In our last article we introduced consumer wearable devices (CWDs) and its use in monitoring heart rate and rhythm. This week we will focus on some other physiological markers that these devices can evaluate.

1. Steps

It has become part of popular culture that for good health one should walk 10,000 steps per day. It is interesting to note, however, that this recommendation has an anecdotal origin. It originated in 1960s Japan where the Yamasa corporation produced a pedometer (a device that counts steps walked) with the nickname manpo-kei (literally translated 10,000 steps meter). This nickname was widely adopted by Japanese walking clubs of the time. Subsequent studies in Japan found that for an average Japanese male this amount of walking led to the burning of 300kcal/day and the setting of this goal for a healthy lifestyle by the Japanese Health Ministry. Currently, when thinking about exercise prescription, most Western organisations recommend a duration of aerobic activity of 150 minutes per week of moderate exercise or 75 minutes per week of intense exercise as opposed to counting steps. It is, however, clear that there are health outcomes that are associated with the number of steps taken daily. Higher step counts are associated with a lower risk of death and a lower risk of cardiovascular disease. A recent meta-analysis combining 15 studies from several countries found that in the population above the age of 60, walking more than 6000-8000 steps was associated with a lower risk of death, whereas for those below the age of 60 years, 8000-10000 steps were required.

There is some data that suggests that simply wearing a CWD that counts steps increases the number of steps that are taken and thus there is the potential for lowering the risk of death and developing cardiac disease in those who wear these devices. CWDs measure step counts by using accelerometers and gyroscopes which respond to motion of the body part to which the device is attached. The accuracy of CWDs for calculating steps can be quite variable, with some devices consistently overestimating or underestimating steps taken when compared to the gold standard using video recording to count steps walked. The accuracy has been found to vary with the manufacturer, the model, the speed of walking, and body position of the CWD. It is important to say, however, that given the relatively wide ranges for which benefit has been shown (6000-8000 or 8000-10000) a device that is extremely accurate may not be required. From a health perspective sufficient evidence exists to encourage those who do not get enough steps in on a day-to-day basis to either increase their step count or follow the duration guidelines for weekly exercise.

2. Oxygen saturation

Several CWDs can measure the blood oxygen saturation. This is done by measuring the absorption of a light source by haemoglobin. There is variation in the amount of light absorbed in proportion to the amount of oxygen that is bound to the haemoglobin molecule and thus the percentage saturation (a measure of the amount of oxygen in the blood) can be calculated. A value of 95 per cent or above is considered to be normal. One important exception is during exercise when studies have found that male competitive athletes can have low saturation levels (less than 93 per cent or 91 per cent depending on the study) in more than 70 per cent of those studied. This feature of CWDs was of much importance during the recent COVID-19 epidemic when low oxygen saturations were used to identify patients who needed hospitalisation or intensification of medical care. Several CWDs have been found to have accuracy similar to medical-grade oximetry devices; however, there has been limited testing comparing CWDs to the gold standard measurement of the blood gas levels in arterial blood.

Given the fact that the measurement of oxygen saturation depends on light penetration of skin, there are concerns about the accuracy of these measurements in dark-skinned individuals. It has been shown that melanin, which is present in those with darker skin, can absorb the projected light leading to inaccuracies in the calculated oxygen saturation. The clinical impact of this has been seen during the COVID-19 epidemic. A retrospective study looking at over 25,000 patients found that medical-grade oximetry devices were more likely to overestimate the amount of oxygen in the blood in black and Hispanic patients when compared to white patients. This overestimation led to a reduced ability to identify black patients who needed more aggressive care when compared to white individuals and translated into 10 per cent less appropriate therapy in these dark-skinned individuals. Given these limitations of measurement of oxygen saturation in black and Hispanic populations, studies are being conducted to improve the accuracy of these devices. Over-reliance should not be placed on a normal oxygen saturation if symptoms suggest significant respiratory issues.

3. Sleep

Sleep is increasingly recognised as an important component of a healthy lifestyle, and it is recommended that most adults get at least 7 hours of sleep per night although this can vary substantially in a particular individual. The scientific evaluation of sleep duration and sleep quality is well established and can be performed using polysomnography. This testing involves sleeping in a medical facility and recording various physiological parameters, including the electrical activity of the brain, eye movements, limb movements, heart rate, and respiration. CWDs do offer the ability to assess some aspects of sleep at home. In evaluating sleep, CWDs assess motion with accelerometers, measuring whether the limbs (most commonly the arms) are moving. In addition to limb movement, different devices will combine additional physiological data to improve discrimination of the duration and the stage of sleep such as heart rate and respiratory rate.

Given the fact that consumer wearable devices are primarily looking at limb movements, there is the tendency for the devices to underestimate sleep in people who have movement disorders during sleep and overestimate sleep duration in those who have limited movements when lying in bed awake. These devices have also been shown to be poor at detecting waking episodes during the middle of the night. When studied against the gold standard of polysomnography, CWDs do fairly well in detecting sleep duration, but they perform less well in classifying sleep stages, tending to overestimate light sleep and deep sleep and underestimate sleep with rapid eye movement.

Sleep apnoea is increasingly recognised as a common condition and serves as a risk factor for some medical conditions, including hypertension and cardiac arrhythmias. Given the ability to recognise falls in oxygen saturation, respirations, and heart rate, CWDs can indicate abnormalities that may suggest a diagnosis of sleep apnoea. There is also the opportunity to use cellphone microphone recording to detect snoring; however, further research needs to be done to define the usefulness of CWDs in this setting. It is important to remember that sleep duration is the primary goal, in terms of sleep quality for the general population. The use of CWDs can be useful in recording and following this over time. People who have markers of poor sleep duration or other abnormalities noted on CWDs can speak with their physicians about a formal medical-grade sleep study.

As we pointed out in the first article, the use of consumer wearable devices is increasing and bringing health data to our fingertips in easily accessible forms. It is important to be aware of some of the limitations of this data, but there is significant potential for individual monitoring of previously inaccessible physiological findings and possible improvement in health outcomes by leveraging this data.

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 info@caribbeanheart.com or call 876-906-2107.