title: Resting and Postexercise Heart Rate Detection From Fingertip and Facial Photoplethysmography Using a Smartphone Camera: A Validation Study.
abstract: BACKGROUND
  Modern smartphones allow measurement of heart rate (HR) by detecting pulsatile photoplethysmographic (PPG) signals with built-in cameras from the fingertips or the face, without physical contact, by extracting subtle beat-to-beat variations of skin color.
  
  
  OBJECTIVE
  The objective of our study was to evaluate the accuracy of HR measurements at rest and after exercise using a smartphone-based PPG detection app.
  
  
  METHODS
  A total of 40 healthy participants (20 men; mean age 24.7, SD 5.2 years; von Luschan skin color range 14-27) underwent treadmill exercise using the Bruce protocol. We recorded simultaneous PPG signals for each participant by having them (1) facing the front camera and (2) placing their index fingertip over an iPhone's back camera. We analyzed the PPG signals from the Cardiio-Heart Rate Monitor + 7 Minute Workout (Cardiio) smartphone app for HR measurements compared with a continuous 12-lead electrocardiogram (ECG) as the reference. Recordings of 20 seconds' duration each were acquired at rest, and immediately after moderate- (50%-70% maximum HR) and vigorous- (70%-85% maximum HR) intensity exercise, and repeated successively until return to resting HR. We used Bland-Altman plots to examine agreement between ECG and PPG-estimated HR. The accuracy criterion was root mean square error (RMSE) ≤5 beats/min or ≤10%, whichever was greater, according to the American National Standards Institute/Association for the Advancement of Medical Instrumentation EC-13 standard.
  
  
  RESULTS
  We analyzed a total of 631 fingertip and 626 facial PPG measurements. Fingertip PPG-estimated HRs were strongly correlated with resting ECG HR (r=.997, RMSE=1.03 beats/min or 1.40%), postmoderate-intensity exercise (r=.994, RMSE=2.15 beats/min or 2.53%), and postvigorous-intensity exercise HR (r=.995, RMSE=2.01 beats/min or 1.93%). The correlation of facial PPG-estimated HR was stronger with resting ECG HR (r=.997, RMSE=1.02 beats/min or 1.44%) than with postmoderate-intensity exercise (r=.982, RMSE=3.68 beats/min or 4.11%) or with postvigorous-intensity exercise (r=.980, RMSE=3.84 beats/min or 3.73%). Bland-Altman plots showed better agreement between ECG and fingertip PPG-estimated HR than between ECG and facial PPG-estimated HR.
  
  
  CONCLUSIONS
  We found that HR detection by the Cardiio smartphone app was accurate at rest and after moderate- and vigorous-intensity exercise in a healthy young adult sample. Contact-free facial PPG detection is more convenient but is less accurate than finger PPG due to body motion after exercise.
authors.affiliation: Division of Cardiology, Department of Medicine and Therapeutics, The Chinese University of Hong Kong and Prince of Wales Hospital, Hong Kong, China (Hong Kong).
authors.forename: Bryan P
authors.identifier: http://orcid.org/0000-0003-0430-5752
authors.initials: BP
authors.lastname: Yan
authors.affiliation: Division of Cardiology, Department of Medicine and Therapeutics, The Chinese University of Hong Kong and Prince of Wales Hospital, Hong Kong, China (Hong Kong).
authors.forename: Christy Ky
authors.identifier: http://orcid.org/0000-0001-7585-3153
authors.initials: CK
authors.lastname: Chan
authors.affiliation: Faculty of Medicine, Newcastle University, Newcastle upon Tyne, United Kingdom.
authors.forename: Christien Kh
authors.identifier: http://orcid.org/0000-0001-8305-3850
authors.initials: CK
authors.lastname: Li
authors.affiliation: Division of Cardiology, Department of Medicine and Therapeutics, The Chinese University of Hong Kong and Prince of Wales Hospital, Hong Kong, China (Hong Kong).
authors.forename: Olivia Tl
authors.identifier: http://orcid.org/0000-0002-7554-2766
authors.initials: OT
authors.lastname: To
authors.affiliation: Division of Cardiology, Department of Medicine and Therapeutics, The Chinese University of Hong Kong and Prince of Wales Hospital, Hong Kong, China (Hong Kong).
authors.forename: William Hs
authors.identifier: http://orcid.org/0000-0002-0802-4969
authors.initials: WH
authors.lastname: Lai
authors.affiliation: Division of Cardiology, Department of Medicine and Therapeutics, The Chinese University of Hong Kong and Prince of Wales Hospital, Hong Kong, China (Hong Kong).|Li Ka Shing Institute of Health Sciences, Faculty of Medicine, The Chinese University of Hong Kong, Hong Kong, China (Hong Kong).
authors.forename: Gary
authors.identifier: http://orcid.org/0000-0001-5510-1253
authors.initials: G
authors.lastname: Tse
authors.affiliation: Cardiio Inc, Cambridge, MA, United States.
authors.forename: Yukkee C
authors.identifier: http://orcid.org/0000-0001-6925-8614
authors.initials: YC
authors.lastname: Poh
authors.affiliation: Cardiio Inc, Cambridge, MA, United States.
authors.forename: Ming-Zher
authors.identifier: http://orcid.org/0000-0002-3510-1923
authors.initials: MZ
authors.lastname: Poh
keywords: heart rate; mobile apps; mobile phone; photoplethysmography; smartphone