ... The low cost, ease-of-use, smart-phone readiness, accurate SO 2 quantification, real time data quality feedback, and long battery life make prolonged monitoring feasible in low resource settings, including typically medically underserved communities, and enable new community and telehealth applications. ( Georgiou et al., 2018 ), respiratory rate ( Johnston and Mendelson, 2004 ;Pimentel et al., 2015 ;Singh et al., 2020 ) and pulse oximetry-derived peripheral capillary oxygen saturation (SpO 2 ) ( Buekers et al., 2019 ;Huang et al., 2014 ;Kim et al., 2017 ). However, a wearable device that monitors cerebral oxygenation is missing from the market, despite the potential benefits of wearable neuromonitoring available to the general population. ... ... Currently, the short source-detector separation is used to quantify heart rate. In future developments, in addition to being used for scalp contamination reduction, the short separation will be used to quantify reflectance photoplethysmography (rPPG) ( Longmore et al., 2019 ) and heart rate variability ( Georgiou et al., 2018 ). ... Article Full-text available Currently, there is great interest in making neuroimaging widely accessible and thus expanding the sampling population for better understanding and preventing diseases. Use of wearable health devices has skyrocketed in recent years, allowing continuous assessment of physiological parameters in patients and research cohorts. While most health wearables monitor the heart, lungs and skeletal muscles, devices targeting the brain are currently lacking. To promote brain health in the general population, we developed a novel, low-cost wireless cerebral oximeter called FlexNIRS. The device has 4 LEDs and 3 photodiode detectors arranged in a symmetric geometry, which allows for a self-calibrated multi-distance method to recover cerebral hemoglobin oxygenation (SO2) at a rate of 100 Hz. The device is powered by a rechargeable battery and uses Bluetooth Low Energy (BLE) for wireless communication. We developed an Android application for portable data collection and real-time analysis and display. Characterization tests in phantoms and human participants show very low noise (noise-equivalent power <70 fW/√Hz) and robustness of SO2 quantification in vivo. The estimated cost is on the order of$50/unit for 1000 units, and our goal is to share the device with the research community following an open-source model. The low cost, ease-of-use, smart-phone readiness, accurate SO2 quantification, real time data quality feedback, and long battery life make prolonged monitoring feasible in low resource settings, including typically medically underserved communities, and enable new community and telehealth applications.
... Wearable technologies such as wristbands offer real-time data on skin conductance and HRV in the context of daily tasks and are found to be a reliable and promising alternative to laboratorybased methods. 69,70 Research on the utility of wearable technologies for assessing physiological reactivity in naturalistic settings may inform advances in psychotherapy, such as interoceptive awareness training through biofeedback. ...
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... Can cause more artifacts in the data due to movement, although there is some evidence to suggest that chest belts could provide reliable and valid results to measure HRV parameters (Flatt et al., 2017;Stone et al., 2021;Wallen et al., 2012;Weippert et al., 2010). Photoplethysmography (PPG) emWave®; ithlete TM Uses optical techniques (light waves) to infer heart rate from the quantification of volume changes in distal blood flow (Georgiou et al., 2018). Often PPG is used in devices such as watches, smartphone cameras, finger sensors, or earlobe clips. ...
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... In 32/178 patients (18%), less than 50% of the data were interpretable (Figure 3). According to a recent systematic review, although the correlation between HRV as measured by Holter and ECG-based wearable devices was excellent at rest, it declined down to 0.85 during movement or exercise [20]. In our study, mobilization of the study patients was not restricted. ...
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Wearable devices represent one of the most popular trends in health and fitness. Rapid advances in wearable technology present a dizzying display of possible functions: from thermometers and barometers, magnetometers and accelerometers, to oximeters and calorimeters. Consumers and practitioners utilize wearable devices to track outcomes, such as energy expenditure, training load, step count, and heart rate. While some rely on these devices in tandem with more established tools, others lean on wearable technology for health-related outcomes, such as heart rhythm analysis, peripheral oxygen saturation, sleep quality, and caloric expenditure. Given the increasing popularity of wearable devices for both recreation and health initiatives, understanding the strengths and limitations of these technologies is increasingly relevant. Need exists for continued evaluation of the efficacy of wearable devices to accurately and reliably measure purported outcomes. The purposes of this review are (1) to assess the current state of wearable devices using recent research on validity and reliability, (2) to describe existing gaps between physiology and technology, and (3) to offer expert interpretation for the lay and professional audience on how best to approach wearable technology and employ it in the pursuit of health and fitness. Current literature demonstrates inconsistent validity and reliability for various metrics, with algorithms not publicly available or lacking high-quality validation studies. Advancements in wearable technology should consider standardizing validation metrics, providing transparency in used algorithms, and improving how technology can be tailored to individuals. Until then, it is prudent to exercise caution when interpreting metrics reported from consumer-wearable devices.
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In this paper, classical time– and frequency-domain variability indexes obtained by pulse rate variability (PRV) series extracted from video-photoplethysmography signals (vPPG) were compared with heart rate variability (HRV) parameters extracted from ECG signals. The study focuses on the analysis of the changes observed during a rest-to-stand manoeuvre (a mild sympathetic stimulus) performed on 60 young, normal subjects (age: $24\pm 3$ years). The objective is to evaluate if video-derived PRV indexes may replace HRV in the assessment of autonomic responses to external stimulation. Video recordings were performed with a GigE Sony XCG-C30C camera and analyzed offline to extract the vPPG signal. A new method based on zero-phase component analysis (ZCA) was employed in combination with a fully-automatic method for detection and tracking of region of interest (ROI) located on the forehead, the cheek and the nose. Results show an overall agreement between time and frequency domain indexes computed on HRV and PRV series. However, some differences exist between resting and standing conditions. During rest, all the indexes computed on HRV and PRV series were not statistically significantly different (p > 0.05), and showed high correlation (Pearson's r > 0.90). The agreement decreases during standing, especially for the high-frequency, respiration-related parameters such as RMSSD (r = 0.75), pNN50 (r = 0.68) and HF power (r = 0.76). Finally, the power in the LF band (n.u.) was observed to increase significantly during standing by both HRV ($28\pm 14$ versus $45\pm 16$ (n.u.); rest versus standing) and PRV ($26\pm 12$ versus $30\pm 13$ (n.u.); rest versus standing) analysis, but such an increase was lower in PRV parameters than that observed by HRV indexes. These results provide evidence that some differences exist between variability indexes extracted from HRV and video-derived PRV, mainly in the HF band during standing. However, despite these differences video-derived PRV indexes were able to evince the autonomic responses expected by the sympathetic stimulation induced by the rest-to-stand manoeuvre.
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This study aimed to comprehensively investigate the reliability of multiple heart rate variability (HRV) parameters, and to explore the influence of artefact removal and breathing condition on HRV reliability. Resting HRV was collected using Polar Team monitors on forty-one participants (age: 19.9±1.2 years; 28 females, 13 males) during two separate days. Within each session, participants performed 10 minutes each of spontaneous and controlled breathing (randomized order). Kubios HRV analysis software was used to analyze 180s data epochs using "low" or "strong" artefact removal. Relative reliability was assessed using intraclass correlation coefficients (ICC2,1) and absolute reliability was quantified using mean-normalized standard error of measurement (SEM%). Time domain and nonlinear parameters produced poor to good inter-session agreement (ICC:0.34-0.68; SEM%: 11.0-39.0) with "low" artefact removal, regardless of breathing condition. Frequency domain parameters demonstrated fair inter-session agreement during controlled breathing (ICC:0.40-0.45; SEM%: 26.0-70.0), but poor agreement during spontaneous breathing (ICC:0.07-0.13; SEM%: 32.0-81.0). Minimal differences in ICCs were observed between "low" and "strong" artefact removal. Thus, this study provides three practical applications: 1) HRV monitoring is most reliable when using time domain and nonlinear parameters, regardless of breathing or filtering condition, but no single parameter is especially reliable. The large variation and poor inter-session reliability of frequency indices during spontaneous breathing are improved by controlling breathing rate; 2) "Low" artefact removal appears superior to more aggressive artefact removal; and 3) Inter-session differences in HRV measurements <30% may be indicative of normal daily variation rather than true physiologic changes.
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Electrocardiographic artifacts are defined as electrocardiographic alterations, not related to cardiac electrical activity. As a result of artifacts, the components of the electrocardiogram (ECG) such as the baseline and waves can be distorted. Motion artifacts are due to shaking with rhythmic movement. Examples of motion artifacts include tremors with no evident cause, Parkinson’s disease, cerebellar or intention tremor, anxiety, hyperthyroidism, multiple sclerosis, and drugs such as amphetamines, xanthines, lithium, benzodiazepines, or shivering (due to hypothermia, fever (rigor due to shaking), cardiopulmonary resuscitation by chest compression (oscillations of great amplitude) and patients who move their limbs during the test, causing sudden irregularities in the ECG baseline that may resemble premature contractions or interfere with ECG wave shapes, or other supraventricular and ventricular arrhythmias. When the skeletal muscles experience shaking, the ECG is “bombarded” by apparently random electrical activity.
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Background and purpose Acute stress in surgery is ubiquitous and has an immediate impact on surgical performance and patient safety. Surgeons react with several coping strategies; however, they recognise the necessity of formal stress management training. Thus, stress assessment is a direct need. Surgical simulation is a validated standardised training milieu designed to replicate real-life situations. It replicates stress, prevents biases, and provides objective metrics. The complexity of stress mechanisms makes stress measurement difficult to quantify and interpret. This systematic review aims to identify studies that have used acute stress estimation measurements in surgeons or surgical trainees during real operations or surgical simulation, and to collectively present the rationale of these tools, with special emphasis in salivary markers. Methods A search strategy was implemented to retrieve relevant articles from MEDLINE and SCOPUS databases. The 738 articles retrieved were reviewed for further evaluation according to the predetermined inclusion/exclusion criteria. Results Thirty-three studies were included in this systematic review. The methods for acute stress assessment varied greatly among studies with the non-invasive techniques being the most commonly used. Subjective and objective tests for surgeons' acute stress assessment are being presented. Conclusion There is a broad spectrum of acute mental stress assessment tools in the surgical field and simulation and salivary biomarkers have recently gained popularity. There is a need to maintain a consistent methodology in future research, towards a deeper understanding of acute stress in the surgical field.
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Heart rate variability (HRV) analysis during exercise is an interesting non-invasive tool to measure the cardiovascular response to the stress of exercise. Wearable heart rate monitors are a comfortable option to measure RR intervals while doing physical activities. It is necessary to evaluate the agreement between HRV parameters derived from the RR series recorded by wearable devices and those derived from an ECG during dynamic exercise of low to high intensity.23 male volunteers performed an exercise stress test on a cycle ergometer. Subjects wore a Polar RS800 device while ECG was also recorded simultaneously to extract the reference RR intervals. A time-frequency spectral analysis was performed to extract the instantaneous mean heart rate (HRM), and the power of low frequency (PLF) and high frequency (PHF) components, the latter centred on the respiratory frequency. Analysis was done in intervals of different exercise intensity based on oxygen consumption. Linear correlation, reliability and agreement were computed in each interval.The agreement between the RR series obtained from the Polar device and from the ECG is high throughout the whole test, although the shorter the RR is, the more differences there are. Both methods are interchangeable when analysing HRV at rest. At high exercise intensity, HRM and PLF still presented a high correlation (ρ>0.8) and excellent reliability and agreement indices (above 0.9). However, the PHF measurements from the Polar showed reliability and agreement coefficients around 0.5 or lower when the level of the exercise increases (for levels of O2 above 60%).