ONCE the province of the professional athlete or the weekend warrior, the use of the consumer wearable devices (CWDs) has become more popular among the general population.
It estimated that by 2022 more than $1 billion of these devices would be in use worldwide. These devices allow the measurement of a variety of physiological data that was once confined to medical or research settings. Examples of these devices include the smart watch (eg, the Apple Watch, Samsung Galaxy watches, Garmin forerunner devices), heart rate monitors worn during exercise and a host of rings and bracelets with the ability to measure heart rate, assess sleep quality and duration, the amount and intensity of exercise and oxygen saturation.
With the increased frequency of use among the general population, our patients often turn to us for an explanation of the data obtained particularly when abnormal findings are seen. From the physician perspective, for a variety of reasons that we will discuss, it can be difficult to assess the clinical importance of these findings. For many types of data there are inadequate definitions of what is normal for a population or an individual. There are often real-world factors that can reduce device accuracy and each company can have propriety algorithms for generation of data that have not been tested against a gold standard.
There are several ways that heart rate can be obtained from a CWD. The two most common are the use of a chest strap to obtain a single lead ECG or the use of plethysmography in which light is used to penetrate the skin and subcutaneous blood vessels. This light is reflected to the device from the blood cells. The intensity and frequency of those reflections is used to generate a heart rate. Most watches, rings and bracelets use the second method to measure heart rate, although some watches can generate a single lead ECG that is user activated. When evaluated in research settings both methods give reasonable approximations of the heart rate when compared to a gold standard ECG; however, in "the real world" the accuracy tends to be less. A chest strap is generally more accurate than devices using plethysmography; however, a chest strap can be affected by motion causing artefact. It can also be affected by the type of exercise, for example, cycling vs running because of transient loss of contact with the skin or challenges with Bluetooth connection. A watch is generally more sensitive to motion resulting in lower accuracy. An important consideration for our population when using plethysmography devices is skin colour with higher doses of melanin as seen in black populations reducing the accuracy of some of these devices.
Most of our patients who follow their heart rates do so in the setting of exercise, where the heart rate is a reasonable proxy for the intensity of exercise, with higher heart rates being associated with more intense exercise. For runners or cyclists who want to exercise aerobically "so-called fat burning zone" using the heart rate to avoid going too hard is easily achievable. A common concern of our patients is the finding of a heart rate that they think is too fast or too slow. It is important to remember that the heart rate over 24 hours is not a number but is a range. On average the heart rate at rest ie, when sitting quietly is 60-100 beats per minute. This, however, can vary with several factors including age, sex, and level of fitness. A very well-trained endurance athlete can have a resting heart rate that is in the 30s as a normal finding. A normal heart rate will vary with activity and with emotional stress so that a rise in heart rate when walking, climbing stairs, being anxious or arguing with a friend or loved one is normal. It is also important to know that the heart rate can fall quite significantly with sleeping given the loss of activation of the "flight or fight" system with heart rates in the 30s being seen in some individuals.
When should we be concerned about the heart rate? Speaking anecdotally, most abnormalities that bring patients to see a cardiologist are found either to be normal findings or an inaccurate measurement by the CWD. If abnormalities in heart rate are associated with symptoms, then these are of more concern. Very slow heart rates and very fast heart rates (particularly at rest) should always be evaluated recognising that most times no significant abnormality will be found. Some CWDs can generate and record a single lead ECG, and these can be useful in trying to ascertain the clinical importance of heart rate abnormalities seen on these devices. Many of these patients will need medical grade rhythm assessment with electrocardiography or prolonged outpatient rhythm monitoring.
Atrial fibrillation is the most common sustained abnormal heart rhythm and is an important cause of stroke and heart disease. Many patients who experience atrial fibrillation will have symptoms of palpitations, but for some this arrhythmia is silent and the first recognition is when the patient presents with a stroke or heart failure. The ability of a smart watch or other CWD to detect atrial fibrillation thus allows for early identification and potentially early diagnosis and treatment. Atrial fibrillation, unlike normal sinus rhythm, is irregular and if an EKG is done there is no organised activity seen from the upper chambers of the heart. By looking at these features some smart watches can detect atrial fibrillation. Several devices have been approved for the detection of atrial fibrillation by either the US Food and Drug Administration or the European Medicines Agency. These include the Apple Watch, the Samsung Galaxy watch, several Fitbit devices and the Alivecor Kardiamobile device.
There is limited data on the real-world efficacy of these devices in detecting atrial fibrillation after approval particularly since smart watches tend to be used most in younger populations as opposed to the middle aged and elderly who are at greater risk of atrial fibrillation. A recent study was published in the Clinical Electrophysiology Journal of the American College of Cardiology comparing the efficacy of five consumer wearable devices in a group of patients who were undergoing evaluation and treatment for atrial fibrillation. Two hundred and eleven patients with an average age of 65 years were provided with smart watches, instructed on how to use them, and followed for approximately one year. The highest sensitivity was found with the Apple Watch 6 and the Samsung Galaxy 3, which detected around 85 per cent of atrial fibrillation episodes. The Fitbit sense detected 79 per cent of episodes. Among all watches rhythms were detected that the device was unable to classify approximately 20 per cent of the time; however, when a recording was available for review by a cardiologist a correct determination could be made in 95 per cent of the tracings. These findings suggest a potential use for consumer wearable devices in the detection of atrial fibrillation in patients at risk particularly with those devices that have undergone validation studies.
In future articles we will look further at other aspects of consumer wearable devices.
Dr Ernest Madu, MD, FACC and Dr Paul Edwards, MD, FACC are consultant cardiologists for the Heart Institute of the Caribbean (HIC) and HIC Heart Hospital. HIC is the regional centre of excellence for cardiovascular care in the English-speaking Caribbean and has pioneered a transformation in the way cardiovascular care is delivered in the region. HIC Heart Hospital is registered by the Ministry of Health and Wellness and is the only heart hospital in Jamaica. Send correspondence to firstname.lastname@example.org or call 876-906-2107.