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Commercially available wearable (ambulatory) pulse oximeters have been recommended as a method for managing patients at risk of physiological deterioration, such as active patients with COVID-19 disease receiving care in hospital isolation rooms; however, their reliability in usual hospital settings is not known.
We report the performance of wearable pulse oximeters in a simulated clinical setting when challenged by motion and low levels of arterial blood oxygen saturation (SaO2).
The performance of 1 wrist-worn (Wavelet) and 3 finger-worn (CheckMe O2+, AP-20, and WristOx2 3150) wearable, wireless transmission–mode pulse oximeters was evaluated. For this, 7 motion tasks were performed: at rest, sit-to-stand, tapping, rubbing, drinking, turning pages, and using a tablet. Hypoxia exposure followed, in which inspired gases were adjusted to achieve decreasing SaO2 levels at 100%, 95%, 90%, 87%, 85%, 83%, and 80%. Peripheral oxygen saturation (SpO2) estimates were compared with simultaneous SaO2 samples to calculate the root-mean-square error (RMSE). The area under the receiver operating characteristic curve was used to analyze the detection of hypoxemia (ie, SaO2<90%).
SpO2 estimates matching 215 SaO2 samples in both study phases, from 33 participants, were analyzed. Tapping, rubbing, turning pages, and using a tablet degraded SpO2 estimation (RMSE>4% for at least 1 device). All finger-worn pulse oximeters detected hypoxemia, with an overall sensitivity of ≥0.87 and specificity of ≥0.80, comparable to that of the Philips MX450 pulse oximeter.
The SpO2 accuracy of wearable finger-worn pulse oximeters was within that required by the International Organization for Standardization guidelines. Performance was degraded by motion, but all pulse oximeters could detect hypoxemia. Our findings support the use of wearable, wireless transmission–mode pulse oximeters to detect the onset of clinical deterioration in hospital settings.
ISRCTN Registry 61535692; http://www.isrctn.com/ISRCTN61535692
RR2-10.1136/bmjopen-2019-034404
Failure to recognize and act on physiological indicators of worsening acute illness in hospital wards is a prevalent problem first recognized over 20 years ago [
This study is part of a phased mixed-methods research project aiming to develop and refine an AMS using wearable devices to aid in the detection of deterioration and improve patient outcomes. The primary objective of this study was to determine the specificity and sensitivity of currently available ambulatory vital sign–monitoring equipment for the detection of hypoxemia. The secondary objective was to determine the effect of motion on data acquisition by the same devices.
This research publication follows the Standards for Reporting Diagnostic Accuracy Studies reporting guidelines [
This was a prospective, observational study in which SpO2 estimates from the study devices were compared with the gold-standard arterial blood oxygen saturation (SaO2) samples and clinical-standard SpO2 estimates collected from arterial blood gas (ABG) samples and a nonambulatory Philips MX450 (Philips, Amsterdam, the Netherlands) pulse oximeter, respectively. The device’s pulse rate estimation accuracy is reported in
Healthy adults (18 years or older) able to give informed consent for participation in the study were recruited consecutively from the Oxford area (United Kingdom) between June 18 and August 8, 2019. The exclusion criteria are described in detail in the study protocol [
The study sessions took place at the Cardiovascular Clinical Research Facility, John Radcliffe Hospital, Oxford, UK. An arterial line was first inserted, under local anesthesia, preferentially into the nondominant radial artery of participants placed in the semirecumbent position (30o head up). Where it was not possible to cannulate the nondominant arm, the dominant arm was cannulated. Participants wore 1 wrist-only device (Wavelet; WaveletHealth, Mountain View, USA) and 3 wrist-worn pulse oximeters with a finger probe: CheckMe O2+ (Viatom Technology Co Ltd, Shenzhen, China), AP-20 (Shenzhen Creative Industry Co Ltd, Shenzhen, China), and WristOx2 3150 with Bluetooth Low Energy (BLE; Nonin Medical Inc, Plymouth, USA) on the same arm. These devices are among the few that make both numeric and waveform data available to other systems. This is a requirement in our research [
An at-rest window was assigned to the period before the first ABG measurement, taken after fitting all the devices. The participants then moved to a chair and were asked to complete a series of consecutive motion tasks: 20 times sit-to-stand (STS), 2-minute tapping at 2 Hz, 2-minute rubbing at 2 Hz, 20 times drinking from a plastic cup, 50 times turning pages, and a set of predefined tablet activity tasks [
Participants moved to a semirecumbent, supine position and wore a tight-fitting silicone facemask connected to a device that reduces the inspired fraction of oxygen, the hypoxicator unit (Everest Summit Hypoxic Generator, Altitude Centre, London, UK). During this phase, oxygen saturation from the clinical-standard Philips MX450 monitor guided the titration of the hypoxicator by a senior anesthetist from the research team, with appropriate resuscitation facilities nearby. In addition, 7% oxygen in nitrogen was used to further lower the fraction of inspired oxygen (FiO2), if required [
Demographic data, including age, sex, height, weight, skin type (Fitzpatrick scale [
The sample size calculation was based on the International Organization for Standardization (ISO) 80601-2-61:2019 guideline for testing the accuracy of pulse oximeters, which requires at least 200 data points balanced across the SaO2 range of 70%-100% from at least 10 subjects. We aimed to collect approximately 30 full data sets (with 7 ABGs being used in both the movement and hypoxia exposure phases, yielding a total of 420 readings, ie, 210 for each phase) to achieve a sufficient number of data points for the primary and secondary outcomes, and to recruit participants varying in their physical characteristics to the greatest extent possible. We excluded participants if incomplete data were collected for any 1 device during testing or if hypoxia was not achieved.
Demographics and baseline vital sign descriptors were summarized using the mean, the median, and the first and third quartiles for continuous variables and proportions for categorical variables. In accordance with the ISO guideline, the accuracy of the SpO2 estimates for each device was determined using the root-mean-square error (RMSE) between the measured values (SpO2i) and the reference values (SaO2i):
The RMSE 95% CI was determined using bootstrapping (random sampling with replacement) with 10,000 repetitions. The ISO guideline requires that valid oximeters present an RMSE below or equal to 4% (and below or equal to 8% when considering the CI). To interpret potential sources of the SpO2 estimation error, the mean bias B and precision S were also calculated as
and
respectively. The latter is also known as the SD of the residuals, which determines the spread of the test SpO2 data around the linear regression model, SpO2fit, which predicts the SpO2 estimates that best fit the reference SaO2 values. The agreement between the test devices and the gold standard was also assessed via Bland-Altman plots. Finally, the mean absolute bias was also analyzed.
The metrics were computed using the median SpO2i from the 40-second window immediately before the stop time from each motion task and the SaO2 value from the ABG taken immediately after the same motion task.
The metrics were computed using the median SpO2i from a 40-second window, including 35 seconds before and 5 seconds after the i-th reference SaO2 value (note that SaO2 readings were taken for the 80%, 83%, 85%, 87%, 90%, 95%, and 100% target values, with the corresponding output of the blood gas analyzer then taken as the reference value). These metrics were also computed for 3 SaO2 subgroups: severe hypoxia, SaO2 lower than 85%; mild hypoxia, SaO2 from 85% to 89%; and normoxia, SaO2 equal to or greater than 90%.
For both phases, one-way ANOVA followed by the Tukey-Kramer test [
To evaluate each device’s diagnostic accuracy in detecting hypoxemia, we determined the sensitivity, specificity, positive and negative predictive values (PPV and NPV), and accuracy (computed from the error matrix) for identifying values of SaO2 below 90%. To consider whether device performance would be more reliable if recalibrated, we calculated the area under the receiver operating characteristic (AUROC) curve for each pulse oximeter and computed the same metrics at the optimal operating value. In addition, 95% CIs for all metrics were determined using bootstrapping.
Due to SpO2 estimation performance issues, Wavelet analysis was removed. Its results can be found in
Prescreening interviews were performed on 51 volunteers (Consolidated Standards of Reporting Trials [CONSORT] flow diagram in
CONSORT flow diagram of the study. ABG: arterial blood gas; CONSORT: Consolidated Standards of Reporting Trials.
Demographics and baseline heart rate, respiration rate, blood pressure, and SaO2a for 33 participants.
Demographics | Mean (median) | Q1b, Q3c | ||
Age (years) | 29.0 (31.18) | 24.0, 36.0 | ||
Sex (female), % | 18 (54.5) | N/Ad | ||
Height (m) | 1.70 (1.70) | 1.6, 1.8 | ||
Weight (kg) | 70.0 (70.7) | 61.0, 80.0 | ||
BMI (kg/m2) | 23.7 (24.3) | 21.5, 26.4 | ||
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Type 1 | 9 (27.3) | N/A | |
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Type 2 | 15 (45.5) | N/A | |
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Type 3 | 2 (6.1) | N/A | |
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Type 4 | 7 (21.2) | N/A | |
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Type 5 | 0 (0) | N/A | |
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Type 6 | 0 (0) | N/A | |
Respiration rate (rpmf) | 15.0 (15.7) | 13.0, 18.0 | ||
Heart rate (bpmg) | 71.0 (70.9) | 62.0, 82.0 | ||
SaO2, % | 100.0 (99.6) | 100.0, 100.0 | ||
Systolic blood pressure (mmHg) | 129.5 (133.8) | 122.8, 142.8 | ||
Diastolic blood pressure (mmHg) | 75.0 (77.4) | 69.8, 86.3 |
aSaO2: arterial blood oxygen saturation.
bQ1: first quartile.
c Q3: third quartile.
dN/A: not applicable.
eFitzpatrick scale.
frpm: respirations per minute.
gbpm: beats per minute.
SpO2 trend for each device during the movement (9:20 AM-9:50 AM) and hypoxia exposure (9:58 AM-10:14 AM) phases of 1 study session. The gold-standard SaO2, derived from ABG samples, are shown as red stars. The different motion tasks and target desaturation intervals are illustrated by brown and blue rectangles at the top, respectively. Wavelet SpO2 data are shown for comparison (results can be reviewed in
The results of SpO2 estimation performance metrics for each device are shown in
Comparison of accuracy (RMSEa) and bias in SpO2b estimation between different motion tasks, for each device, for 33 participants.
Performance metrics | At rest | STSc | Rubbing | Tapping | Drinking | Turning pages | Tablet use | ||
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Available SpO2 points, n | 32 | 30 | 32 | 30 | 31 | 27 | 31 | N/Ae |
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RMSE, % (95% CI) | 0.82 (0.55-1.06) | 4.68 (1.47-7.72) | 11.96 (9.44-14.23) | 12.21 (9.31-14.74) | 1.96 (1.48-2.46) | 8.52 (6.18-10.75) | 8.01 (1.15-13.72) | N/A |
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Mean bias, % | –0.21f,g | –0.9h,i | –9.91f,g,j,k | –9.82g,i | –1.45j | –6.46 | –2.22k | <.001 |
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Mean |bias|, % | 0.6k,l,m | 2.15f,g | 9.91f,i,j,k | 9.85g,h,l,n | 1.57h,j | 6.46m | 2.56i,n | <.001 |
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Precision, % | 0.81f | 4.31h | 6.91 | 7.49f,h,j,k | 1.37j | 5.57 | 7.89k | <.001 |
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Available SpO2 points, n | 30 | 31 | 31 | 30 | 32 | 32 | 32 | N/A |
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RMSE, % (95% CI) | 1.68 (1.21-2.12) | 3.5 (1.49-5.37) | 8.45 (5.86-10.88) | 3.99 (2.28-5.69) | 2.43 (1.9-2.96) | 7.83 (5.9-9.8) | 4.2 (2.86-5.47) | N/A |
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Mean bias, % | –1.06 | –1.37 | –6.19j | –2.65h,j | –1.93 | –6.04h | –2.94 | .001 |
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Mean |bias|, % | 1.29 | 1.92 | 6.31 | 2.71 | 1.97 | 6.06 | 2.98 | .005 |
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Precision, % | 1.33 | 3.08 | 5.84 | 3.06 | 1.41 | 5.11 | 2.86 | .13 |
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Available SpO2 points, n | 33 | 33 | 33 | 32 | 32 | 32 | 32 | N/A |
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RMSE, % (95% CI) | 1.11 (0.92-1.28) | 2.31 (1.9-2.67) | 9.49 (7.04-11.86) | 7.15 (3.07-10.3) | 1.17 (1.0-1.36) | 6.64 (3.81-9.03) | 1.97 (1.29-2.68) | N/A |
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Mean bias, % | 0.89f | 1.97h | –5.37f,h,i,j,k | –1.75i | 0.84j | –3.04 | 0.4k | <.001 |
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Mean |bias|, % | 0.97h | 2.02 | 6.6h,j | 3.33 | 1.06j | 4.03 | 1.51 | .002 |
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Precision, % | 0.63g,k | 0.78f | 6.77f,i,j,k | 7.16g,h | 0.8h,j | 6.04 | 1.82i | <.001 |
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Available SpO2 points, n | 32 | 33 | 29 | 29 | 32 | 24 | 33 | N/A |
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RMSE, % (95% CI) | 1.18 (0.84-1.51) | 2.33 (1.26-3.41) | 9.5 (7.29-11.5) | 7.17 (4.66-9.35) | 1.27 (0.95-1.57) | 6.28 (4.25-8.27) | 3.91 (1.49-5.62) | N/A |
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Mean bias, % | –0.71f | –0.4h | –7.52f,h,i,j,k | –4.56i | –0.86j | –4.51 | –1.81k | .002 |
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Mean |bias|, % | 0.92k,l | 1.38f,g | 7.52f,i,j,k | 4.69g,l,m | 1.02h,j | 4.56 | 2.02h,m | <.001 |
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Precision, % | 0.97f | 2.12h | 5.98 | 5.7f,h,j,k | 0.93j | 4.35 | 3.06k | .001 |
aRMSE: root-mean-square error.
bSpO2: peripheral oxygen saturation.
cSTS: sit-to-stand.
dOne-way ANOVA followed by the Tukey-Kramer test was used to evaluate differences in the mean bias and mean absolute bias between tasks. The Levene test was used in the case of precision.
eN/A: not applicable.
f-nDifferent from each other; for example, for CheckMe O2+, the mean bias of the tapping motion task was different from that of the turning page task and that of the rubbing task (paired differences coded as j and h, respectively).
Comparison of the mean bias (SpO2–SaO2) and precision between devices for each movement type. The number of points available per device is presented below each bar. For each task, one-way ANOVA followed by the Tukey test was used to evaluate differences in the mean bias between devices. *Different from other values. +Different from each other. SaO2: arterial blood oxygen saturation; SpO2: peripheral oxygen saturation; STS: sit-to-stand.
Comparison of accuracy (RMSEa) and mean bias of SpO2b estimation between devices during the hypoxia exposure phase. There were 215 SaO2 target windows in this phase.
Performance metrics | Philips MX450 | CheckMe O2+ | WristOx2 3150 | AP-20 | |
Available SpO2 points, n | 215 | 207 | 209 | 214 | N/Ac |
RMSE, % (95% CI) | 2.67 (2.31-3.06) | 3.20 (2.85-3.56) | 3.33 (2.85-3.86) | 2.86 (2.44-3.25) | N/A |
Mean bias, % | 0.49d | –0.22 | –1.92e | –0.3d | <.001 |
Mean |bias|, % | 1.92 | 2.42 | 2.40 | 2.00 | <.02 |
Precision, % | 2.62d | 3.16d | 2.73 | 2.83 | <.02 |
aRMSE: root-mean-square error.
bSpO2: peripheral oxygen saturation.
cN/A: not applicable.
dDifferent from each other.
eDifferent from other values.
(a-d) Bland-Altman plots for the Philips MX450, AP-20, CheckMe O2+, and WristOx2 3150 SpO2 estimation, respectively. The mean bias and limits of agreement values are shown at the left of their respective dashed lines. The solid line represents y=0 (no bias). SaO2: arterial blood oxygen saturation; SpO2: peripheral oxygen saturation.
Comparison of the mean bias (SpO2–SaO2) and precision between devices for the 3 SaO2 subgroups: severe hypoxia, SaO2<85%; mild hypoxia, SaO2=85%-89%; and normoxia, SaO2=90%-100%. The number of points available per device is presented below each bar. For each subgroup, one-way ANOVA followed by the Tukey test was used to evaluate differences in the mean bias between devices. *Different from other values. +Different from each other. SaO2: arterial blood oxygen saturation; SpO2: peripheral oxygen saturation.
Comparison of accuracy (RMSEa) and mean bias of the device’s SpO2b estimation between 3 SaO2c subgroups: severe hypoxia (SaO2<85%), mild hypoxia (SaO2 85%-89%), and normoxia (SaO2≥90%).
Performance metrics | <85% | 85%-89% | 90%-100% | ||||||
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60 | 76 | 79 | N/Af | |||||
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Available SpO2 points, n | 60 | 76 | 78 | N/A | ||||
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RMSE |
2.99 (2.44-3.57) | 2.73 (2.28-3.16) | 2.88 (1.88-3.78) | N/A | ||||
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Mean bias |
0.28 | –0.54 | –0.52 | .18 | ||||
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Mean |bias| |
2.33 | 2.07 | 1.68 | .17 | ||||
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Precision |
2.74 | 2.71 | 2.86 | .16 | ||||
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Available SpO2 points, n | 56 | 75 | 76 | N/A | ||||
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RMSE |
3.52 (2.86-4.18) | 3.10 (2.46-3.83) | 3.05 (2.54-3.53) | N/A | ||||
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Mean bias |
0.67d | –0.11d | –0.99 | .01 | ||||
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Mean |bias| |
2.74 | 2.35 | 2.26 | .39 | ||||
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Precision |
3.5 | 3.14 | 2.92 | .28 | ||||
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Available SpO2 points, n | 60 | 76 | 79 | N/A | ||||
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RMSE |
2.80 (2.18-3.33) | 2.54 (2.08-3.02) | 2.70 (1.88-3.56) | N/A | ||||
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Mean bias |
1.26 | –0.05g | 0.42g | .02 | ||||
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Mean |bias| |
2.13 | 1.97 | 1.72 | .44 | ||||
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Precision |
2.16 | 2.57 | 2.69 | .24 | ||||
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Available SpO2 points, n | 59 | 74 | 76 | N/A | ||||
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RMSE |
2.69 (2.28-3.08) | 3.49 (2.92-3.99) | 3.61 (2.37-4.64) | N/A | ||||
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Mean bias |
–1.36 | –2.47 | –1.83 | .06 | ||||
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Mean |bias| |
2.21 | 2.83 | 2.12 | .13 | ||||
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Precision |
2.13 | 2.48 | 3.03 | .99 |
aRMSE: root-mean-square error.
bSpO2: peripheral oxygen saturation.
cSaO2: arterial blood oxygen saturation.
dFor each device, one-way ANOVA followed by Tukey test was used to evaluate differences in the mean bias and mean absolute bias between subgroups. The Levene test was used in the case of precision.
eABG: arterial blood gas.
fN/A: not applicable.
gDifferent from each other.
Performance metrics of each pulse oximeter for detecting hypoxemia (SaO2a<90%). The metrics are shown at a 90% SpO2b cut-off and for the determined optimal SpO2 cut-off.
Device | Cut-off, % | AUROCc, mean (95% CI) | Sensitivity, mean (95% CI) | Specificity, mean (95% CI) | PPVd, mean (95% CI) | NPVe, mean (95% CI) | Accuracyf, mean (95% CI) | ||||||
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Philips MX 450 | 90.0 | N/Ag | 0.86 (0.80-0.92) | 0.93 (0.87-0.99) | 0.96 (0.92-0.99) | 0.79 (0.71-0.88) | 0.89 (0.84-0.93) | |||||
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CheckMe O2+ | 90.0 | N/A | 0.87 (0.81-0.93) | 0.85 (0.76-0.93) | 0.91 (0.86-0.96) | 0.80 (0.71-0.88) | 0.87 (0.82-0.91) | |||||
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WristOx2 3150 | 90.0 | N/A | 0.97 (0.93-0.99) | 0.80 (0.70-0.89) | 0.89 (0.84-0.94) | 0.94 (0.87-0.99) | 0.91 (0.86-0.95) | |||||
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AP-20 | 90.0 | N/A | 0.91 (0.85-0.95) | 0.89 (0.82-0.96) | 0.94 (0.89-0.98) | 0.85 (0.76-0.92) | 0.90 (0.86-0.94) | |||||
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Philips MX 450 | 90.7 | 0.94 (0.90-0.98) | 0.97 (0.94-0.99) | 0.86 (0.78-0.94) | 0.93 (0.88-0.97) | 0.94 (0.88-0.99) | 0.93 (0.90-0.97) | |||||
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CheckMe O2+ | 89.0 | 0.92 (0.87-96) | 0.78 (0.71-0.85) | 0.88 (0.80-0.95) | 0.92 (0.86-0.97) | 0.70 (0.60-0.79) | 0.82 (0.76-0.87) | |||||
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WristOx2 3150 | 88.0 | 0.94 (0.89-97) | 0.88 (0.82-0.94) | 0.86 (0.78-0.94) | 0.92 (0.87-0.96) | 0.81 (0.72-0.89) | 0.88 (0.83-0.92) | |||||
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AP-20 | 91.0 | 0.94 (0.89-98) | 0.95 (0.91-0.98) | 0.84 (0.75-0.92) | 0.91 (0.86-0.96) | 0.91 (0.84-0.97) | 0.91 (0.87-0.95) |
aSaO2: arterial blood oxygen saturation.
bSpO2: peripheral oxygen saturation.
cAUROC: area under the receiver operating characteristic.
dPPV: positive predictive value.
eNPV: negative predictive value.
fAccuracy = (True positives + True negatives)/n, where n is the total number of examples.
gN/A: not applicable.
hThe optimal SpO2 cut-off is the best compromise between sensitivity and specificity to detect hypoxemia (SaO2<90%).
ROC curves in detecting hypoxemia (SaO2<90%) during the hypoxia exposure phase. ROC: area under the receiver operating characteristic; SaO2: arterial blood oxygen saturation.
Several studies have been published on both the usefulness and the potential issues of pulse oximetry in the clinical setting using nonambulatory devices. In this study, we compared the performance of wearable pulse oximeters and 1 nonambulatory pulse oximeter using gold-standard arterial blood samples drawn from healthy adult participants. Availability of waveform data was a requirement that limited the selection of devices. Our provision of waveform data will allow clinical staff to assess the reliability of the signal. However, a risk with all continuous-monitoring systems is that they increase the burden on clinical teams by providing excess data. Further work is required to determine the usefulness of these systems in clinical practice and how continuous-monitoring data should be summarized in the electronic patient record.
In tests of finger-based devices, WristOx2 3150 significantly underestimated SaO2 (mean bias –1.92% [SD 2.73%];
From the 7 motion tasks, tapping, rubbing, turning book pages, and using a tablet challenged the finger-based wearable devices the most (the first 2 are also analyzed by Louie et al [
The sample size calculation for our study was based on the ISO 80601-2-61:2019 guidelines to evaluate the accuracy of pulse oximeters in detecting changes in SpO2, not to identify differences in performance between pulse oximeters and between activities. The study was not designed to generalize results to the wider population, for example, for patients with darker skin types or with acute illness.
We chose to sample ABGs at the end of each task to avoid accidental removal of the cannula. However, it became clear during our study that the ABGs could have been sampled while the motion task was occurring, perhaps better representing that interval reference SaO2. Preliminary analysis of the difference between ABGs taken immediately after the STS motion task and those taken at the midpoint of that motion, for 15 patients, showed that the SaO2 dropped by an average of 1.87% (SD 0.87%,
The accuracy of SpO2 estimation by finger-worn pulse oximeters was within that required by the ISO 80601-2-61:2019 guideline (≤4%). The accuracy was degraded by motion but not more than that with usual-care bedside monitors. All finger-worn pulse oximeters were capable of detecting hypoxemia, their performance being comparable to that used in nonambulatory standard care.
Our findings support the use of wearable, finger-based, wireless transmission–mode pulse oximeters to detect the onset of clinical deterioration in the hospital, possibly earlier than intermittent vital-sign measurements. The continuous assessment of SpO2, especially values below 90%, may be helpful to manage the care of ambulatory in-hospital patients who have been infected with the SARS-CoV-2 virus [
Analysis of the pulse rate estimation in wearable pulse oximeters.
Results of Wavelet’s SpO2 estimation.
arterial blood gas
ambulatory monitoring system
area under the receiver operating characteristic
Consolidated Standards of Reporting Trials
Bluetooth Low Energy
beats per minute
electrocardiogram
fraction of inspired oxygen
International Organization for Standardization
negative predictive value
positive predictive value
root-mean-square error
receiver operating characteristic
respirations per minute
arterial blood oxygen saturation
peripheral oxygen saturation
sit-to-stand
This work was supported by the National Institute for Health Research (NIHR) Biomedical Research Centre (BRC), Oxford, UK. MS, LY, SV, PW, and CA were funded/supported by the NIHR BRC. The views expressed are those of the authors and not necessarily those of the National Health Service (NHS), the NIHR, or the Department of Health. MAFP was funded by a Drayson research fellowship. AS is currently supported by an NIHR doctoral research fellowship (NIHR-DRF-2017-10-094).
PW and LT report significant grants from the National Institute of Health Research (NIHR), UK, and the NIHR Biomedical Research Centre (BRC), Oxford, UK, during the conduct of the study. PW and LT report modest grants and personal fees from Sensyne Health, outside the submitted work. LT works part-time for Sensyne Health and holds share options in the company. PW also holds shares in the company.