AFE clinical grade vital signs for disease detection
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 their importance.
Unfortunately, most people who have continuous vital signs monitoring are likely already in a clinical setting and being treated for an acute condition. Instead of using vital signs as an indicator of the effectiveness of disease treatment and patient recovery, the model for future healthcare will be to use continuous and remote monitoring of vital signs as a tool to identify potential indicators of disease onset, allowing interventions to take place in clinicians as early as possible before serious disease develops.
It is envisioned that the ever-increasing integration of clinical grade sensors will ultimately allow the development of disposable and wearable patches for vital sign health that are discarded and replaced periodically, such as contact lenses.
Although many portable health and fitness devices include functions for measuring vital signs, the integrity of their readings can be questionable for several reasons, including the quality of the sensors used (most are not clinical grade. ), the location to which they are mounted and the quality of contact with the body when worn.
While these devices are sufficient to satisfy the desire for occasional self-observation by non-healthcare professionals using a convenient and comfortable portable device, they do not meet the standards of performance and accuracy required for healthcare professionals. Skilled health professionals correctly assess an individual’s health and make an informed diagnosis.
On the other hand, the devices currently used to provide clinical grade vital sign observation over extended time intervals can be cumbersome and uncomfortable, with varying degrees of portability. In this design solution, we examine the clinical significance of four vital sign measurements – blood oxygen saturation (SpO2), heart rate (HR), electrocardiogram (ECG), and respiratory rate (RR) – and consider the best type of sensor to provide clinical data – the level readings of each.
Blood oxygen saturation
Healthy individuals generally have a blood oxygen saturation level in the region of 95 to 100%. However, SpO2 levels of 93% or less can be an indication that an individual is suffering from respiratory distress – a common symptom of COVID-19 patients, for example – making it an important vital sign for healthcare professionals. to be monitored regularly. Photoplethysmography (PPG) is an optical measurement technique that uses multiple LED emitters to illuminate blood vessels below the skin’s surface and photodiodes receptors to detect the reflected light signal, thereby allowing SpO2 to be calculated. Although they have become a common feature in many wearable devices worn on the wrist, PPG light signals are prone to interference from motion artefacts and transient variations in ambient lighting that can potentially cause erroneous readings, which can lead to erroneous readings. means that these devices do not provide clinical quality measurements. In a clinical setting, SpO2 is measured using a finger-worn pulse oximeter (Figure 2), typically attached continuously to the finger of a motionless patient. Although there are portable battery-powered versions, they are only convenient for taking intermittent measurements.
Heart rate and ECG
A healthy heart rate (HR) is generally considered to be between 60 and 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 are almost the same, as blood is pumped throughout the body with each contraction of the heart muscle. However, some serious heart conditions can cause a difference between HR and heart rate.
For example, in the case of cardiac arrhythmias such as atrial fibrillation (Afib), not all muscle contractions in the heart pump blood throughout the body – instead, blood can pool in the chambers. of the heart itself, a potentially fatal event. Afib can be difficult to detect because it sometimes occurs intermittently and only for short, transient intervals.
The importance of being able to detect and treat this disease is demonstrated by the fact that, according to the World Health Organization, a quarter of all strokes in people over 40 are caused by Afib. Since PPG sensors take optical measurements assuming the heart rate is the same as the pulse rate, it cannot be relied on to detect Afib. This requires that the electrical activity of the heart be recorded continuously 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. Although these use fewer electrodes than static ECG monitors used in clinical settings, they can be somewhat bulky and uncomfortable to wear, especially while sleeping.
12 to 20 breaths per minute is the expected respiratory rate (RR) for most healthy people. An RR rate exceeding 30 breaths per minute may be an indicator of respiratory distress caused by fever or other reason. While some portable solutions infer RR using accelerometer or PPG techniques, clinical-grade measurement of RR is performed either using the information contained in an ECG signal or using ‘a bioimpedance sensor (BioZ) which characterizes the electrical impedance of the skin using two or more electrodes attached to a patient’s body.
While FDA-approved ECG functionality is available in some high-end portable health and fitness devices, bioimpedance sensing is a feature that is typically not provided because it requires the inclusion of an integrated circuit. separate BioZ sensor. In addition to RR, BioZ sensors also enable bioelectric impedance analysis (BIA) and bioelectric impedance spectroscopy (BIS), both of which are used in the measurement of compositional levels of muscle, fat and fat. body water. BioZ sensors also enable impedance cardiography (ICG) and are used to measure the galvanic skin response (GSR), which can be a useful indicator of stress.
The block diagram of a clinical grade AFE vital signs integrated circuit that integrates the functions of three separate sensors – PPG, ECG, and BioZ – in a single package is shown in Figure 1.
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 being programmable from two high current 8-bit LED drivers. The receive path has two low noise, high resolution read channels which each include independent 20-bit ADCs and an ambient light cancellation circuit, providing over 90dB of ambient rejection at 120Hz. The PPG channel features up to 113 dB of SNR which supports Sp02 measurements at just 16 µA.
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