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.
尽管许多健康和健身可穿戴设备都包括生命体征测量功能，但由于几个原因，它们的读数的完整性可能值得怀疑，包括所使用的传感器质量（大多数不是临床等级），安装的位置以及质量的质量磨损时的身体接触。
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.
血氧饱和度
健康个体通常在95-100％的地区具有血氧饱和度。但是，SPO2的水平为93％或更低，这表明个人正在经历呼吸窘迫 - 例如，CoVID -19患者的常见症状 - 使医疗专业人员定期监测的重要体征是重要的生命体征。Photoplethysmmography（PPG）是一种光学测量技术，它使用多个LED发射器来照亮皮肤表面下的血管和光电二极管接收器以检测反射的光信号，从而允许计算SPO2。尽管它已成为许多腕上戴可穿戴设备的共同特征，但PPG光信号容易受到运动伪像的干扰和环境照明的瞬时变化，这可能会引起虚假读数，这意味着这些设备不提供临床级测量。在临床环境中，SPO2是使用手指磨损的脉搏血氧仪（图2）通常连续连接到固定患者手指上的。尽管存在电池供电的便携式版本，但它们仅用于进行间歇性测量。
心率和心电图
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.
例如，在心律不齐的情况下，例如心房颤动（AFIB），而不是心脏内部的每一个肌肉收缩都会在体内泵入血液，而是血液可以在心脏本身的腔体内积聚，这可能会危及生命。AFIB可能很难检测到它有时间歇性发生，并且仅用于短暂间隔。
据世界卫生组织称，能够检测和治疗这种情况的重要性证明了这一事实是，40岁以上的人中有四分之一是由AFIB引起的。由于PPG传感器对HR与脉搏率相同的假设进行了光学测量，因此不能依靠它们检测AFIB。这要求心脏的电活动在扩展间隔内连续记录 - 心脏电信号的图形表示称为心电图（ECG）。
Holter Monitor是为此目的最常见的临床级便携式设备。尽管这些电极的用电极少于临床环境中使用的静态ECG监测器，但它们的穿着可能会很麻烦和不舒服，尤其是在睡觉时。
Respiration Rate
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.
尽管在某些高端健康和健身可穿戴设备中可以使用FDA批准的ECG功能，但生物阻抗感应是通常无法提供的功能，因为它需要包含单独的Bioz传感器IC。除RR外，Bioz传感器还可以实现生物选择性阻抗分析（BIA）和生物电阻抗光谱（BIS）（BIS），均用于测量身体肌肉，脂肪和水分水平。Bioz传感器还可以实现阻抗心脏摄影（ICG），并用于测量电流皮肤反应（GSR），这可能是应力的有用指标。
3合1传感器
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.
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