Exploring photometry applications and trends

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By eeNews Europe

Pulse oximetry has been an essential tool in helping to save lives and improving the quality of life for well over 50 years. Pulse oximeters monitor the oxygen saturation and heart rate of the wearer noninvasively. Advanced units are able to display the pulsatile nature of the arterial blood vessels. The pulse oximeter is an important medical tool that aids in the diagnosis of cardiac and vascular anomalies in neonatal, pediatric, and adult patients. Analysis of the pulsatile signal gives trained professionals a wealth of information about the respiratory, circulatory, and cardiovascular systems — all without taking a blood sample.

Pulse oximeters are also being used in the home. They are routinely used to determine home oxygen liter-flow and to monitor respiration and heart rate during exercise. Serious athletes and professional trainers monitor oxygen levels to optimize efficiency during strenuous workouts.

This year’s Consumer Electronics Show (CES) in Las Vegas witnessed many new heart rate monitor devices that are being made available to consumers. Many of these devices measure heart rate by acquiring the electrical activity of the heart, predominantly with a chest strap or wristwatch. Chest strap heart monitors have been around a long time, and they work well — but tend to be uncomfortable. The typical wrist-based heart rate monitors is bio-potential-based, and requires contact with both hands to take a measurement. The ideal device would be a passive, but continuously measuring vitals, including heart rate. The device needs to be comfortable to wear and positioned on the body in a location that does not interfere with daily activities. Ideal locations include the wrist, ears, forehead, forearm, calf, neck or ankle.

Principle of operation

The operating principle for a pulse-oximeter being used to measure the blood oxygen saturation level is to project light (normally LED) sources of different wavelengths through the body tissue to a photo-detector. Typically, the LED wavelengths being used for pulse-oximeters are red (~660 nm) and infrared (~905 nm). These two wavelengths of light, are absorbed by the blood at different rates, dependent upon the blood oxygen level.

When the blood is oxygenated red light is absorbed to a lesser extent than when using the infrared (IR) light. When the blood is deoxygenated, red light is absorbed to a greater extent than the IR light. This can be seen in Figure 1, which shows light absorption by wavelength and whether the blood is oxygenated (HbO2) or de-oxygenated (Hb).

Figure 1: Oxygenated versus deoxygenated blood light absorption of IR and RED

Common body locations for taking pulse-oximeter measurements are through the tissue at the fingertip, earlobe, or foot (normally on infants). LEDs intensity, tissue thickness, skin color, sensor placement, and the oxygenated and deoxygenated blood light absorption all need to be considered during measurement.

The oximeter calculates oxygen saturation by taking the ratio of the absorption of red and infrared light, separating the time invariant parameters (intensity of light, skin color, tissue type, and deoxygenated blood) from the time varying parameters (oxygenated blood). Normal oxygen saturation values for a healthy individual range between 95 and 100 percent. The measured signal is pulsatile in nature due to the arterial blood vessels which expand and contract corresponding to each heartbeat. There are two types of pulse-oximeter measurements: transmissive and reflective. Reflective is normally used on the chest or forehead locations. Transmissive, which is more common, is normally used for finger, earlobe, or infant foot locations.

Placement of the LED light source and the photo-diode, along with separation of the light and detector, differentiate these two device types. The transmissive method projects a light through the tissue and a photodiode measures the light, which makes it through to the opposite side. In the reflective method, the light source and photodiode are the same surface. The signal levels for reflective are much smaller, so the user needs to be mindful of where it’s placed and how the circuit is designed.

Pulse-oximeter trends
There are several new market trends driving growth in the pulse-oximeter market. These include: home monitoring / telehealth, emerging markets, and sports and fitness. Additionally, technological advances in electronic hardware and software are fueling market growth.

Home monitoring / telehealth — Home monitoring enables the elderly to stay in their own homes rather than moving to some form of assisted living. Telehealth enables continuous monitoring and aids physicians to determine treatment and drug effectiveness. In the case of the elderly, who typically have more than half a dozen different drugs, oximeters could be used to monitor drug interactions and whether the user has taken the prescribed medications.

Emerging markets — To date, many developing countries in Asia, Africa and other regions have minimal availability of medical diagnostic equipment such as electrocardiogram (ECG), blood pressure meter, pulse oximeter, and many others. Historically this has been due to cost. However, advances in electronic technology have changed this by creating significant improvements in integration and reduction in costs. The increase in integrated analog front-end devices has reduced time-to-market and electrical know-how needed to build some equipment. The circuits often are designed to interface directly to the sensor with minimum external components. What used to take more than a dozen chips and over six months is now done with one chip and as little as one month.

Sports and fitness — Information on the number of steps, distance, speed, power, cadence, skin conductivity and heart rate, are all parameters that are useful in the calculation of activity, calories burned, efficiency, and so on. Until recently, the only way to get continuous heart rate was with the use of a bio-potential chest strap. Heart rate monitor watches have been available for several years and provides periodic measurement. But in order for the user to be able to read their heart rate, they need to wear the heart monitor on one wrist, then either press a button or touch an electrode on the watch face using the other hand.

Monitoring the heart rate continuously can provide significantly more valuable information, such as active, resting and sleeping rates. Over time, it is possible to quantify improvements in fitness levels. Watches that use an optical method of measuring heart rate have the distinct advantage of being continuous and passive. The biggest hurdles to overcome are power, motion cancellation, and ambient light cancellation.

1.    Motion — When a user is active, motion artifacts are introduced into the signal. This motion creates an error in the heart rate measurement. Advanced algorithms are needed to process the data prior to calculation of the heart rate. It is commonplace to use some form of multi-sensor motion chip to identify motion and compensate the heart rate measurement.

2.    Ambient light cancellation — Normally in a clinical environment, the ambient light levels are at a more constant level. For sports and fitness, ambient light interferers are more varied. In many cases, the user can be running through woods on days with very bright sunlight. This gives a wide range of ambient light levels ranging from the shadows of the trees to direct sunlight.

3.    Power — As with all portable battery-powered devices, battery life is very important for product success. A device that requires frequent recharging is likely to be less successful than a comparable device with a long battery life. In order to significantly reduce the power of an optical-based heart rate monitor, there are several parameters that need to be optimized. LED’s are the dominant power hog for a photo-based continuous heart rate monitor. Duty cycle of the LED on time, intensity and frequency of measurements significantly impact the average LED power.

Engineering advances
Software — Along with the availability of lower-power microcontrollers, significant advancements have been made in the development of physiological algorithms that can be used to calculate physical parameters. Example: using the ECG and pulse-oximeter information, along with other sensor information, it is possible determine the user’s energy level and warn when he/she is approaching exhaustion.

Hardware — Previously clinical pulse-oximeter and optical heart rate monitors have been discrete component solutions. Variations in component and manufacturing tolerances require calibration after the device is manufactured. Advances in electronic technology have enabled design of fully integrated pulse oximeter and optical heart rate monitor solutions.

For example, the Texas Instruments integrated analog front-end for pulse oximeters (AFE4490) and integrated analog front-end for heart rate monitors and low-cost pulse oximeters (AFE4400) devices are leading the way, by providing a fully integrated analog front-end (AFE) for pulse-oximeter and photo-based heart rate monitor solutions. This family of devices will be enhanced as additional versions are introduced in the near future.

In addition to the low-noise receiver channel, LED transmit section with LED current control capability, and diagnostics for LED and photodiode fault detection, this AFE integrates a completely configurable timer module. This timer module offers programmability and flexibility in controlling the various timing edges for sampling and conversion of LEDs and ambient phases. The advantage of the timer module is that it completely off-loads the pulse sequencing and timing control usage of the timers from the host processor to synchronize the LED sampling and data conversions. Thus, loading on a microcontroller is reduced and operation in lower power modes is maximized. The timer module also provides high resolution of control on the LED turn on/off times, which allows for optimization of power versus performance. The device’s integrated fault detection provides warning of open circuit and short circuit of the system LEDs and photodiode.

Figure 2: A block diagram of an integrated analog front-end

Another unique feature offered by these AFE devices is the ambient light cancellation scheme. The receiver of the AFE4400 provides digital samples corresponding to ambient duration. The host processor can run an algorithm to process these ambient values and estimate the amount of ambient light leakage. Based on the algorithm, the host processor sets the value for the ambient cancellation digital-to-analog converter (DAC). The ambient cancellation stage can subtract the ambient component and gain up the pleth components of the received signal. The gain settings of this stage helps to achieve lower input referred noise.

Figure 3: Block diagram representing the ambient cancellation loop (closed by the host processor) using the integrated AFE devices
Click on image to enlarge

When compared to existing discrete solutions, integrated analog front-end devices like TI’s AFE44xx family hold over eleven different circuits into a single IC. This integration provides significant improvements: 60 percent less cost, reduced size by 90 percent, and reduced power by at least 50 percent. Consequently, the overall bill of materials is also 90 percent less. These rich savings and reductions make it possible for more cost-effective pulse-oximeter products to be developed that can reach the growing worldwide markets.


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About the authors

Robert John Burnham
is HealthTech Strategic Marketing Manager at TI where he is responsible for new product definition, business case, marketing strategy, system engineering and customer support. Robert has more than three decades of engineering experience with at least 20 patents to his credit. Robert received his BSEE with Honors from Anglia University, UK.

Antoine Lourdes Praveen Aroul is an applications engineer at Texas Instruments where he is responsible for designing new semiconductor products. He also supports silicon evaluation reviews and customers through the complete design-in process. Antoine received his PhD in Computer Engineering degree from the University of Texas at Dallas, Richardson, Texas

Lijoy Philipose is an applications manager with more than 15 years of engineering experience. Currently, Lijoy is responsible for TI’s HealthTech business unit where he develops and builds reference examples for end equipment, provides customer technical support, and trains analog applications engineers and customers on using TI analog and mixed-signal parts to build end products. Lijoy received his BSEE degree from the University of Illinois at Urbana Champaign, Illinois.

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