An interventional study of Smartphone simultaneous facial and fingertip photoplethysmography (research app) and Audio-guided paced breathing in Respiratory Sinus Arrhythmia, Heart Rate Variability and Healthy Volunteers, sponsored by BELL Therapeutics Inc.. Not yet recruiting at 1 site in South Korea. Open to participants aged 19 Years to 39 Years, including healthy volunteers. Per ClinicalTrials.gov, last updated 2026-11-06.
Sponsored by BELL Therapeutics Inc. · Not applicable, Interventional, and Other
This study examines how accurately a smartphone can measure the rhythmic rise and fall of heart rate that occurs with breathing (respiratory heart rate variability).
Healthy adult volunteers attend one laboratory visit of about 90 minutes in Seoul, Republic of Korea. While seated, each participant breathes freely for 5 minutes, then follows audio-guided breathing at seven slow rates between 4 and 7 breaths per minute in random order and finally at 15 breaths per minute, with 2-minute rests between conditions (31 minutes of recording and 16 minutes of rest). During every condition, a single iPhone measures the face with its front camera (contact-free measurement) and a fingertip lightly placed over its rear camera lens (contact measurement) at the same time; no video is stored. A chest ECG and a respiratory belt record continuously throughout the session as references.
The main question is how closely the breathing-related heart rate variability amplitude measured from the face agrees with the ECG during slow paced breathing. The study also compares the fingertip measurement with the ECG, reports how often the smartphone measurements fail to produce a usable value, evaluates standard heart rate variability measures, and explores whether shorter recordings would give similar agreement and how breathing rate affects agreement.
The smartphone app is a research prototype used only for data collection. This study does not test a treatment, and its measurements are not used to diagnose or treat participants.
Design
Single-visit, single-group, within-participant method-comparison study. Each participant completes nine breathing conditions:
Pacing uses audio cues with an inhalation:exhalation time ratio of 4:6 at a comfortable depth, without breath holding. Conditions are separated by rests of at least 2 minutes, during which the fingertip is lifted and the cameras and flashlight are stopped. Planned recording time is 31 minutes plus 16 minutes of rest (47 minutes). Consent, sensor setup, at least 5 minutes of seated rest before recording, countdowns, data checks and a short questionnaire are additional; the visit lasts about 90 minutes. Participants are asked to avoid alcohol and strenuous exercise for 24 hours and caffeine, smoking and meals for 2 hours before recording. If these are not met, recording starts later or the visit is rescheduled; the times are recorded, and no participant is excluded on this basis after recording has started.
Order of conditions
The order of the seven slow rates is generated for each assignment number with a fixed random seed and stored before the participant's visit, balancing the position of each rate across participants as far as possible. The order is not changed after data are viewed. Free breathing is always first so that it is recorded before any paced breathing, and 15 breaths/min is always last so that faster breathing does not precede the slow rates. Differences between these two conditions and the slow rates may therefore include order and time effects; for 15 breaths/min these include carry-over from slow breathing and fatigue late in the session.
Measurements
A single iPhone 17 Pro running a research data-collection app uses the front camera (face) and the rear main camera (fingertip) simultaneously on a shared device clock. The fingertip is placed lightly over the rear camera lens while the flashlight beside the lens stays on and uncovered. No video from either camera is stored. For the face, the app continuously tracks regions on both cheeks and converts them into region-averaged signals. For every frame, the app saves the timestamp, the mean color values of the measurement regions, the detected face position and the camera settings, together with the extracted signals, beat candidates, device events and the time of every breathing cue. Further processing (beat detection, fiducial points and delay correction) follows an analysis specification frozen before the first participant is enrolled. A chest ECG (reference for beat timing) and a respiratory belt (reference for breathing rate and phase, and for adherence to pacing) record continuously from before the first condition until after the last. Start and end synchronization events are recorded to align the smartphone with the reference devices. Device model, app build and camera settings are fixed before the first participant is enrolled.
Analysis principles
Questionnaire
After all nine conditions, participants complete a short study-specific questionnaire once; it is not a validated instrument. Six items are rated from 0 to 10 for the average experience of the visit: difficulty keeping the face within the camera view, difficulty keeping the fingertip on the rear camera, fingertip discomfort, fingertip warmth or heat, being bothered by having the face filmed, and difficulty following the audio guide. For each method separately, participants then rate their satisfaction with the method as experienced that day (7-point item) and report any inconvenience. They are then handed the smartphone used in the session, may handle it as they like, and rate for each method how much they would want to use it alone for 5 minutes every night at home before bed (7-point items); they then choose the one method they would use (face, fingertip, or no preference), give the reason, and add suggestions. Face and fingertip items are presented in one of two orders (face first or fingertip first), assigned to each participant in advance from a random allocation table in blocks of four; 'no preference' is always the last option. The two difficulty ratings, their within-participant difference and the chosen method are outcome measures; the other items are reported descriptively. Items are analyzed separately without a total score, and missing responses are not imputed.
Sample size
The target is at least 20 evaluable participants, and up to 36 may be enrolled. Enrollment beyond 20 is intended to narrow the confidence intervals of the bias and limits of agreement and to allow for withdrawal or unusable reference recordings. The final number is determined by the recruitment and measurement schedule and budget, not by interim results: agreement is not analyzed before data collection ends, and checks during the study are limited to data completeness and recording integrity. All evaluable participants are analyzed; no subset is selected. The target of 20 reflects feasibility and is not derived from a power or precision calculation, because the variability and acceptable limits are not yet established.
Scope
Test-retest reliability, treatment effects of slow breathing, and confirmation of a physiological resonance frequency are outside the scope of this study.
96 studies on the registry are indexed under Hypoventilation; 21 are open to participants now.
This study's planned enrollment of 36 is below the median of 52 across 63 interventional studies indexed under Hypoventilation.
Browse Hypoventilation studies →BELL Therapeutics Inc. is the lead sponsor of 4 studies on the registry; 2 are open to participants now.
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Go through the criteria one by one — yes, no or not sure — and take the list to your doctor or the study team. Kept in this browser only.
Exclusion Criteria:
Participants are not asked to stop any medication for this study; current medications are recorded.
Underlined terms in the criteria open these too. Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Before you join, the study team explains what taking part involves — its purpose, what happens, the risks and possible benefits — and answers your questions, so you can decide freely. Consent is usually recorded on a form (the informed consent form, ICF) signed by you or, where the law allows, by someone legally authorised to decide for you.
You can keep asking questions during the study, you are told of new information that matters, and you can choose to leave at any time.
Informed consent
Before you join, the study team explains what taking part involves — its purpose, what happens, the risks and possible benefits — and answers your questions, so you can decide freely. Consent is usually recorded on a form (the informed consent form, ICF) signed by you or, where the law allows, by someone legally authorised to decide for you.
You can keep asking questions during the study, you are told of new information that matters, and you can choose to leave at any time.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Single group. Each participant completes free breathing (5 min), then paced breathing at 4.0, 4.5, 5.0, 5.5, 6.5 and 7.0 breaths/min (3 min each) and 6.0 breaths/min (5 min) in randomized order, and finally paced breathing at 15 breaths/min (3 min), with 2-minute rests. Throughout each condition the face and fingertip are measured simultaneously by one smartphone, while a chest ECG and a respiratory belt record continuously as references.
Device: Smartphone simultaneous facial and fingertip photoplethysmography (research app) · Behavioral: Audio-guided paced breathing
Research-prototype iOS data-collection app on a single iPhone 17 Pro. The front camera measures the face for contact-free remote photoplethysmography; the rear main camera, with the flashlight on, measures a fingertip lightly placed over the lens for contact photoplethysmography. No video is stored; the app tracks both cheek regions and saves region-averaged signals with frame timestamps for later analysis. The app plays the breathing guide. It is not used for diagnosis, and readings shown during recording are not used as study results.
Fixed-rate audio cues at 15.0 and at 4.0, 4.5, 5.0, 5.5, 6.0, 6.5 and 7.0 breaths/min, with inhalation taking 40% and exhalation 60% of each cycle, at a comfortable depth without breath holding. A free-breathing condition without cues is included. Participants may rest or stop at any time.
Log Ratio of Facial Remote Photoplethysmography to ECG Respiratory Heart Rate Variability Amplitude During Slow Paced Breathing
ln(A\_pp face / A\_pp ECG) for each slow paced-breathing condition (4.0, 4.5, 5.0, 5.5, 6.0, 6.5 and 7.0 breaths/min). A\_pp is the peak-to-peak amplitude (ms) of the first harmonic of inter-beat intervals at the guided breathing frequency, estimated separately for the face and the ECG within the same window (60-180 s after the start of each 3-minute condition and 60-300 s for the 5-minute 6.0 breaths/min condition). Unit: natural-log ratio (unitless; 0 means no difference); +0.1 means a facial amplitude about 10% larger than the ECG amplitude. The reported value is the bias, the mean log ratio pooled across participants and the seven rates, with repeated-measures 95% limits of agreement and participant-level confidence intervals. Conditions without a valid facial estimate are counted in the facial measurement failure outcome.
Time frame: Seven slow paced-breathing conditions (3 minutes each; 6.0 breaths/min 5 minutes) during a single study visit (Day 1)
Log Ratio of Fingertip Contact Photoplethysmography to ECG Respiratory Heart Rate Variability Amplitude During Slow Paced Breathing
ln(A\_pp fingertip / A\_pp ECG), with A\_pp, analysis windows and summary defined as in the primary outcome: the bias pooled across participants and the seven slow rates, with repeated-measures 95% limits of agreement and participant-level confidence intervals. Unit: natural-log ratio (unitless; 0 means no difference). Conditions without a valid fingertip estimate are counted in the fingertip measurement failure outcome.
Time frame: Seven slow paced-breathing conditions during a single study visit (Day 1)
Within-Participant Difference Between Facial-to-ECG and Fingertip-to-ECG Absolute Log Ratios of Respiratory Heart Rate Variability Amplitude During Slow Paced Breathing
For each participant, \|ln(A\_pp face / A\_pp ECG)\| minus \|ln(A\_pp fingertip / A\_pp ECG)\| in the same window, averaged over the slow paced-breathing conditions in which both sensors give a valid estimate. Unit: natural-log ratio (unitless); positive values mean a larger facial error. The reported value is the mean across participants with a participant-level 95% confidence interval. Each sensor is also compared with the ECG separately, because agreement between face and fingertip does not imply agreement with the ECG.
Time frame: Seven slow paced-breathing conditions during a single study visit (Day 1)
Percentage of Attempted Conditions Without a Valid Facial Estimate
Percentage of all originally attempted breathing conditions, pooled across participants, in which the facial pathway yields no valid estimate under the quality rules fixed before evaluation data are analyzed. The denominator is every attempted condition, including conditions whose ECG is unsuitable as a reference. Failed conditions are not removed, and the reason for each failure is reported.
Time frame: All nine conditions during a single study visit (Day 1)
Percentage of Attempted Conditions Without a Valid Fingertip Estimate
Percentage of all originally attempted breathing conditions, pooled across participants, in which the fingertip pathway yields no valid estimate under the quality rules fixed before evaluation data are analyzed. The denominator is every attempted condition, including conditions whose ECG is unsuitable as a reference. Failed conditions are not removed, and the reason for each failure is reported.
Time frame: All nine conditions during a single study visit (Day 1)
Percentage of ECG Beats Without a Matching Facial Pulse Beat (Missed Beats)
Percentage of ECG beats (R peaks) in the analysis windows of all originally attempted conditions, including conditions counted as measurement failures, for which no facial pulse beat is detected within 150 ms of the R peak after the median delay between the ECG and the pulse signal in that condition is subtracted, with each beat matched at most once; a pulse beat detected but displaced beyond this tolerance leaves its ECG beat unmatched. Results at tolerances of 100 ms and 200 ms are reported as a sensitivity analysis. ECG segments unsuitable as a reference are excluded from the denominator and reported separately. The reported value is the mean of participant-level percentages with a 95% confidence interval. Errors computed only from matched beats do not show missed beats.
Time frame: Analysis windows of all nine conditions during a single study visit (Day 1)
Percentage of ECG Beats Without a Matching Fingertip Pulse Beat (Missed Beats)
Percentage of ECG beats (R peaks) in the analysis windows of all originally attempted conditions, including conditions counted as measurement failures, for which no fingertip pulse beat is detected within 150 ms of the R peak after the median delay between the ECG and the pulse signal in that condition is subtracted, with each beat matched at most once; a pulse beat detected but displaced beyond this tolerance leaves its ECG beat unmatched. Results at tolerances of 100 ms and 200 ms are reported as a sensitivity analysis. ECG segments unsuitable as a reference are excluded from the denominator and reported separately. The reported value is the mean of participant-level percentages with a 95% confidence interval. Errors computed only from matched beats do not show missed beats.
Time frame: Analysis windows of all nine conditions during a single study visit (Day 1)
Percentage of Facial Pulse Beats Without a Matching ECG Beat (Extra Beats)
Percentage of facial pulse beats detected in the analysis windows of all originally attempted conditions that have no matching ECG beat (spurious beats), using the same one-to-one matching as for missed beats: within 150 ms of the R peak after the median delay between the ECG and the pulse signal in that condition is subtracted, with each beat matched at most once. A beat displaced beyond this tolerance from its ECG beat also counts as unmatched. Results at tolerances of 100 ms and 200 ms are reported as a sensitivity analysis. Beats in ECG segments unsuitable as a reference are excluded and reported separately. The reported value is the mean of participant-level percentages with a 95% confidence interval.
Time frame: Analysis windows of all nine conditions during a single study visit (Day 1)
Percentage of Fingertip Pulse Beats Without a Matching ECG Beat (Extra Beats)
Percentage of fingertip pulse beats detected in the analysis windows of all originally attempted conditions that have no matching ECG beat (spurious beats), using the same one-to-one matching as for missed beats: within 150 ms of the R peak after the median delay between the ECG and the pulse signal in that condition is subtracted, with each beat matched at most once. A beat displaced beyond this tolerance from its ECG beat also counts as unmatched. Results at tolerances of 100 ms and 200 ms are reported as a sensitivity analysis. Beats in ECG segments unsuitable as a reference are excluded and reported separately. The reported value is the mean of participant-level percentages with a 95% confidence interval.
Time frame: Analysis windows of all nine conditions during a single study visit (Day 1)
Log Ratio of Facial to ECG Root Mean Square of Successive Differences (RMSSD) During Free Breathing
ln(RMSSD face / RMSSD ECG). RMSSD (ms) is computed from facial pulse intervals and from ECG RR intervals in the same window, using only consecutive valid interval pairs. Unit: natural-log ratio (unitless; 0 means no difference). The reported value is the bias (mean across participants) with 95% limits of agreement, for the primary 60-300 s window and, to indicate whether free breathing could be shortened to 3 minutes, for the 60-180 s window of the same recording.
Time frame: 5-minute free-breathing condition (60-300 s window, and 60-180 s window for the 3-minute comparison) during a single study visit (Day 1)
Log Ratio of Fingertip to ECG Root Mean Square of Successive Differences (RMSSD) During Free Breathing
ln(RMSSD fingertip / RMSSD ECG). RMSSD (ms) is computed from fingertip pulse intervals and from ECG RR intervals in the same window, using only consecutive valid interval pairs. Unit: natural-log ratio (unitless; 0 means no difference). The reported value is the bias (mean across participants) with 95% limits of agreement, for the primary 60-300 s window and, to indicate whether free breathing could be shortened to 3 minutes, for the 60-180 s window of the same recording.
Time frame: 5-minute free-breathing condition (60-300 s window, and 60-180 s window for the 3-minute comparison) during a single study visit (Day 1)
Log Ratio of Facial to ECG Standard Deviation of Normal-to-Normal Intervals (SDNN) During Free Breathing
ln(SDNN face / SDNN ECG). SDNN (ms) is computed from facial pulse intervals and from ECG RR intervals in the same window, using only valid intervals. Unit: natural-log ratio (unitless; 0 means no difference). The reported value is the bias (mean across participants) with 95% limits of agreement, for the primary 60-300 s window and, to indicate whether free breathing could be shortened to 3 minutes, for the 60-180 s window of the same recording.
Time frame: 5-minute free-breathing condition (60-300 s window, and 60-180 s window for the 3-minute comparison) during a single study visit (Day 1)
Log Ratio of Fingertip to ECG Standard Deviation of Normal-to-Normal Intervals (SDNN) During Free Breathing
ln(SDNN fingertip / SDNN ECG). SDNN (ms) is computed from fingertip pulse intervals and from ECG RR intervals in the same window, using only valid intervals. Unit: natural-log ratio (unitless; 0 means no difference). The reported value is the bias (mean across participants) with 95% limits of agreement, for the primary 60-300 s window and, to indicate whether free breathing could be shortened to 3 minutes, for the 60-180 s window of the same recording.
Time frame: 5-minute free-breathing condition (60-300 s window, and 60-180 s window for the 3-minute comparison) during a single study visit (Day 1)
Log Ratio of Facial to ECG Respiratory Heart Rate Variability Amplitude at 15 Breaths/Min
ln(A\_pp face / A\_pp ECG) in the 60-180 s window of the 15 breaths/min condition, with A\_pp defined as in the primary outcome. Unit: natural-log ratio (unitless; 0 means no difference). The reported value is the bias (mean across participants) with 95% limits of agreement.
Time frame: 3-minute 15 breaths/min condition during a single study visit (Day 1)
Log Ratio of Fingertip to ECG Respiratory Heart Rate Variability Amplitude at 15 Breaths/Min
ln(A\_pp fingertip / A\_pp ECG) in the 60-180 s window of the 15 breaths/min condition, with A\_pp defined as in the primary outcome. Unit: natural-log ratio (unitless; 0 means no difference). The reported value is the bias (mean across participants) with 95% limits of agreement.
Time frame: 3-minute 15 breaths/min condition during a single study visit (Day 1)
Log Ratio of Facial to ECG Respiratory Heart Rate Variability Amplitude With a Shorter Settling Period or Shorter Recording (Exploratory)
The primary measure, the bias of ln(A\_pp face / A\_pp ECG) pooled across participants and the seven slow rates, recomputed with less of the start excluded (from 0, 15 or 30 seconds instead of 60 seconds to the end of the third minute) and with fixed 120-second windows starting at 0, 15, 30 and 60 seconds, which correspond to conditions of 2, 2.25, 2.5 and 3 minutes (60-180 s is the primary window). For 6.0 breaths/min the first 3 minutes of the 5-minute recording are used. This separates the effect of the excluded settling period from that of analysis length. Unit: natural-log ratio (unitless; 0 means no difference). Overlapping windows are not treated as independent samples, and a shorter recording is not called sufficient without pre-specified agreement limits.
Time frame: 0-180, 15-180 and 30-180 s windows and 120-second windows starting at 0, 15, 30 and 60 s, in the first 3 minutes of the seven slow paced-breathing conditions during a single study visit (Day 1)
Log Ratio of Facial to ECG Respiratory Heart Rate Variability Amplitude at 6.0 Breaths/Min: 3-Minute and 5-Minute Windows
Bias of ln(A\_pp face / A\_pp ECG) (mean across participants, with 95% limits of agreement) at 6.0 breaths/min from the 60-180 s window, as in a 3-minute condition, and from the 60-300 s primary window of the same recording, to indicate whether this condition could be shortened to 3 minutes. Unit: natural-log ratio (unitless; 0 means no difference). The 3-minute comparison is limited to this condition and to free breathing (RMSSD and SDNN outcomes) and is not extended to the other rates.
Time frame: 5-minute 6.0 breaths/min condition (60-180 s and 60-300 s windows) during a single study visit (Day 1)
Percentage of Participants Whose Facial and ECG Estimates Give the Same Breathing Rate With Maximal Respiratory Heart Rate Variability Amplitude (Exploratory)
For each participant, the rate between 4.0 and 7.0 breaths/min with the largest A\_pp is identified separately from the face and from the ECG. The outcome is the percentage of participants for whom the two rates are the same, among participants with valid estimates at all seven rates from both sources. A maximum at 4.0 or 7.0 breaths/min is flagged as a grid boundary. This rate is not treated as a physiological resonance frequency or an optimal treatment rate.
Time frame: Seven slow paced-breathing conditions during a single study visit (Day 1)
Percentage of Participants Whose Fingertip and ECG Estimates Give the Same Breathing Rate With Maximal Respiratory Heart Rate Variability Amplitude (Exploratory)
For each participant, the rate between 4.0 and 7.0 breaths/min with the largest A\_pp is identified separately from the fingertip and from the ECG. The outcome is the percentage of participants for whom the two rates are the same, among participants with valid estimates at all seven rates from both sources. A maximum at 4.0 or 7.0 breaths/min is flagged as a grid boundary. This rate is not treated as a physiological resonance frequency or an optimal treatment rate.
Time frame: Seven slow paced-breathing conditions during a single study visit (Day 1)
Absolute Difference Between Belt-Measured and Target Breathing Rate During Paced Breathing
Absolute difference (breaths/min) between the breathing rate measured by the respiratory belt and the target rate, averaged over the eight paced conditions for each participant and summarized as the mean across participants. Values for each target rate are supplementary.
Time frame: All eight paced conditions during a single study visit (Day 1)
Number of Participants With Symptoms Requiring a Pause or Stop
Symptoms such as dizziness, tingling, chest discomfort, breathlessness, or warmth or pain at the fingertip that lead to pausing or stopping a recording.
Time frame: During the single study visit (Day 1)
Participant-Rated Difficulty Keeping the Face Within the Camera View (0-10 Numeric Rating)
Single study-specific questionnaire item (not a validated scale), rated once for the average experience across all measurements of the visit: difficulty keeping the face within the camera view, on a 0 to 10 numeric rating scale (0 = not at all difficult, 10 = very difficult). Higher scores mean greater difficulty, a worse outcome. Reported as the mean with a 95% bootstrap confidence interval, and the median and interquartile range.
Time frame: End of the single study visit (Day 1), after all nine conditions
Participant-Rated Difficulty Keeping the Fingertip on the Rear Camera (0-10 Numeric Rating)
Single study-specific questionnaire item (not a validated scale), rated once for the average experience across all measurements of the visit: difficulty keeping the fingertip on the rear camera, on a 0 to 10 numeric rating scale (0 = not at all difficult, 10 = very difficult). Higher scores mean greater difficulty, a worse outcome. Reported as the mean with a 95% bootstrap confidence interval, and the median and interquartile range.
Time frame: End of the single study visit (Day 1), after all nine conditions
Within-Participant Difference in Rated Difficulty Between Fingertip and Facial Measurement
For each participant, the fingertip-contact difficulty rating minus the face-position difficulty rating (both 0 to 10, as above). Possible range -10 to 10; positive values mean that keeping the fingertip in place was rated more difficult. This is the only pre-specified comparison of the questionnaire. Reported as the mean difference with a 95% percentile bootstrap confidence interval (2,000 participant-level resamples).
Time frame: End of the single study visit (Day 1), after all nine conditions
Number of Participants Choosing Each Measurement Method for Hypothetical Nightly Home Use
Single study-specific questionnaire item asked once at the end of the visit, after both methods have been experienced and rated: which method the participant would choose to measure for 5 minutes every night at home before bed (facial measurement, fingertip measurement, or no preference). Before this item, participants are handed the smartphone used in the session and may handle it as they like. The face and fingertip options appear in the participant's assigned order (face first or fingertip first), with 'no preference' always last. Reported as the number and percentage of participants choosing each option, with 95% Wilson confidence intervals. This is a stated hypothetical choice after simultaneous measurement, not a comparison of methods used independently.
Time frame: End of the single study visit (Day 1), after all nine conditions
Plan to share: No — Individual participant data will not be shared. No video is stored. The smartphone signals and the sensor recordings (ECG and respiratory belt) are stored only for analysis by the study team and will not be provided to other researchers or organizations.
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Start is estimated for Nov 2026; primary completion for Dec 2026, 0 months later.
Of 35 interventional studies with no phase under Hypoventilation, half reached primary completion within 15 months of starting (the median), counting those still running; a quarter within 5 and three quarters within 26. This study's planned 0 months is below that.
How these figures are worked out, and which studies are left out of them.
From the registry record's own update history. This site started tracking changes on Sep 25, 2026; for anything earlier, see the record history on ClinicalTrials.gov ↗
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BELL Therapeutics Inc.