DN Staff

August 12, 1996

10 Min Read
How to deal with data

In the early days of medicine, doctors got only as much information as they went after. If they needed a pulse, they took a pulse right there. Temperature: They'd stick a thermometer in the patient's mouth and wait a couple of minutes. This form of data acquisition was truly "real time."

Medical electronics today can do data acquisition at speeds of hundreds of kilobits per second. Then it becomes a problem of what to do with all this data--how you transmit, record, store, analyze, and archive it.

"It's becoming easier and easier to collect a lot of data," says Gene Salber, engineering manager at medical equipment maker Braemar. "Sometimes it's more difficult to keep track of it all and make sure you keep the data associated with the correct patient."

Three of the latest methods for dealing with this data overload are: using flash memory, taking advantageof wireless communication, and integrating the IEEE's Medical Information Bus.

Storage in a flash. Cardiologists use devices called Holter recorders to record a patient's heart activity in order to identify beat irregularities, or arrhythmias. Until recently, these were electromechanical devices that used tape as the storage medium.

In the late 1940s, Dr. Norman Holter's pioneering efforts to study the electrical activity of the heart required an 85-pound backpack radio transmitter. In the 1950s, the transistor allowed Holter monitors to use a miniaturized tape recorder. By attaching electrodes to the chest surface, a clinician could record electrocardiographic signalsfor 6 to 10 hours. The next step: lightweight tape-based 24-hour recorders.

Braemar, Burnsville, MN, a leading supplier of tape mechanisms for Holter ambulatory cardiac monitors, recently introduced the tapeless DL700 Ambulatory Electrocardiographic Monitor, significantly advancing the technology. The new monitor uses solid-state FlashDisk storage cards from SanDisk, Santa Clara, CA. The cards replace the tape mechanism, using 10 megabytes to store 24 hours of patient heart activity.

Flash memory is solid-state data storage that uses semiconductor chip technology instead of a rotating platter, as in traditional disk drives. Because it's solid state, flash memory involves no moving parts. And unlike DRAM, flash doesn't need power to retain data.

"Tape systems eventually will give you problems due to mechanical wear and tear," notes Gene Salber, engineering manager at Braemar. "With the solid-state system we get with flash memory, why there's really nothing there to wear out."

"With tape you have to play the analog tape back in a playback system and digitize it before uploading it to a computer," adds Braemar design engineer Doug Ehrreich. "So we're digitizing it right up front, storing the information digitally, then plugging the PCMCIA card into a standard computer slot--you don't need a special playback device."

Braemar engineers considered using DRAM for data storage, but if the unit lost battery power during the 24-hour recording period, all the data would be lost as well. Flash avoids that pitfall. Mechanical disk drives weren't even considered because they consume considerably more power than flash memory and are subject to losing data due to crashes.

The DL700 is one of the first electronic devices to provide such a long period of nonvolatile storage on removable flash-memory cards. The patient wears the monitor in a pouch or on a belt, with the electrical leads going under the shirt, for the 24-hour duration, so the unit's light weight (7.6 oz) and compact 6 x(GREEK: )3.5 x(GREEK: )0.95-inch size are important to keep patients comfortable.

Patients also sleep better thanks to flash-memory-based monitors. The DL700 runs silently; tape-based Holter monitors require small motors to continuously move the tape.

Because the card adheres to the PC Card ATA storage standard, clinicians can eject a patient's card and insert it into a standard PCMCIA card slot in a desktop computer for scanning and analysis. Meanwhile, the recorder itself can move on to an-other patient.

"The ATA interface also helped speed the design cycle because we didn't have to add any special interface chips or write software drivers," adds Ehrreich.

Holter recorders must be shock and vibration resistant to avoid accidental damage or interrupted operation. The nonvolatile flash memory is solid state and has no moving parts. There are no tapes to break or motors to cause speed variations, or tape head alignment problems, which can cause decreased amplitude signals and timing skew. Flash-based units also offer faster playback than tape units.

Wireless monitoring. Instead of storing data, a portable patient monitor from Criticare Systems, Milwaukee, WI, uses wireless technology to send data to a central station.

Multiple Parameter Telemetry (MPT) is a device that lets patients roam hospital halls while the staff monitors their heart, blood pressure, and pulse oximetry. The transmitter also has an LCD screen that displays readings on demand to a clinician, thus eliminating the need for portable ECG monitors in a patient's room, as well as stand-alone pulse oximiters and blood-pressure monitors.

The technology that makes it possible: the 900-MHz Hummingbird spread-spectrum transceiver from Xetron, a Cincinnati-based subsidiary of Northrop Grumman. The 3.85 x(GREEK: )2.25-inch low-power device operates at 19.2 kbps.

Spread-spectrum technology was originally developed by the military to ensure reliable RF communications--even in the presence of high noise and deliberate jamming attempts. The technology spreads data across a wide frequency band to reduce the impact of interference on transmission.

The Hummingbird uses frequency-hopping spread-spectrum technology. Frequency-hopping radio transmissions produce a narrow-band signal that is constantly in motion, jumping among more than 50 frequencies many times a second in a prearranged pattern. The receiver tracks the transmission pattern of the incoming RF signal. The resulting data flow is spread over numerous frequencies, minimizing errors due to any individual frequency's interference or jamming.

Xetron engineers made the design even more robust by adding forward error correction. This scheme adds extra check information to the data stream. The receiver uses it to acknowledge and correct errors, thus eliminating the need to retransmit packets with bad data.

The Hummingbird doesn't require a license to operate because it doesn't use a fixed frequency. Different communications ranges are available by using higher- or lower-power units. In fact, Criticare can configure its transmitter and central station to monitor patients at home.

Take the bus. A new IEEE open standard for a "Medical Information Bus" (MIB) will let bedside devices and hospital computers from multiple vendors interoperate without custom software or hardware interfaces. To help speed development of MIB medical equipment, LinkTech Inc., Bohemia, NY, is offering IEEE 1073 boards, chip sets, and modules. Designers can use these components to make their medical equipment MIB compliant.

"The advantage of MIB," says Dr. Benoit Dawant of Vanderbilt University, Nashville, TN, "is that it will greatly simplify the acquisition of the data from multiple devices manufactured by different companies." Dawant is helping to implement MIB in the university's Medical Center. Many equipment manufacturers use proprietary interfaces, which makes interconnecting the devices and automatically collecting data from them a decidedly nontrivial matter.

IEEE standard 1073, now also an ANSI standard, defines a data communication interface between bedside medical devices and hospital patient-care computers. The standard is based on the ISO Open System Interconnect (OSI) 7-layer model. Thus, it will provide plug-and-play operation entailing all functional levels from the connectors and cabling through to the software.

"There's a large desire in the medical community to get clinical data into hospital computer systems," says Bob Kennelly, medical line product manager at LinkTech. The main reason: having computer-based patient records.

Today, that transfer is possible only through RS-232C ports. These ports are not standard on medical devices--people use the pins in different ways. Kennelly claims you can blow up a laptop computer by plugging in an infusion pump.

IEEE 1073 defines an active star topology that involves two types of communications stations: a device communication controller (DCC) and a bedside communication controller (BCC). A DCC provides an embedded 1073 into a bedside medical device such as an infusion pump or cardiac monitor. For interfacing with existing equipment, hospitals can use an external converter box.

The BCC is the hub of the network. It interfaces with one or more DCCs. In applications such as anesthesiology workstations, a BCC may be embedded into a local host computer that performs functions such as displaying and recording data. In other applications, the BCC can connect to one or more hospital computer systems over a local-area network. These computers could be involved in different functions within the hospital, such as patient care, pharmacy management, and billing.

"Plug-and-play is more important in medicine than it is anywhere else in the world right now," says Dr. Reed Gardner of McKay-Dee Hospital in Ogden, UT, which is implementing MIB. "If you buy a PC or Macintosh, it'll hook up to a modem and you can communicate. We need to be able to do the same in medicine."

Why flash memory works

In the late 1980s, flash-memory technology was developed as an extension of ultraviolet erasable programmable read-only memory (EPROM). Flash memory is nonvolatile, unlike DRAM and SRAM (dynamic and static random-access memory), requiring no power to retain data. It is also electrically reprogrammable, unlike EPROM.

Flash-memory benefits include:

- Unlike rotating disk drives, flash data storage devices have no moving parts that are subject to mechanical failure.

- Flash provides nonvolatile read/write memory that requires no battery to retain data indefinitely.

- Flash is noiseless, considerably lighter, more rugged and reliable, and consumes much less power than rotating disk drives. It also offers faster access time, and can be up to 30% faster than a typical disk drive.

- Flash can be used to store all kinds of data--from voicemail and video clips to e-mail, photographs, and images--on a device small enough to fit in your wallet.

These characteristics suit flash for portable data-processing products that must operate under various environmental conditions. Examples include: handheld data-collection devices; desktop, laptop, and notebook PCs; PDAs (personal digital assistants); digital cameras; cellular phones; pagers; and music samplers.

Don't overlook electromagnetic compatibility

With the onset of the European Union EMC Directive and Medical Device Directive, medical-device designers have to get up to speed on electromagnetic compatibility.

Because electronic devices are everywhere in hospitals, designers must make sure they don't emit levels of electromagnetic energy that interfere with other equipment. Interference in MRI imaging systems, malfunctions in patient-connected equipment, and even problems with electric wheelchairs and scooters are becoming more than amusing subplots on hospital shows such as "ER."

Also, many medical devices in the U.S. have long been exempt from EMI (electromagnetic interference) requirements such as the Federal Communication Commission Part 15 Rules and Regulations for Computing Devices. Thus, many devices now need to be retrofitted with EMI shielding and gasketing.

Two companies that specialize in the technology--and do compliance testing--are Instrument Specialties, Delaware Water Gap, PA, and Chomerics, Woburn, MA. Give them a call.

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