Don't throw a 32-bit Pentium microprocessor at medical-device
problems when an 8-bit chip can do the job. For example, the lower-cost chips
already perform as glucose, blood-pressure, heart, and respiratory monitors.
They can also drive a medical device's display or simplify the user interface.
And they can automate and control functions for drug-infusion pumps,
ventilators, pulse oximeters, and small motors. This makes these products easier
to use and more reliable--benefits that easily translate into home-use equipment
and less expensive medical care.
Some companies are even starting to use 8-bit chips to control the infusion of chemotherapy drugs. Doctors say that chemo works better if you can change the rate of drug delivery. In this case, the processor does more than automate a function--it lets a doctor or nurse program a treatment. More examples follow:
Scooters go digital. Curtis PMC, Dublin, CA, just introduced its first line of controllers for scooters used by arthritis sufferers and others with crippling diseases that incorporate 8-bit Motorola 68HC11 microcontrollers. "This is our first digital design," says Project Manager Pete Andriola. "All our earlier designs are analog, and that's not uncommon in this industry."
Typically, a scooter user has enough physical ability to control a throttle and brake, but normally has to walk with a cane. Wheelchairs are usually 5 to 10 times more expensive and can't maneuver in some places that scooters can. Scooter users can travel at 10 or 12 mph from the house to the store. Some of the bigger units have enclosures for use in all weather conditions--not unlike small electric cars.
The move from analog to digital is driven by the regulatory environment. The safety requirements that scooters have to meet in Europe in 1996, with similar measures slated for the U.S. in the near future, have dictated a much higher level of built-in self-checking and of fault handling and error protection.
"Analog controllers that conform to these specifications essentially double the part count by requiring almost full redundancy," says Andriola. "In the analog controllers, the control functions are performed by piles of op amps and comparators, which you'd have to double." This results from having to build two independent control sections and have them declare a fault whenever they disagree with each other.
Controller lowers cost. "We were pleasantly surprised that as you go up in current levels and complexity of controller, the microcontroller designs become less expensive," reports Andriola.
Fault-detection features are relatively straightforward to accomplish with a microcontroller. In the high end of this product, Curtis went from 350 parts to 250 parts. At the middle and high ends of the product line, using the 68HC11 decreased the cost of the product, giving the controller more features, and letting the company meet the regulations.
At the very bottom of the product line a slight cost increase is necessary to meet the regulations. Still, you get the bonus of the extra features, such as indoor and outdoor configurations and variable speed and acceleration. Now that they're available, Andriola thinks people will discover that they need them.
At best in the past, scooters have had a high/low speed switch. "Now, we can implement a feature we call multimode, where instead of just selecting speeds, a user can select a whole set of parameters for indoor or outdoor use. In other words," explains Andriola, "not only will the speed change, but the acceleration rate, braking distance, and current limits could be changed. This gives the end user much more control over the feel of the vehicle than would be possible with analog components."
Another benefit: The OEM could set up a scooter for a patient while he or she was being trained by a therapist to use the vehicle. Following the training period, the user could get a brisker feel and travel at higher speeds.
The microcontroller-based design also gives OEMs increased flexibility. Instead of buying three or four different scooter controllers, an OEM can buy one controller and configure it three or four different ways. Configuring means setting such things as top speed, rates of acceleration and braking, and current limit. Previously, these specs would have required different controllers or at least tweaking multiple potentiometers on an analog circuit. This can be an expensive and not terribly reliable solution.
Home-to-office link. Another microcontroller-based design just hitting the market is the E-FAX Voyager from Blakbag Technology, Houston, TX. The Voyager--a portable, battery-powered ECG (electrocardiogram) monitor--can communicate with a doctor's office via a telephone link. The company has dubbed this process "transtelephonic monitoring."
The chip behind the design is the PIC17C42 from Microchip Technology, Chandler, AZ. This high-end, 8-bit microcontroller has a one-time-programmable (OTP) memory, draws low current, and can go to "sleep" when not active. These features make it a good choice for such battery-powered applications, claims Microchip Marketing Manager Jerry Corbin.
Patients who experience sporadic chest pain, palpitations, dizziness, or passing out would require such a unit. Usually in such cases, a doctor will run an ECG in the office and not see anything unusual. Then the patients might go home with a Voyager to capture data when the symptom occurred again.
The patient wears the unit, about the size of a TV remote control, on a belt. Two electrodes connect the unit to the patient's chest and gather data continuously as a loop recorder. The patient hits the save button, which captures the symptom's data anywhere from 10 seconds to 5 minutes before the button was pushed. The unit continues recording to save the event and its aftereffects.
Next, the patient transmits the recorded information to a doctor. By putting the Voyager unit under a telephone handset, dialing a phone number, and pressing the send button, the unit sends a combination of analog and digital data to the E-FAX Retriever in the doctor's office. Daraius Hathiram, VP of engineering at Blakbag, calls this connection a bidirectional acoustic link. "It's not really a modem connection," he explains.
After receiving the call, the office receiver pages the doctor, then faxes the ECG strip to some prespecified location. Alternately, the doctor can retrieve the fax from a remote location. The doctor then calls the patient and takes the appropriate action.
The primary reason for choosing Microchip's part, says Hathiram, was its low power: "The part takes samples 200 times a second, but goes to sleep between samples--that's really the unique property we're utilizing to get low power. It also has a very simple instruction set."
High-risk babies. Amid the joy parents feel when bringing a newborn home from the hospital is concern for the baby's health. This concern is especially strong if the baby falls into a "high-risk" category.
Physicians consider infants to be at high risk if they are premature; have low birth weight; survived an apparent life-threatening event, such as apnea, low heart rate, or low blood oxygen saturation; or the baby's family has a history of SIDS (sudden infant death syndrome).
Apnea means that the baby stops breathing. This occurs most commonly during sleep. One of the early theories behind SIDS was that victims probably had apnea episodes prior to their deaths, although researchers can't prove a direct correlation between apnea and subsequent SIDS. They do know, however, that frequent and prolonged apnea episodes can lead to health problems.
Peace of mind at home. If an infant falls into a high-risk category, a doctor will prescribe a home monitor, such as the SmartMonitor from Healthdyne Technologies, Marietta, GA. Parents rent the unit from a local medical supply company. Average monitoring period is four to five months, or until the baby outgrows the high-risk stage.
The SmartMonitor consists of a monitoring unit with a Hitachi 64180 8-bit microprocessor and a foam belt with carbon-impregnated silicon rubber electrodes on either side of the baby's chest. The unit monitors both breathing and heart rate.
The microprocessor reads signals from an analog board connected to the patient and decides whether to trigger an alarm. It also controls the memory system. An on-chip watchdog timer will sound an alarm if something is wrong with the processor itself, thus increasing the unit's reliability. Says Healthdyne Electrical Engineer Gary Holder: "Eight bits are sufficient. We're dealing with fairly slow signals--heart rate and respiration."
Saving young lives. The apnea delay is normally set at 20 seconds because babies have short pauses between breaths. "The whole idea," says Healthdyne Technologies' Marketing Director Steve Combs, "is that if the baby is having a problem, he or she usually responds well to gentle physical stimulation from the care-giver. For example, moving the arm or placing a hand on the baby's chest, picking up the baby, or, if required, CPR. Left alone, babies can get into a degenerative state where they don't respond properly. But if you can interrupt that, you can save their lives."
The foam-belt electrodes pick up the ECG signals to measure heart rate. "We apply a 32-kHz square wave to the baby's chest cavity," says Combs. "The baby's chest impedance is roughly 200 ohms at that frequency. We measure changes in chest impedance as the baby breathes. Air intake increases impedance during inhalation, and impedance goes down during exhalation. It's roughly a 1-ohm change that we're looking for."
Healthdyne's previous design had mechanical switches to control setpoints. With the new design, the processor can keep the continuous-buffer event-recording memory system in a standby mode until an alarm occurs. At that point, it directs the unit to capture 15 seconds of waveforms from the baby prior to the onset of an event and then record the event and an additional 30 seconds or more after the alarm condition is corrected. Before, there was no way to save the pre- event data.
Combs says that the processor also let the designers use a 2-line LCD user interface with a simple 3-button control to set the alarm and record parameters. "It gives us flexibility to make changes in software without changing tooling or circuit boards."