Network technologies

Equipment location tracking in hospitals

When deployed in a hospital, mobile technologies such as real-time location services (RTLS) and barcode scanning can offer significant benefits in terms of increasing the speed and improving the accuracy of patient care.

In such large buildings or campuses, knowledge of the location of expensive or critical equipment — eg IV pumps, computers on wheels or laptops — is essential to protect both the investment in technology and, more importantly, the patient.

For example, if an obligatory upgrade is required to the firmware of medical equipment, and the bio-medical engineer cannot find the equipment, then it may remain in use while faulty. In terms of patient welfare, simply fitting a battery-powered transmitter to vulnerable patients allows their location to be quickly determined and alerts hospital staff if they move into dangerous, restricted or offsite areas where exit alarms can be sounded immediately should a transmitter tag pass a pre-determined ‘pinch-point’.

Location tracking is therefore becoming a crucial application in hospitals today — particularly given its importance and the ease of implementing the technology. Most location applications require battery-powered location tags to be fitted to equipment or patients. Tags come in a variety of sizes, and features vary between vendors.

Tag features

The most common features include LEDs and audible alerts to take account of the fact that location information is typically only accurate up to a few metres — this is because of the nature of a variable radio frequency environment, which makes it difficult to be more precise. As a result, bio-medical engineers searching for equipment use the LED or audible alert when they are in the general area as a prompt to identify the exact device they need to locate.

Other features found in tags include basic two-way communication capabilities, temperature sensing (particularly useful if used to track the storage of drugs), and the support of other telemetry readings (pressure, liquid level, etc).

Location accuracy

Depending on the application — whether it is used for locating equipment or acting as an exit monitor — location accuracy will typically need to be in the range 3-5m for a room or ward or 1m at a door or entry point.

In some respects, location sensing at a door, or a ‘choke point’, is relatively straightforward. Tags used for this application are typically battery powered, have a standard Wi-Fi wireless interface, and are triggered by an ‘exciter’ — a low frequency (125MHz) signal from a transmitter in the immediate vicinity of the choke point. Once excited, the tag immediately transmits its location to a software application that creates an alert.

For general location tracking, around a hospital building rather than at a choke point, most location-based services will be based upon the Received Signal Strength Methodology. At its simplest, this method uses the signal strength measured at an access point (AP) for the client device to be tracked; the RF power loss between transmitter and receiver is related to the distance travelled. Received signal strength indication (RSSI) results in an estimate of distance but not direction.

To determine the location of a client, at least three access points in the network must be able to detect and measure the client tag's signal strength. Its accuracy is then dependent on two factors.

The first is the accuracy of each measurement. Unfortunately, there are many sources of inaccuracy in deriving distance estimates from an RSSI measurement. The RSSI figure itself may be accurate, but especially with mobile clients, the orientation of the antenna, whether there is a human body in the path, the presence of walls, cubicles or metal objects can all have a large effect on the path loss. This translates into uncertainty about the exact distance.

The second is that the longer the distance being measured, the more inaccurate the measurement. It is reasonable to make the assumption that the accuracy of an RSSI-based distance estimate is proportional to the actual distance from AP to client: doubling the distance from AP to client tends to double the uncertainty of the measurement.

However, many hospitals will deploy location services at a similar time to voice-over-IP telephony solutions such as ‘nurse call’ systems. The greater density of AP coverage required for both solutions (typically 150% coverage) are complementary and improve the inherent accuracy of the location solution.

Some vendors have developed systems that take raw RSSI measurements and process them by applying probability theory in conjunction with survey measurements. This approach can provide very accurate results under certain conditions, but it requires a walk-around — a survey of the RF (radio frequency) conditions within a hospital using specialist equipment, and usually an extra computing platform in the network.

While techniques such as analysis of building material and walk-around calibration can improve the accuracy of RSSI measurements, they add considerable expense and complexity to the network installation, and the improvement in accuracy erodes over time, as the environment changes.

Instead of RF fingerprinting the building with a walk-around, requiring the network manager to mark up the floor plan with RF obstacles and construction materials, the wireless grid is utilised for self-calibration.

A high density of access points allows many measurements to be made, contributing to more accurate location tracking in three ways:

• many more APs will be in range of the client, resulting in more measurements and a more accurate location estimate;
• those many APs that are not at that moment providing wireless service will be constantly in AM mode, enabling long-term, time-averaged RSSI measurement.

The result is superior to building material modelling and similar to client calibration, but without the major drawbacks: no walk-arounds are required and changes in the RF plan are accounted for by the infrastructure.

Conclusion

The use of real time location services offers an opportunity to drastically improve patient care within a hospital in numerous ways, from locating equipment in need of updates or service to tracking assets or patients within the building or across campus.

Modern ‘thin’ access point wireless LANs inherently support location services, and for many new implementations, expanding the network to support accurate location definitions will be a relatively easy upgrade.

Given the ease of deployment and solutions currently available, wireless location tracking can become a vital networking technology in today’s hospital environment, both in terms of boosting the efficiency of managing equipment in a complex organisation and enhancing patient care.

Roger r Hockaday