The importance of physiological vital signs as indicators of human health has long been understood by medical professionals, but the current COVID-19 pandemic has also raised public awareness of its importance.
Unfortunately, most people who find themselves undergoing continuous vital sign monitoring may already be in a clinical setting where they are being treated for acute illness.Instead of using vital signs as an indicator of the effectiveness of disease treatment and patient recovery, the future model of healthcare will employ continuous and remote vital sign monitoring as a tool to identify potential indicators of disease onset, allowing clinicians to intervene in the development of severe disease. The earliest opportunity before.
It is envisioned that the increasing integration of clinical-grade sensors will eventually enable the development of disposable, wearable vital signs health patches that can be regularly disposed of and replaced, such as contact lenses.
While many health and fitness wearables include vital signs measurement capabilities, the integrity of their readings can be questioned for a number of reasons, including the quality of the sensors used (most are not clinical grade), where they are installed, and where the sensors the quality of.Physical contact while wearing.
While these devices are adequate for the desire of non-health professionals for casual self-observation using a convenient and comfortable wearable device, they are not suitable for trained medical professionals to properly assess individual health and make informed diagnoses.
On the other hand, devices currently used to provide clinical-grade vital sign observations over longer time intervals can be bulky and uncomfortable, and have varying degrees of portability.In this design solution, we review the clinical significance of four vital sign measurements—blood oxygen saturation (SpO2), heart rate (HR), electrocardiogram (ECG), and respiration rate (RR)—and consider providing clinical Best Sensor Type - Readings for each grade.
Blood oxygen saturation levels in healthy individuals are usually around 95-100%.However, a SpO2 level of 93% or lower may indicate that an individual is experiencing respiratory distress—such as a common symptom in patients with COVID-19—making it an important vital sign for regular monitoring by medical professionals.Photoplethysmography (PPG) is an optical measurement technique that uses multiple LED emitters to illuminate blood vessels beneath the skin's surface and a photodiode receiver to detect the reflected light signal to calculate SpO2.While it has become a common feature of many wrist-worn wearables, the PPG light signal is susceptible to interference from motion artifacts and transient changes in ambient lighting, which can lead to false readings, meaning these devices do not provide clinical-grade measurements .In a clinical setting, SpO2 is measured using a finger-worn pulse oximeter (Figure 2), usually continuously attached to a stationary patient's finger.While battery powered portable versions exist, they are only suitable for making intermittent measurements.
A healthy heart rate (HR) is generally considered to be in the range of 60-100 beats per minute, however, the time interval between individual heartbeats is not constant.Commonly referred to as heart rate variability (HRV), this means that the heart rate is an average measured over several heartbeat cycles.In healthy individuals, the heart rate and pulse rate are nearly the same, because with each contraction of the heart muscle, blood is pumped throughout the body.However, some serious heart conditions can cause heart and pulse rates to differ.
For example, in arrhythmias such as atrial fibrillation (Afib), not every muscle contraction in the heart pumps blood throughout the body - instead, blood accumulates in the chambers of the heart itself, which can be life-threatening .Atrial fibrillation can be difficult to detect because it sometimes occurs intermittently and only for short brief intervals.
According to the World Health Organization, Afib causes one in four strokes in people over the age of 40, a fact that demonstrates the importance of being able to detect and treat the disease.Since PPG sensors make optical measurements under the same assumption as HR and pulse rate, they cannot be relied upon to detect AF.This requires continuous recordings of the heart's electrical activity -- a graphical representation of the heart's electrical signals called an electrocardiogram (ECG) -- over long time intervals.
Holter monitors are the most common clinical grade portable devices used for this purpose.While they use fewer electrodes than static ECG monitors used in clinical settings, they can be bulky and uncomfortable to wear, especially while sleeping.
12-20 breaths per minute is the expected respiratory rate (RR) for most healthy individuals.An RR rate above 30 breaths per minute may be an indicator of respiratory distress due to fever or other causes.While some wearable device solutions use accelerometer or PPG technology to infer RR, clinical-grade RR measurements are performed using information contained in the ECG signal or using a bioimpedance (BioZ) sensor that uses two sensors to characterize The electrical impedance of the skin.One or more electrodes attached to the patient's body.
While FDA-cleared ECG functionality is available in some high-end health and fitness wearables, bioimpedance sensing is a feature that is not typically available because it requires the inclusion of a separate BioZ sensor IC.In addition to RR, the BioZ sensor supports Bioelectrical Impedance Analysis (BIA) and Bioelectrical Impedance Spectroscopy (BIS), both of which are used to measure the compositional levels of body muscle, fat and water.The BioZ sensor also supports impedance electrocardiography (ICG) and is used to measure galvanic skin response (GSR), which can be a useful indicator of stress.
Figure 1 shows a functional block diagram of a clinical-grade vital signs AFE IC that integrates the functionality of three separate sensors (PPG, ECG, and BioZ) into a single package.
Figure 1 MAX86178 ultra-low-power, 3-in-1 clinical-grade vital signs AFE (Source: Analog Devices)
Its dual-channel PPG optical data acquisition system supports up to 6 LEDs and 4 photodiode inputs, with the LEDs programmable through two high-current, 8-bit LED drivers.The receive path has two low-noise, high-resolution readout channels, each including independent 20-bit ADCs and ambient light cancellation circuitry, providing over 90dB of ambient rejection at 120Hz.The SNR of the PPG channel is as high as 113dB, supporting SpO2 measurement of only 16µA.
The ECG channel is a complete signal chain that provides all the key features needed to collect high-quality ECG data, such as flexible gain, critical filtering, low noise, high input impedance, and multiple lead bias options.Additional features such as fast recovery, AC and DC lead detection, ultra-low power lead detection and right leg drive enable robust operation in demanding applications such as wrist-worn devices with dry electrodes.The analog signal chain drives an 18-bit sigma-delta ADC with a wide range of user-selectable output sample rates.
BioZ receive channels feature EMI filtering and extensive calibration.BioZ receive channels also feature high input impedance, low noise, programmable gain, low-pass and high-pass filter options, and high-resolution ADCs.There are several modes for generating input stimuli: balanced square wave source/sink current, sine wave current, and sine wave and square wave voltage stimulation.A variety of stimulation amplitudes and frequencies are available.It also supports BIA, BIS, ICG and GSR applications.
FIFO timing data allows all three sensor channels to be synchronized.Housed in a 7 x 7 49-bump wafer-level package (WLP), the AFE IC measures only 2.6mm x 2.8mm, making it ideal for design as a clinical-grade wearable chest patch (Figure 2).
Figure 2 Chest patch with two wet electrodes, supporting BIA and continuous RR/ICG, ECG, SpO2 AFE (Source: Analog Devices)
Figure 3 illustrates how this AFE can be designed as a wrist-worn wearable to provide on-demand BIA and ECG with continuous HR, SpO2, and EDA/GSR.
Figure 3: Wrist-worn device with four dry electrodes, supporting BIA and ECG, with continuous HR, SpO2, and GSR AFE (Source: Analog Devices)
SpO2, HR, ECG and RR are important vital sign measurements used by healthcare professionals for diagnostic purposes.Continuous vital signs monitoring using wearables will be a key component of future healthcare models, predicting disease onset before symptoms appear.
Many of the currently available vital signs monitors produce measurements that cannot be used by healthcare professionals because the sensors they use are not clinical grade, while others simply do not have the ability to accurately measure RR because they do not include BioZ sensors.
In this design solution, we demonstrate an IC that integrates three clinical-grade sensors - PPG, ECG, and BioZ into a single package and show how it can be designed into chest and wrist wearables , to measure SpO2, HR, ECG, and RR, while also providing other useful health-related functions, including BIA, BIS, GSR, and ICG.In addition to being used in clinical-grade wearables, the IC is ideal for integration into smart clothing to provide the type of information that high-performance athletes need.
Andrew Burt is Executive Business Manager, Industrial and Healthcare Business Unit, Analog Devices
Post time: Aug-05-2022