Originally Published MDDI June 2005
Conserving power is essential for implanted devices. New low-power mixed-signal ASICs are ideal for such applications.
|Table I. Relevant U.S. and European standards (click to enlarge).|
Wireless systems enabled by low-power radio technology are delivering mobility, comfort, and higher levels of patient care. Both skin-surface and implantable medical electronics devices can benefit from transceivers that use the latest mixed-signal application-specific integrated circuit (ASIC) technology to provide desired rates of data transfer over short ranges.
Electronics used in medical environments, especially implanted devices, need to demonstrate low power consumption and high-reliability design. These characteristics are imperative because the devices are difficult to access and the consequences are serious if the devices malfunction or fail. In addition to cardiac pacemakers, low-power radio devices that use ASICs are already included in several other applications, such as blood-glucose monitoring and body-temperature sensing.
There are two protocols that relate to medical electronics devices of this type. The first is medical implant communications services (MICS). MICS applies to implantable technologies that need to communicate outside the body periodically or when there is a deviation from specified parameters. The second is wireless medical telemetry service (WMTS) for nonimplanted (or on the skin) patient monitoring systems. Telemetry devices must communicate on a far more regularly timed basis with a suitably equipped remote location, such as a nurses' station.
MICS is set to completely replace magnetic-inductive coupling techniques, thereby providing faster data transfer and longer range in pacemaker and defibrillator applications. Magnetic-inductive coupling supports only one-way communication at data rates of around 50 Kb/sec over a range of only a few inches, whereas MICS solutions that use a radio-frequency (RF) link can achieve up to 250 Kb/sec at a range of around 6 ft.
The Federal Communications Commission (FCC) allocated the 402–405 MHz frequency band to MICS in 1999. This particular band was carefully chosen for a number of reasons. First, it enables the design of a low-power transmitter and antenna arrangement to achieve a reasonable transmission range while still being small enough for implanted applications. Second, the frequency range does not pose an interference risk because no other radios operate in the same band. And finally, MICS-band frequencies have propagation characteristics that are conducive to transmission within the human body.
|Figure 1. Powering down receiver circuits conserves additional power (click to enlarge).|
International frequency bands for implantable devices need to be harmonized so that patients with implanted medical devices can obtain care wherever they are in the world. The MICS band is already compatible with devices governed by the European Telecommunications Standards Institute (ETSI). ETSI recently standardized the frequencies and electromagnetic compliance requirements of ultra-low-power active medical implants (ULP-AMIs). Table I outlines the relevant standards for the United States and the European Union.
To avoid interference problems, implantable devices using MICS are subject to a number of operating restrictions. Under normal situations, an implanted MICS device may not transmit. The external communications must monitor the relevant channels and cannot begin a communication session unless the chosen channel is free of transmissions from other MICS devices in the area. Standards also prohibit regularly scheduled transmissions that are not instigated by a change in the patient's medical condition. If, however, the device detects a medical implant event, it may immediately begin communications.
An event is defined as an occurrence that requires the transmission of data to protect the health of the person using the implant. For a cardiac rhythm management (CRM) device, a dozen different conditions could qualify as an implant event. A typical example would be uneven beating at the top of the heart coupled with racing at the bottom. This type of event may be painless to the patient, but it is significant enough to merit reporting via the radio link.
MICS Extends beyond Pacemakers
The range of implanted devices in common use will soon extend beyond CRM devices to include candidates such as glucose meters, insulin pumps, vascular blood pressure monitors, incontinence control devices, and neuromodulators. As with CRM devices, each of these device candidates tries to accomplish two tasks. First, they monitor and report on some physiological parameter, such as blood sugar. Second, they try to control the parameter (e.g., through the release of insulin) as guided by a set of device-programmable controls. It is in the automated control and external monitoring of a physiological parameter that the true benefit of a MICS-enabled device becomes clear.
Controlling a medical condition requires different algorithms or coefficients for every individual and may even require additional information from the body. The adaptability of a medical implant device is a key point. Because medical implants must be designed with adaptability in mind, the advantages of wireless communication become critically important.
To minimize the cost, risk, and patient trauma associated with repeated surgical procedures, it is essential to maximize the battery life of an implanted device. Battery life should extend five to seven years; therefore, every joule of energy must be carefully conserved. The low-power, fully integrated mixed-signal ASICs used in the latest implanted devices help to achieve this.
Chip-based RF transceivers employ several different techniques to keep the power down. One technique is to keep the power-hungry transmitter circuits powered off when they are not in use. An additional technique is to periodically power down the receiver circuits to conserve additional power (see Figure 1).
The entire duty cycle can be completed in less than 100 microseconds, which makes it possible to achieve very low average power consumption while monitoring the MICS channel for transmitted messages. Both techniques require a rapid-start oscillator that can wake up the receiver and (if needed) the transmitter in an extremely short time.
|Figure 2. Low-power MICS radio block diagram (click to enlarge).|
Both the amplitude shift key/on-off key (ASK/OOK) and frequency shift key (FSK) methods are popular modulation schemes in narrow MICS-channel applications. Twin independent receive channels are used to help improve the reliability of MICS transmissions so that power is not wasted with retransmissions. More-sophisticated transceivers also contain baseband clock and data recovery circuits. These circuits postprocess the demodulated incoming data stream to produce both a sampled-data bitstream and a clock signal. The process helps improve transmission reliability by synchronizing the data-processing clock with the incoming data (see Figure 2).
With the rapid increase in the number of wireless devices, FCC has occasionally had to shift limited-frequency spectrum users into or out of various frequency bands to avoid interference problems. Of primary concern are devices that may handle mission-critical applications, such as medical telemetry devices. Therefore, in October 2000, FCC set aside up to 14 MHz of radio spectrum primarily for wireless medical telemetry services (WMTS), including the 608–614 MHz band for medical device use.
Before a portion of the RF spectrum was set aside, WMTS devices operated on an unlicensed basis in the vacant TV channel spectrum (174–216 MHz and 470–668 MHz) or on a secondary licensed basis in the private land mobile radio band (450–470 MHz). The secondary status meant that medical telemetry devices had to endure interference from TV broadcasts or from land mobile-radio transmissions. Furthermore, medical telemetry devices could not interfere with TV or radio transmissions under any circumstance.
The advent of digital television signals anticipated intensive use of the TV spectrum and threatened to preclude medical telemetry operations entirely. To solve the problem, FCC set aside a different portion of the RF spectrum for WMTS operations. The solution also meant that current WMTS equipment was effectively rendered obsolete.
To soften the blow, FCC provided a transition period for telemetry equipment to move to the new frequency bands. After the transition period ended in October 2003, FCC, FDA, and the American Hospital Association (AHA) all strongly recommended that WMTS operation take place only in the prescribed frequency bands. All WMTS equipment built after October 2002 must operate in the new bands.
Because telemetry systems transmit real-time physiologic data, it is critical to avoid lost or delayed data. In the unlicensed industrial, scientific, and medical (ISM) bands where medical telemetry devices can also operate, an increasing number of radios raise the likelihood of signal loss and interference.
ISM-band radios are required to use spread-spectrum techniques such as direct sequencing or frequency hopping. Both techniques, operating together on the same band, can cause interference on secondary users like medical telemetry systems. In the WMTS band, both types of spread-spectrum techniques are used but do not coexist, thereby avoiding interference problems.
The key benefit of a WMTS system is the mobility it brings to the patient; not being tethered to monitoring equipment by cables allows far greater freedom of movement. The benefits to nursing staff are significant too. Nurses do not need to spend time removing and replacing wired sensors. In addition, data from several patients can be easily monitored from a central point such as a nursing station. Hospitals also benefit. The total cost of a WMTS system is significantly less than a conventional setup because of its simplified wiring.
Low-Power Wireless Sensors
The design of a WMTS system bears many similarities to that of a MICS system. Typically at its core is an integrated mixed-signal ASIC or application-specific signal processor that supports low-power, low-voltage, and high-reliability requirements. A WMTS system often includes low-power radio-enabled sensors that are affixed to the patient and monitor for changes in the body's condition. These sensors are networked to patient-monitoring equipment.
|Figure 3. WMTS wireless telemetry sensor block diagram (click to enlarge).|
A wireless sensor should offer several features, such as bi-directional digital communication, a time stamp of the measurements, and multivariable sensor configuration.
Due to the low power requirements, a wireless telemetry sensor is often implemented as a mixed-signal ASIC consisting of a sensor interface, an embedded signal processor, and radio communications (see Figure 3). Like MICS systems, WMTS devices employ additional techniques to keep power-hungry transmitter circuits powered down when not in use. Receiver circuits can also be switched off when not in use.
Duty-cycled transmitters have the additional benefit of reduced radiation exposure. For many years, the general public has been concerned about the possible health effects of exposure to RF radiation. High levels of RF fields are known to cause a variety of physical effects on the human body. At the operation frequencies of MICS and WMTS devices, the known health effects center on tissue heating. A measure of this heating effect is known as specific absorption rate (SAR). To address consumer health and safety aspects, many authorities now require products to meet SAR limits before being placed on the market. FCC requires that all WMTS transmitters that operate within 20 cm of a person's body be routinely evaluated to demonstrate compliance with FCC's SAR limit. Because medical telemetry devices are typically worn on the body, the sensors are classified as portable transmitters and are subject to the guidelines listed in Table II.
The Future for MICS and WMTS
|Table II. FCC guidelines for controlled radiation exposure (click to enlarge).|
Because of the enormous benefits to both patients and medical staff, MICS and WMTS systems are expected to proliferate. In the case of implantable MICS-based technology, networked communications will be seen very soon. Devices will be able to talk to both portable recording and monitoring equipment worn by the patient and to the fixed diagnostic equipment in the hospital environment. Standardized protocols will help ensure that programming equipment made by one manufacturer will be able to talk to implanted devices made by another.
The effect on hospital resources could be quite extensive. For example, patients recovering from minor surgery could be allowed to go home early, freeing up hospital beds and nursing staff. MICS and WMTS devices ensure continued monitoring once patients leave the hospital. In addition, a patient's physiological state can be monitored, which is particularly important when a physician needs to observe a patient's new drug therapy or the after-effects of anesthesia.
Although it brings great benefits to patients and healthcare providers, the ability to contact and communicate with a device inside the human body also has important ethical and privacy issues. Moving forward, it is essential that all data are securely encrypted to ensure complete privacy of patient data. There are also clear safety issues that can arise due to the ability to instruct an implanted device to start a new or different treatment protocol.
The next likely development for WMTS is home use. FCC currently does not permit home use because of the difficulty this may cause in coordinating various operating frequencies. Two transmitters using the same frequency in close proximity could result in harmful interference. However, if suitable technology and experience can negate these concerns, then FCC may change its rules, permitting home use of WMTS and all the benefits that it brings.
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