Smartphone Camera Measurement of Breathing-Related Heart Rate Variability Compared With ECG in Healthy Adults

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.

Inclusion Criteria: * Adults aged 19 to 39 years * Able to understand the study explanation and give written informed consent * Able to remain seated for about 90 minutes and to follow audio-guided breathing at rates as slow as 4 breaths per minute Exclusion Criteria: * Diagnosed cardiac arrhythmia or conduction disorder (for example, atrial fibrillation) * Implanted cardiac pacemaker or implantable cardioverter-defibrillator * Current cardiopulmonary symptoms (for example, chest pain, shortness of breath or palpitations) * Any condition that, in the investigator's judgment, makes slow paced breathing unsafe or uncomfortable * Skin lesion, wound or dressing on the measured fingertip or on the facial measurement area * Severe hand tremor that makes it difficult to keep a fingertip still Participants are not asked to stop any medication for this study; current medications are recorded.