Guest blogger, Jess Goodman M.D., is the current President of VitalSines Inc., a company which manufactures health monitoring systems. Dr. Goodman is a physician with 30+ years of experience and currently serves on the University of Toronto Teaching Staff.
There has been an explosion of interest and in wrist worn personal health monitoring technology over the last few years. Wrist worn health monitors are becoming increasingly sophisticated, with some having optical sensors in their base allowing continuous heart rate sensing without need for a chest strap. The watch shown below illustrates this concept. TI’s AFE4400 and the AFE4490 Oximeter System on a Chip (SoC) allows a wristwatch optical sensor to acquire far more information than heart rate alone.
Use of an optical sensor in a wristwatch provides an ability to continuously monitor human health in ways never before possible. This four part blog series will examine how the AFE4400 and the AFE4490 oximeter SoC supports highly innovative approaches to continuous capture of the arterial pulse signal from the wrist and other areas of the body. The blog will also examine how these innovations will lead to changes in the way people care for their health and the way human life is understood and visualized.
VitalSines, a company I founded, has prototyped a wristwatch personal health monitor called the Biowatch using the AFE4490 along with a 10 axis motion sensor, 4GB Flash memory and Bluetooth. In later blog entries, I’ll use the BioWatch to show how people will soon be able to view their health on a 24/7 basis with Internet server assisted data analysis, storage, visualization and communication. The view of personal health we will have in ten years is beyond our ability to imagine today.
The AFE4400 and the AFE4490 SoC functions as a pulse oximeter. Oximeters use red and infrared LED’s along with a photodiode to obtain the arterial pulse signal. The arterial pulse signal is conventionally used to determine blood oxygen saturation and heart rate. The AFE4400 and the AFE4490 integrates a number of separate components necessary to perform pulse oximetry in a way that optimizes signal quality, offers precise control over signal characteristics, minimizes power drain, lowers cost and reduces size. This allows the arterial pulse signal to be used in new ways that may even allow continuous blood pressure readings to be obtained using the optical signal alone.
Before discussing applications of the AFE4400 and the AFE4490 it is important to understand why it is such an advance in the art of oximetry. Innovative characteristics of the AFE4400 and the AFE4490 allow high quality acquisition of the pulse signal from the wrist and other body surfaces:
- Differential transimpedance amplifier - Most oximeters use a single transimpedance amplifier to convert a photodiode’s current output to a voltage. Because an oximeter uses both red and infrared LED’s, it is necessary to sample the photodiode output with each LED turned on and the off in an alternating way. This allows high frequency noise to be digitized along with the pulse signal, a problem called aliasing. Aliasing corrupts the pulse signal, reduces its information content and decreases its usefullness. Use of a Differential Transimpedance Amplifier samples both sides of a photodiode at the same time, cancelling out high frequency noise. This provides a pulse signal with greater information content, more useful for sophisticated signal analysis.
- High LED current output – The wrist and other body surfaces are much more challenging for oximetry than the fingernail. It is necessary to illuminate these areas well and the AFE4400 and the AFE4490 allows use of LED currents as high as 200ma.
- Short LED on times - To minimize current drain and prevent heating of the skin it is important to turn LED’s on for very short periods. The AFE4400 and the AFE4490 allows LED on times as short as 50 microseconds.
- DC Bias removal – The pulse signal is a small wave riding on a much larger voltage (DC) Bias as shown in the diagram below. The pulse signal has amplitude of only 2% of the voltage range. This small signal needs to be isolated and amplified in order to be useful. The AFE4400 and the AFE4490 avoids any high pass filtering (that degrades the signal) in order to achieve this objective. It does so by injecting just enough current into the pulse signal to remove DC Bias and also uses a very high resolution analog to digital converter (22 bits) to capture the pulse signal with exceptional quality.
Ambient light removal – With both LED’s off the AFE4400 and the AFE4490 samples photodiode output to allow simple removal of the effects of ambient light. This allows an oximeter to be used in a highly mobile fashion, moving from brighter areas to darker areas without degradation of pulse signal quality.
In my next blog I’ll discuss using the AFE4400 and the AFE4490 to support a virtual hospital home medical monitoring model for chronic illness such as congestive heart failure and emphysema (COPD).