Pulse oximetry, a noninvasive means of monitoring a patient's blood oxygen saturation level, stands to benefit greatly from a move away from costly discrete-component solutions toward more-integrated designs. But integration requires hard decisions on which processing architecture to use.
Pulse oximetry depends on pulse intensity, or pulsatile flow. Thus, good blood flow in the area being monitored is essential-any restrictions are likely to cause measurement errors. Pulse oximetry can also give an indication of pulse rate, so this feature is found on most pulse oximeters. The operating range of typical pulse oximeters is from 70 to 100 percent saturation, with the readings becoming unreliable below about 70 percent.
Pulse oximetry is based on the variation of light absorption during an arterial pulse event. Two sources of light, one in the visible-red spectrum (660 nanometers) and one in the infrared spectrum (940 nm), are transmitted alternately through the test area, which typically is the fingertip or earlobe. The amount of light absorbed during these pulsatile events is related to the amount of oxygen in the blood. A microprocessor calculates the ratio of absorption of the two frequencies and compares the result with a table of saturation values stored in its memory. This yields the blood oxygenation level.
The typical elements of a pulse oximeter unit are a microprocessor, memory (EPROM and RAM), two digital-to-analog converters to control the LEDs, filtering and amplification for the received signal from the photodiode and an analog-to-digital converter to digitize the received signal for presentation to the microprocessor. The LEDs and photodiode are contained in a small probe designed to be attached to the patient's fingertip or earlobe. The unit also incorporates a display and a plotter. The elements are typically discrete components.
Integrated analog microcontrollers featuring a mixture of A/D and D/A converters, microcontroller cores, memory options and other peripheral functions are available from several manufacturers. The only external components required are those for filtering, signal conditioning and amplification. If the chosen MPU is sufficiently powerful, the signal conditioning could be done in the digital domain, further simplifying the system.
How many bits?
Choosing an integrated solution can be challenging, however, as these analog microcontrollers come in a variety of flavors: 8-, 16- and 32-bit CISC and RISC architectures, with a variety of options for A/Ds, D/As and other peripherals.
In terms of MCU, designers should first consider an 8-bit core like the venerable 8051. These cores are well-known, well-understood and easy to use. Plenty of code and support are readily available, as is a set of proven tools.
While 16- and 32-bit MCU cores are not yet popular in data acquisition systems, they are gaining acceptance. Manipulating wide data is easier with these wide-bus MCUs than it would be with an 8-bit MCU, so calculations involving 12-, 16- and 24-bit A/D data are easier and often faster. With their fast operating speed, these wide-bus MCUs can also be more efficient than their 8-bit counterparts at performing complex mathematical routines. Tool vendors are beginning to offer tool sets and support for these 16-/32-bit integrated solutions.
For a simple pulse oximeter, an 8-bit core could well be sufficient, but if further functionality such as complex filtering, mathematics or data manipulation is required, then the 16-/32-bit device might be a better option. The ideal solution would be a 16-/32-bit MCU with internal program flash and RAM for storing the operating program, saturation lookup tables and captured data; two D/As to drive the light sources; and a multichannel A/D with at least 12-bit resolution to digitize the data received by the photodiode and to monitor other parameters such as battery life. Some on-board configurable glue logic could be an advantage as well.
Aude Richard (firstname.lastname@example.org) and Brian Moss (email@example.com) are applications engineers with Analog Devices Inc. (Limerick, Ireland).