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Wireless Advances Are Afoot

Wireless foot controls, such as the one shown here for cataract surgery, avoid some of the problems associated with cabled controls.

Historically, medical device manufacturers that require foot controls have used a cabled unit that plugs into the console of the device being operated. Typical applications include ultrasonic diagnostic systems, surgical microscopes, medical camera systems, x-ray equipment, and ophthalmic surgery systems.

Until recently, a cabled unit has been the only available option. Although these types of units have been acceptable, many OEMs recognized that the cable presents some limitations. In a physical sense, the cable poses a tripping hazard to personnel and limits the location of the foot control relative to the location of its host medical device. In addition, the cable is the most frequent point of failure—either from excessive stress at the strain relief or due to damage from being rolled over by chairs, examination tables, or equipment carts. Finally, the cable makes it difficult to clean and store the foot control.

As a result of these issues, field experiences, and a proliferation of wireless equipment (e.g., cell phones, patient monitoring systems, computers, etc.), many medical device OEMs have wondered whether it would be possible to eliminate the cable. This article presents major factors to consider when selecting a wireless technology for a medical-grade foot control.

Nature of the Medical Device

As previously noted, the diverse potential uses for wireless controls range from relatively low-risk applications, such as medical camera systems capturing reference images, to relatively high-risk applications, such as x-ray equipment and laser-based systems. In this context, manufacturers must consider the consequences of the loss of a desired signal, as well as the consequences of the equipment being activated by a random signal (noise) rather than through the purposeful actuation of the foot control. 

These concerns are similar to those that arise when using a cabled foot control, during which time control may be lost due to a cart or exam table cutting the cable, as an example. Although the loss of a signal in an image-capturing system is an inconvenience, it has no serious consequence for a patient. In this case, a unidirectional communications protocol may be appropriate. In these protocols, a transmitter signals the receiver, but there is no communication from the receiver. Unidirectional protocols often have the benefits of less-frequent battery recharging and replacement.

Alternatively, the integrity of communications between a foot control and a C-arm x-ray system or laser-based surgical instrument may have more significant consequences for consideration. Here it may be more prudent to use a bidirectional communications system—capable of two-way communication between the foot control and the medical device receiver. Systems such as Bluetooth, ZigBee, or some other similar proprietary protocol may offer noise immunity, greater encryption possibilities (for pairing the transmitter with its receiver), and the ability to verify the integrity of the communications link in real time.

In addition, some medical device applications may require the transmission and reception of ancillary, noncontrol data. These data could include the OEM’s identification, medical device identification, and real-time state of battery condition (voltage, current, charge status, temperature, number of experienced recharge cycles, etc.). Such parameters could be important as assessed risk increases or if a surgical procedure is interrupted due to a power loss.

Power Consumption

Because foot controls are most often battery operated, factors such as the duration of a typical procedure, the number of procedures per day, the duty cycle, and required data transfer rates affect the battery type (e.g., rechargeable versus nonrechargeable). These factors also affect the size and number of cells required to satisfy the power and battery service parameters of the application. Consideration of the wireless technology’s inherent power requirements and its ability to minimize power consumption (e.g., having a sleep mode), can help OEMs come up with the most practical battery candidates. See the sidebar, "Battery Selection Considerations," for more elements that need to be considered.

A component of the wireless setup is an RF transmitter-receiver module with an integral antenna.

Potential for Interference

How well does the technology minimize or eliminate the potential for interference? While medical device design engineers can control the generation and emission of random noise from their own equipment, they have no control over its presence in a user’s environment.

For many medical applications, total elimination of potential interference is impractical. The possibility of noise—for example, electromagnetic interference (EMI) signals from sources such as similar systems, patient monitoring systems, and other electrical equipment in proximity to the medical device—must always be considered. Note the following:

?    The need for the use of two or more like systems in the same local environment may require a wireless protocol capable of pairing the foot control and its controlled medical device in real time. Such bidirectional communication ensures that foot control A for device A will not operate device B and vice versa. 
?    Infrared (IR) wireless systems are typically not recommended for use in the presence of plasma screens (due to their emission of light in the IR portion of the spectrum).
?    Improperly applied ZigBee-based system signals may be masked by selected wireless local area networks.
?    Depending on its frequency, the possibility of random noise (such as from an electrosurgical generator) may dictate use of a wireless protocol unaffected by such an EMI presence (such as a frequency-hopping or similarly robust technology).

These issues can be addressed through the programming of data frame formats in a specific form acceptable to the receiver (e.g., such as identifying the medical device manufacturer, the specific transmitter source, signal encoding, etc.), and choosing a wireless protocol that is sufficiently robust to operate reliably in the environment. 

Desired Transmission Distance

The desired transmission distance may affect both the power requirements of the wireless protocol and the risk assessment (i.e., the farther the transmission distance capability, the greater the risk of inadvertent actuation of the controlled medical device).

In addition, OEMs must be aware of possible signal loss due to barriers in the signal path. For most medical devices operated via a foot control, the transmission distance is generally well below 10 m because the user is typically in close proximity to the medical device being controlled. Nevertheless, the wireless protocol chosen should be capable of reliable operation in the target environment.

Response Time

The wireless protocols typically considered for medical foot controls have response times well within those required (e.g., less than 250 milliseconds). However, when using a sleep mode to conserve power, system wake-up time must be considered. It is important that the system wake-up time does not exceed the desired control response time.

Cost

Table I. (Click to enlarge) A comparison of wireless technologies for foot controls.

Cost is typically be driven by the technical requirements of the application and the level of acceptable risk. For example, applications having lower levels of assessed risk may be satisfied by the typically lower overall cost (batteries, support circuitry, base technology) of unidirectional wireless protocols such as IR and single-frequency technologies. Where risk aversion is high, data density is high, or real-time pairing of the transmitter-receiver is desirable, higher-cost wireless protocols may be more appropriate.

Wireless Technology Possibilities

With the apparent need and potential benefits as drivers, a number of commercially available wireless technologies can now be considered. These include the following:

?    Infrared (IR).
?    915 MHz (available in the ISM band for medical applications).
?    DECT (digitally enhanced cordless telecommunications).
?    WLAN 802.11.
?    ZigBee.
?    Bluetooth.
?    Steute RF 2.4-MED.
?    Other proprietary protocols.

Each of these has its own unique attributes, some of which are summarized in Table I. In low-risk applications, most wireless technologies may suffice depending on cost-performance requirements.

Desired Features

Note that wireless systems can also be designed as hybrids, i.e., capable of also functioning as a conventional cabled device if needed. In addition to the requirements for a conventional cabled foot control, the following are among the most common features OEMs should seek when considering a wireless human interface:

?    Ability to meet the functional control requirements.
?    Compliance with all relevant radio-frequency standards in the countries of use, e.g., IEC 60601-1 and 60601-1-2, EN 60950, EN 50371, etc.
?    Geographic acceptance (here defined as acceptance in the countries in which the OEM wishes to market its equipment).
?    Maximum operating time between battery recharging or battery replacement.
?    Ease of periodic system maintenance for recharging, battery replacement, cleaning, transmitter-receiver pairing, etc.
?    Robust construction, e.g., IP X6 to IP X8.
?    Ability to monitor battery charge status in real time.

Other Issues

Medical device OEMs considering the use of wireless controls may have many queries about the choice of a technology for their application. Many questions revolve around safety and reliability, power management, and the receiver module.

Safety and Reliability. Although interest among both OEMs and their customers has been high, early adoption of wireless foot controls has been slow due to concerns about safety and reliability. These concerns most often revolve around false signals (EMI), crosstalk between like systems, and signal loss. Current technologies (especially bidirectional, frequency-hopping protocols such as Bluetooth and Steute 2.4-MED) effectively address these issues. Their frequency-hopping characteristics have made them highly immune to outside interference. With proper formatting of their data frames, these wireless options can eliminate the possibility of crosstalk with encryption for the manufacturer, serial number, type of device, and device identity—such that the pairing of the transmitter (foot control) and receiver (medical device) eliminates communication with other like systems or interference from other wireless systems or stray EMI.

In addition, with suitable programmed software, bidirectional communication permits constant confirmation of the married pair, immediate safe actions in the event of signal loss, and real-time monitoring of the power-supply status (to visually or audibly alert the user well in advance of the need for battery charging or replacement). 

Other safety features that can be provided include recharge cycle monitors, which track the number of recharge cycles undergone, and floor sensors, which intentionally interrupt the communications link if the foot control is picked up from the floor for more than some preset time period. In addition, OEMs can also look for options that allow system shutdown in the event of power loss or loss of communications and periodic or permanent display of the battery charge status.

This is an example of a wireless foot control module for an electrosurgical generator.

Power Management and Batteries. Currently available battery chemistries, coupled with today’s wireless technologies, provide an array of combinations with which to address each specific application requirement. In addition, the use of sleep modes with acceptably fast wake-up times enables the conservation of battery power during periods of noncontrol activity. Such conservation can extend the interval between battery recharging or replacement to weeks and months.

Foot control design can be such that recharging can be accomplished simply by plugging in a medical-grade wall recharger into a sealed recharging connector. Similarly, battery replacement can be fast and easy by using a sealed battery compartment that can be quickly accessed without tools. 

As previously mentioned, several wireless technologies enable the monitoring of battery charge status, the number of experienced recharges, voltage, current, and temperature  in real-time. Such monitoring allows safe, timely actions to be taken before a power supply and system operation have been compromised.

Receiver Module. If an OEM needs to add a receiver module to a medical device to facilitate wireless operation, typical questions revolve around the location, integration, interfacing, and power.

Essentially there are two locations for the receiver: integrated within the medical device console or in an externally mounted enclosure. The choice may depend on whether the wireless control is offered as an upgrade or postsystem sale add-on, the material of construction of the medical device console (for antenna location consideration), and the wireless technology selected (line-of-sight technologies such as IR or omnidirectional radio-frequency systems such as Bluetooth, ZigBee, and Steute RF 2.4-MED). 

The receiver requires power either from the host system power supply, a medical-grade plug-in power source, or from its own batteries, if practical. When line powering of the receiver is required, it can be provided from the host medical device’s power supply directly to an internally located receiver, or via a pin on the host’s female connector, to which an externally mounted receiver is plugged in.

The outputs of the wireless receiver can be formatted for interface compatibility to communicate with the host system, whether these are discrete analog and digital signals, USB, RS-232, or RS-485. Thus the receiver outputs can mimic whatever has been provided previously via a cabled foot control.

The questions that are paramount are a function of the application and the operating characteristics desired by the OEM’s technical and marketing staff. However, the traits previously mentioned are generally among those most frequently asked for when making an informed decision and technology selection.

Benefits and Drawbacks to the OEM

As with a conventional cabled foot control, there are benefits as well as new issues to address for the OEMs opting for a wireless solution. Among the benefits are the following:

?    Ability to increase revenues. Offering wireless controls may present the opportunity to expand an OEM’s revenues by addressing customer wants and by providing an optional wireless upgrade for their installed base of devices.
?    Ability to enhance technical image. With the increasing use of wireless technology, OEMs that offer this feature can be perceived as technical innovators or leaders. 
?    Elimination of cabled foot control problems. Manufacturers can avoid problems such as the tripping hazard, cable damage, and cleaning and storage issues.

Among the new issues to address are the following:

?    Periodic recharging/replacement of batteries. This a function of the usage, battery energy density, number of batteries, use of a sleep mode, etc.
?    Need to maintain transmitter-receiver pairs. This is important for safety as well as logistical purposes if foot controls are collected in a group for cleaning or storage. 
?    Higher cost than a cabled unit. The degree of cost increase is a function of the wireless technology selected, the number of discrete actuators, the type of controls signals required (analog and digital), and other desired design features.

Conclusion

Wireless devices continue to proliferate the medical device space and device users continue to embrace wireless controllers. As a result, one can expect to see more medical device OEMs offering such foot controls, either as standard equipment or as an optional upgrade. A number of OEMs are already offering such controls for medical devices such as x-ray systems, medical cameras, electrosurgical generators, fluoroscopy systems, orthopedic surgery systems, and ophthalmic surgery equipment.

Peter Engstrom is managing director of Steute Meditech Inc. (Ridgefield, CT). Maurizio Lauria is product manager at the company.

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