Keywords: medical monitoring, medical instruments, fieldbus, demonstrator, Profibus
For the purposes of various databases and information systems are off-the-shelf solutions available. There are already many standards for platform independent representation and interchange of static or quasi-static medical records (patient data, insurance data, medical images etc.) [1]. These communication standards are based on the upper layers of the OSI model, and the lower layers, like the physical transmission, the media access and the routing methods are not defined. This approach allows the adaptation of such high level protocols on many existing commercial or semi-professional transmission technologies, like 802.x LANs. According to its economical price lots of hospitals and medical centers used and use the commercial 802.3 Ethernet to interconnect their departments.
Clinical monitoring and intensive patient care mean a much different problem. In these applications the various bed side medical equipments are to be networked at a lower level of the communication hierarchy. This requires a dynamic, real-time, deterministic, fault-tolerant and secure data exchange. The 802.x standards are lack of some or more of these features [2]. According to a survey in 1989 [3] the main requirements of bed side medical monitoring systems are:
As it can be established, to fulfill all of these requirements we must define the lower layers of the OSI model. Although many vendors are offering their monitoring systems, they are in lack of an open communication technology so in lack of the interchangeability of devices various vendors. Since 1992 IEEE has a draft proposal of the Medical Instrumentation Bus (MIB) [4]. The final standard is not yet approved. Another problem is that MIB is very poor hardware supported. Although a standard MIB device interface is in the range of $100, only a couple of vendors offer a MIB connectable equipment. Until MIB is not an approved standard vendors are not interested in the producing of MIB devices. On the customers' side it can be observed that no one like to invest money in devices described in an evolving, subject to change draft standard. So the old and unique communication solutions stay and mean a stable and more economical solution for both manufacturer and customer.
To introduce the ideas described above, we decided to realize a distributed demonstrator system based on an open fieldbus communication technology. The demonstrator system will be installed in the Biomedical Engineering Laboratory at the TU Budapest, Department of Control Engineering and Information Technology. Due to its open architecture the system will serve optimally our educational purposes. This work runs under the INCO Copernicus #960161 project of the European Union.
As central monitoring station:
The sensory and actuary components representate the bed side euipments of a single patient. Beside them the system can be expanded with additional elements due to future enhancement.
The main goal of the demonstrator is the ensure the real-time, deterministic and secure data exchange demand of the monitoring purpose. To achieve a "real-time" monitoring and to keep the required data transmission rate under a reasonable value we agreed to reach 10 polling cycle per second pro device. The transmission rate can be reduced using various preprocessing and signal-compressing algorithms in the external interface modules.
| "Slow" vital signals (temperature, pulse, CO2) | "Fast" vital signals (EEG, ECG, blood pressure) | |
| Bandwidth | 0 - 10 Hz | 0 - 1000 Hz |
| Required resolution | 12 bit | 12 bit |
| Data / polling cycle | approx. 10 bytes | approx. 100 bytes |
| Signal preprocessing | none | intensive |
"Fast" signals have a spectra of up to 1 kHz, they supply a much larger amount of data. To get a reasonable amount to be transmitted, intensive signal preprocessing is required. This task can be fulfilled only with the use of more powerful hardware, like industrial PCs (marked as Type II slaves).
To be economical signals of more than one bed side device can be measured with both a Type I and a Type II slave. In addition the slaves should have an appropriate fieldbus link.
Due to the educational purposes of the demonstrator an artificial patient will be used. The artificial patient is a desktop PC. This PC contains D/A cards to produce the required vital signals, and A/D cards to close the control loop from the side of the actuators. The various physiological or pathological vital signals to be generated can be selected on a user-friendly graphical interface.
To easily measure their effect, the actuators are motorized potentiometers with appropriate FDL.
A powerful standard desktop PC plays the role of the central master monitoring station. Its task is to manage the communication, to gather the information form the Type I and Type II devices, to make decisions, to control the actuators, to visualize the status and vital signals of the cared patient on a user-friendly graphical interface and to store the data in a database. These tasks require a group of parallel executed real time activities. So the monitoring station will run a graphical user interface based operating system capable of multitask process handling with preemptive scheduling and a secure file system.
Number of devices in the system
Due to the specification of the demonstrator the system consists of a central monitoring station (a master unit) and 6 distributed devices. These are the external modules, that interfacing the vital signals onto the fieldbus, and the actuators (infusion pump and gas inhalator represented by the motor-stepped potentiometers). For the proper capability to expand the system in the future, but keep the required transmission speed under an affordable level the maximum number of the slaves is fixed to a reasonable number of 10.
Data transfer rate
For a satisfactory reaction time at least 10 telegrams per second from each slave should be transmitted. Calculations have shown, that due to the intensive signal preprocessing a serial data transfer rate of 100 kbit/s is enough for the full system.
Data security
The transferred data should be protected at least with a CRC code. The main controller should realize all kind of eventual defects in the communication media or in the slaves and manage the defects in a plug & play manner.
Distance and media
The distributed system should support distances as high as maximal 100 meters and the transmission media should be due to the ease of installation a single twisted pair.
One of the most powerful open industrial communication technology is the family of PROFIBUS standards. Since this is the market-leader European fieldbus and supported by hundreds of vendors, we decided to use it in our demonstrator.
The family of PROFIBUS standards
PROFIBUS is a vendor independent, open field bus standard for various applications like industrial automation, production control, building automation and in our case, as a new area: biomedical applications. The original PROFIBUS standard was the German standard DIN 19245 Part I and Part II [5]. The bus is specified based upon the OSI layer model. The Part I describes the functionality of the lower two layers, and Part II deals with the complex functionality of the layer 7 (Fieldbus Message Specification, FMS).
By the year 1990 Part III of DIN 19245 published [6], and a simple, but yet powerful application extension over the layer 2 - without the FMS layer - and a standardized user interface was described. This new PROFIBUS standrard is the so called PROFIBUS DP (Decentralized Periphery).
In the middle of the 90's a new Part IV of the DIN 19245 was released. The branch of PA (Process Automation) devices based upon the DP, and this standard offers a transmission media for the explosion secured (Ex) areas, but at a lower transmission speed. Since 1996 PROFIBUS is a European standard EN 50170.
Since PROFIBUS hasn't got the OSI layers 2-6, it is impossible to realize a complex system with routing and multiple sessions, but it is not required at all, since medical monitoring as other "on-the-field" tasks, interconnect a limited number of devices with the main aspects of deterministic and reliable data transmission.
As our main aim is to use a vendor-independent, fully standardized, and easy-to-use communication protocol, we decided to use the PROFIBUS DP.
PROFIBUS DP cares a patient
The PROFIBUS DP was developed for its simple standardized user interface and its main goal is to realize a fast cyclical data transfer, upto 246 bytes/data per message and upto 12 Mbit/s between the various devices connected to the bus. The transmission media can be either a single twisted pair or an optical fiber at higher speeds. A mono-master DP structure will be used. All system components except the artificial patient are linked to the fieldbus. The central monitoring station is linked as the DP master, the other devices are the slave ones.