**2. Vital signs remote monitoring**

This section details some vital signs and how they are captured from the body. Vital signs monitoring is essential to observe how healthy a person is. Logically, we have lower and upper thresholds for each vital sign, that is, limits that a particular metric should operate. Also, these thresholds variate in accordance with the age of the person, their health status, if they have chronic diseases, and so on. The vital signs discussed here are relevant to give us insights sequels regarding the long COVID-19, which is especially important for patients with chronic diseases. The literature shows us that five health parameters are essential vital signs to detect the evolution of COVID- 19. **Figure 1** depicts them appropriately. They are: (i) heart rate; (ii) heart rate variability; (iii) body temperature; (iv) peripheral oxygen saturation; and (v) respiratory rate.

#### **Figure 1.**

*Five vital signs are captured: (1) respiratory rate; (2) heart rate and heart rate variability; (3) body temperature; and (4) peripheral oxygen saturation.*

### **2.1 Vital sign 1: respiratory rate**

This vital sign analyzes the breathing rate per minute (BRPM). The literature states that an expected respiratory rate is between 12 and 20 BRPM. Regarding the idea of reducing the BRPM, there is a more significant change that a particular person has COVID-19. This occurs because COVID-19 attacks the lungs, turning challenging to breathe well and regularly. A wearable device collection can monitor BRPM through piezoresistive and inertial sensors [4]. Moreover, when analyzing the literature, we observe that most approaches require sensors in the patient's chest, abdomen, neck, or nose. We also observe the growth of new solutions that explore algorithms to derive the respiratory rate from optic sensors embedded in smartwatches and wristbands.

#### **2.2 Vital sign 2: temperature**

Body temperature is one of the most used signals to understand a person's health. The temperature is primarily used to observe that a person has an inflammation process, the starting of a disease, or particle reactions against viruses or bacteria [5]. Thus, fever is the second most common symptom of a COVID-19 infection. Usually, a body temperature over 37.3°C characterizes fever. As said, this could indicate that the body is trying to fight an illness or infection. Body temperature is captured in different ways: axillary, orally, and rectally using traditional thermometers.

Recent solutions use technologies that can measure skin temperature too. The skin temperature frequently varies to regulate and stabilize the core temperature. The use of imaging and infrared devices became common to fastly check the body temperature of individuals in a touchless manner. Other strategies apply skin carbon nanotube (CNT) printed adhesives that provide more precise temperature detection. However, CNT-based sensors require a computing unit to acquire data from them to make them available for processing.

#### **2.3 Vital sign 3: heart rate**

Heart rate measures the beats per minute (BPM) of an individual's cardiac cycle [6]. It varies throughout the day physiologically in a healthy individual, according to physical activity, consumption of caffeinated foods, and emotions, for example. However, the appropriate heart rate range for an individual at rest is from 50 to 90 BPM, which may be lower in people who practice physical activities [7]. Segundo [8] with a 1°C rise in body temperature, there is approximately the same amount as an 8.5 BPM increase in heart rate. For example, traditional methods for measuring heart rate include the electrocardiogram (ECG) and radial pulse palpation. However, such methods, in addition to depending on a professional to be measured, also have little mobility for access and difficulty monitoring daily activities. The ECG is an accurate method, but it needs to be in a specific place to perform it since the measurement through the radial pulse can be imprecise. Thus, current initiatives seek to explore different strategies of bioelectric sources to measure heart rate accurately. Smart bands often use photoplethysmography (PPG) to measure heart rate [9]. When the heart beats, capillaries expand and contract based on changes in blood volume. PPG is a non-invasive optical technique capable of measuring blood volume variations in the capillary structure [10]. Thus, continuous monitoring can predict some pathologies, such as arrhythmias, anemia, and hyperthyroidism. In the context of COVID-19, the heart is one of the organs affected by the virus. It can attack the organ of someone who already has a previous cardiac pathology or even an acute form of a healthy heart [11]. Thus, some pathologies such as myocarditis, arrhythmias, and cardiac arrest may be present in infected patients [12]. Thus, early detection of heart rate can help in the perception of a sign of severity in patients with the disease, in addition to facilitating the early detection of symptoms at home and in hospitals.

### **2.4 Vital sign 4: heart rate variability**

Heart rate variation (HRV) is the variation between the time interval between two beats in the cardiac cycle and is one of the main ways to assess the proper functioning of the heart and the regulation of the autonomic nervous system [13], so it is a relevant measure to identify and assist in the prognosis of various pathologies. It has a large interpersonal variation, and a high value represents a more significant resistance to stress, while a low value may indicate illness, stress, depression, or anxiety, and low values may provide an early indication that the individual is suffering from infection [14]. In the context of COVID-19 infection, it is a relevant parameter to indicate how the patient's prognosis will be. Recent studies carried out with patients over 70 years old showed that those with high HRV had greater survival and low HRV showed greater survival. Intensive care unit admission rate [15]. The main way to detect HRV is through continuous monitoring in the hospital or the ward through the electrocardiogram. Hence, the lack of mobility is the main difficulty in continuous monitoring. In this way, wearable smart bands that rely on heart rate measurements, calculating the root mean square of successive differences between normal heartbeats (RMSSD), it is possible to determine and measure HRV using heart rate measurements, and thus help to monitor patients both in hospital and domestic environments, mitigating the problem of mobility [16].

#### **2.5 Vital sign 5: oxygen saturation**

Blood oxygen saturation (SpO2) level measures the percentage of oxygen carried by hemoglobin molecules in an individual's peripheral blood. It may be decreased when an infection occurs, such as COVID-19, in which inflammatory cytokines prevent the efficient gas exchange from occurring in the respiratory membranes. Rates below 95% of oxygenated blood indicate a warning that the individual may be starting to become short of breath. Oxygen levels commonly remain at the same rate during all daily activities. Standard approaches to compute SpO2 use PPG signals composed of red and infrared light sensors applied to the extremities of the body [17].
