The importance of the physiological vital signs as indicators of human health have long been understood by medical professionals but the current COVID-19 pandemic has also heightened public awareness of their significance.
Unfortunately, most people who find themselves undergoing continuous vital signs monitoring are likely already in a clinical setting being treated for an acute condition. Instead of using vital signs as an indicator for the effectiveness of disease treatment and patient recovery, the model for future healthcare will be to employ continuous and remote vital signs monitoring as a tool to identify potential indicators of disease onset, allowing interventions to take place by clinicians at the earliest opportunity before serious illness develops.
It is envisaged that ever increasing integration of clinical-grade sensors will eventually enable the development of disposable, wearable vital-signs health patches that are disposed of and replaced periodically such as contact lenses.
While many health and fitness wearables include vital signs measurement functions, the integrity of their readings can be questionable for several reasons, including the quality of sensors used (most are not clinical grade), the location at which they are mounted, and the quality of body contact while worn.
Although these devices are sufficient to satisfy the desire for casual self-observation by non-health professionals using one convenient and comfortable wearable, they do not meet the standards of performance and accuracy required for trained medical professionals to properly assess the health of an individual and make an informed diagnosis.
On the other hand, devices currently used to provide clinical-grade vital signs observation over extended time intervals can be bulky and uncomfortable, with 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 the best type of sensor to provide clinical-grade readings of each.
Blood Oxygen Saturation
Healthy individuals typically have a blood oxygen saturation level in the region of 95-100%. However, SpO2, levels of 93% or lower can be an indication that an individual is experiencing respiratory distress – a common symptom of COVID-19 patients, for example – making it an important vital sign for medical professionals to monitor regularly. Photoplethysmography (PPG) is an optical measurement technique that uses several LED transmitters to illuminate blood vessels under the surface of the skin and photodiode receivers to detect the reflected light signal, thereby allowing SpO2 to be calculated. While it has become a common feature in many wrist-worn wearables, PPG light signals are prone to interference from motion artifacts and transient variations in ambient lighting that can potentially cause spurious 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 the finger of a stationary patient. While battery-powered, portable versions exist, they are only practical for making intermittent measurements.
Heart Rate and ECG
A healthy heart rate (HR) is usually considered to be in the range of 60-100 beats per minute, however, the time interval between individual beats is not constant. Often referred to as heart rate variability (HRV), this means that heart rate is an average value measured over several beat cycles. In a healthy individual, HR and pulse rate are almost identical, because blood is pumped around the body with each contraction of the heart muscle. However, some serious cardiac conditions can cause the HR and pulse rate to differ.
For example, in the case of cardiac arrhythmias such as atrial fibrillation (Afib), not every muscle contraction within the heart pumps blood around the body – instead blood can accumulate within the chambers of the heart itself, a potentially life-threatening occurrence. Afib can be difficult to detect as it sometimes occurs intermittently and only for short transient intervals.
The importance of being able to detect and treat this condition is evidenced by the fact that, according to the World Health Organization, one quarter of all strokes in people over the age of 40 are caused by Afib. Since PPG sensors make optical measurements on the assumption that HR is identical to pulse rate, they cannot be relied on to detect Afib. This requires the electrical activity of the heart to be continuously recorded over an extended interval – the graphical representation of the heart’s electrical signal is called an electrocardiogram (ECG).
The Holter monitor is the most common clinical-grade portable device for this purpose. While these use fewer electrodes than static ECG monitors used in clinical settings, they can be somewhat cumbersome and uncomfortable to wear, particularly when sleeping.
12-20 breaths per minute is the respiration rate (RR) expected for most healthy people. A RR rate exceeding 30 breaths per minute may be an indicator of respiratory distress caused by a fever or some other reason. While some wearables solutions deduce RR using accelerometer or PPG techniques, clinical-grade measurement of RR is performed using either using the information contained in an ECG signal, or by using a bioimpedance (BioZ) sensor that characterizes the electrical impedance of the skin using two or more electrodes attached to the body of a patient.
While FDA-approved ECG functionality is available in some high-end health and fitness wearables, bioimpedance sensing is a feature that is typically not provided as it requires the inclusion of a separate BioZ sensor IC. Apart from RR, BioZ sensors also enable bioelectrical impedance analysis (BIA) and bioelectrical impedance spectroscopy (BIS), both used in the measurement of body muscle, fat, and water composition levels. BioZ sensors also enable impedance cardiography (ICG) and are used to measure galvanic skin response (GSR), which can be a useful indicator of stress.
The functional block diagram of a clinical-grade vital-signs AFE IC that integrates the functions of three separate sensors – PPG, ECG and BioZ- into a single package, is presented in Figure 1.
Figure 1 The 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 being programmable from two high-current, 8-bit LED drivers. The receive path has two low-noise, high-resolution readout channels that each include independent 20-bit ADCs and ambient light cancellation circuit, providing in excess of 90dB of ambient rejection at 120Hz. The PPG channel exhibits up to 113dB SNR that supports Sp02 measurements at only 16µA.
Next: three in one sensing
The ECG channel is a complete signal chain that provides all of the critical features necessary to collect high-quality ECG data such as flexible gain, critical filtering, low noise, high input impedance, and multiple lead-biasing options. Additional features, such as fast recovery, AC and DC lead-off detection, ultra-low-power lead-on 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-selected output sample rates.
The BioZ receive channel has EMI filtering and extensive calibration features. The BioZ receive channel also has high input impedance, low noise, programmable gain, lowpass and high-pass filter options, and a high-resolution ADC. There are several modes for generating input stimulus: balanced square-wave source/sink current, sine-wave current, and both sinewave and square-wave voltage stimuli. A wide range of stimulus magnitudes and frequencies is available. It also supports BIA, BIS, ICG and GSR applications.
FIFO timing data allows all three sensor channels to be synchronized. This AFE IC is available in a 7 x 7 49-bump wafer-level package (WLP) with package dimensions of only 2.6mm x 2.8mm, making it ideal to be designed into, for example, 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 into 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 which healthcare professionals use for diagnostic purposes. Continuous vital sign monitoring using wearable devices will be a key component in the model for future healthcare, allowing disease onset to be anticipated before symptoms develop.
The measurements produced by many currently available vital-sign monitors cannot be used by healthcare professionals because they use sensors that are not clinical grade while others simply do not have the capability to accurately measure RR because they do not include a BioZ sensor.
In this design solution we presented an IC that integrates three clinical-grade sensors – PPG, ECG and BioZ into a single package and showed how it can be designed into chest and wrist-worn wearables to measure SpO2, HR, ECG and RR while also providing additional useful health -related features including BIA, BIS, GSR and ICG. Apart from application in clinical-grade wearable devices, this IC is also ideal for integration into smart clothing to provide the type of information required by high-performance athletes.
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