Heart Rate Apps Show Inconsistent Results

There is a huge variability among commercially available photoplethysmography (PPG) heart rate apps, even those using the same technology, according to a new study.

Researchers at Heart Clinic Zurich and University Hospital Zürich conducted a study to test the clinical diagnostic accuracy of four iPhone-based heart rate measuring apps that use the built-in camera and flash. These included the Instant Heart Rate (IHR) and Heart Fitness (HF) apps, which work via contact PPG, and the What’s My Heart Rate (WMH) and Cardio Version (CAR) apps, which are based on non-contact PPG. Measurements were compared to electrocardiogram (ECG) and pulse oximetry-derived heart rate.

In all, app-based PPG was performed on 108 randomly selected patients. The results showed substantial differences in accuracy between the four apps, with non-contact apps performing less well than contact apps, particularly at higher heart rates and lower body temperatures. The overall accuracy of PPG app-measured heart rate, reported as mean absolute error in beats per minute compared to ECG, was 2 for pulse oximetry, 4.5 for IHR, 2 for HF, 7.1 for WMH, and 8.1 for CAR. The study was published on May 2, 2017, in the European Journal of Preventive Cardiology.

“While it's easy to use the non-contact apps - you just look at your smartphone camera and it gives your heart rate - the number it gives is not as accurate as when you have contact with your smartphone by putting your fingertip on the camera,” said senior author cardiologist Christophe Wyss, MD, of Heart Clinic Zurich. “But the performance of the two contact apps was also different. The one contact app was excellent, performing almost like a medically approved pulse oximeter device, but the other app was not accurate, even though they use the same technology.”

“Heart rate apps come installed on many smartphones and once people see them it is human nature to use them and compare their results with others. The problem is that there is no law requiring validation of these apps, and therefore no way for consumers to know if the results are accurate,” concluded Dr. Wyss. “Consumers and interpreting physicians need to be aware that the differences between apps are huge and there are no criteria to assess them.”

As the heart pumps blood to the periphery a pressure pulse is created, which distends the arteries and arterioles in the subcutaneous tissue. The change in volume caused by the pressure pulse is detected by illuminating the skin with the light from a light-emitting diode (LED), and then measuring the amount of light either transmitted or reflected to a photodiode, generating a PPG.