In recent years, the offer of wearable devices that allow monitoring the heart rate with the help of a smartphone has grown substantially. The best example of this type of device is the Apple Watch. Its heart rate monitoring features have been tested and validated in several clinical studies. Apple Watch is currently certified as a medical device by the Food and Drug Administration (FDA), the regulator for medicines and medical devices in the U.S. How does Apple Watch measure heart rate? Apple Watch uses two types of technology to assess heart rate, photoplethysmography and capture of electrical activity like what is done on an electrocardiogram. Photoplethysmography Photoplethysmography is a technology that allows identifying changes in light absorption by tissues and that indirectly detects variations in the blood volume of blood capillaries. In Apple Watch, the light sensors present on the back of the watch and which contact the wrist, use photoplethysmography to measure heart rate and its variability. Several studies have shown that measuring heart rate with Apple Watch, both at rest and during exercise, is reliable and reproducible for most people. However, the relationship between the actual heart rate and the measurement by the Apple Watch is not always perfect. So, when in doubt, it may be necessary to confirm the heart rate using another method , particularly during exercise. Capture of cardiac electrical activity This technology, more exclusive to the Apple Watch and only available in the latest models (from the 4 series), evaluates the heart rate by measuring the electrical activity of the heart , using electrodes on the digital crown and on the back of the watch. By placing a finger on the digital crown, it is possible to create a closed circuit between the chest and the arm , which allows the capture and registration of electrical impulses from the heart , producing a tracing similar to that of an electrocardiogram. In the case of Apple Watch, the recorded cardiac tracing is a simplified version of an electrocardiogram, like in the image bellow, because of the twelve channels of cardiac electrical activity it is only possible to evaluate one. The cardiac tracing can be recorded and later shared and discussed with a doctor. Although it is a simpler version of an electrocardiogram, it may allow the detection and in some vessels to identify the type of arrhythmia. Apple Watch and atrial fibrillation Apple Watch also allows detection of atrial fibrillation. Atrial fibrillation is one of the most frequent arrhythmias and is associated with an increased risk of stroke, especially when undiagnosed and untreated. Several studies have investigated the Apple Watch ability to detect atrial fibrillation. The best known is the Apple Heart Study, which included more than 400 thousand participants who used the Apple Watch. Of these, 2161 received automatic notifications about the possibility of having an irregular pulse and atrial fibrillation. Arrhythmia was confirmed in 34% of participants who agreed to undergo continuous electrocardiogram monitoring for seven consecutive days, immediately after receiving a positive alert. The automatic detection of atrial fibrillation using the Apple Watch is not consensual in the scientific community , so any positive r esult should be discussed with your physician . Additionally, it is important to emphasize that the Apple Watch should not be used to diagnose other heart problems , such as acute myocardial infarction or angina pectoris and that in the presence of suspicious symptoms, medical help should always be sought. References: Apple Watch. Helping your patients identify early warning signs . Boudreaux BD, Hebert EP, Hollander DB, Williams BM, Cormier CL, Naquin MR, et al. Validity of wearable activity monitors during cycling and resistance exercise . Med Sci Sports Exerc 2018;50:624-33. Falter M, Budts W, Goetschalckx K, Cornelissen V, Buys R. Accuracy of apple watch measurements for heart rate and energy expenditure in patients with cardiovascular disease: Cross-sectional study . J Med Internet Res 2019;7:e11889. Isakadze N, Martin SS. How useful is the smartwatch ECG? Trends Cardiovasc Med 2020 30:442-8. Perez MV, Mahaffey KW, Hedlin H, Rumsfeld JS, Garcia A, Ferris T, et al. Large-scale assessment of a smartwatch to identify atrial fibrillation . N Engl J Med 2019;381:1909-17. Author : José Ferreira Santos November 2020 More about the cardiology team at Hospital da Luz network. The content published in the Health Guide is intended for the general public and is provided for informational and educational purposes only. It does not replace individual medical assessment, diagnosis, or personalized clinical advice from a qualified healthcare professional. The content is produced or reviewed by physicians and other healthcare professionals at Hospital da Luz and is based on the knowledge available at the time of publication or the most recent update, as indicated at the beginning of each item.
