A COMPUTER-CONTROLLED SYRINGE DRIVER FOR USE DURING ANAESTHESIA

Transcription

1 Br.J. Anaesth. (1986), 58, A COMPUTER-CONTROLLED SYRINGE DRIVER FOR USE DURING ANAESTHESIA A Modification of the Graseby MS16A A. T. COHEN The continuous infusion of drugs during the course of general anaesthesia is becoming more popular. This is because of the increasing number of suitable short-acting drugs, and the improvements and reduced cost of suitable delivery devices. The features of an ideal infusion device for use during general anaesthesia are that it should be: reliable, compact, easy to set up and use, battery powered; should use standard disposables (syringes/infusion sets); function independent of gravity; and should have a computer interface. The Graseby Medical MS16A pump (fig. 1) satisfies the majority of these requirements. It is a compact battery-operated syringe driver of the type used by diabetic patients. Its small size and light weight make it convenient to use in the ambulant patient, and it accommodates any size of syringe from 2 ml to 20 ml. The infusion rate (mm h" 1 ) of plunger travel is set by adjusting the two rotary dials on the front of the pump. Independent evaluation has found it to be reliable and accurate (Rithalia and Tinker, 1981; Health Equipment Information, 1982). Unfortunately, it lacks a facility for computer control, a feature important for the development of closed-loop administration systems, or for the automated delivery of multi-stage infusions of drugs. Commercial computer-controlled pumps are available, but are expensive. This paper describes a modification of the Graseby MS16A pump and an interface to allow its control by microcomputer. The interface was designed to be connected via the serial (RS232) port that is found on most microcomputers. It could easily be modified to use a parallel port. ANDREW T. COHEN, F.F-A.R.CS., Department of Anaesthesia, St James' University Hospital, Beckett Street, Leeds LS9 7TF. SUMMARY The requirements of a drug infusion device for use in theatre are discussed. A modification of the Graseby MS16A syringe pump and an inten'ace circuit permitting its remote control by microcomputer are described. The pump is controlled via a standard computer interface (RS232) which makes it a unique and powerful research tool. Aspects of safety are considered. MATBRIALS AND METHODS The disposable syringe is fixed on the infusion pump using a restraining loop. The plunger of the syringe rests against an actuator which moves intermittently with each rotation of a screw-bearing assembly. The screw rotates of a revolution until an internally attached cam makes or breaks a microswitch, causing it to stop. The frequency of rotation is determined by the control circuits of the pump, the majority of which are contained within a single chip (IC1P). This generates an oscillating signal which is fed into a divider chain incorporating the switches set by altering the rotary dials of the pump. The motor is thus energized at intervals that depend on this setting. In order to allow external control of the device, a method of influencing the divider chain had to be devised. Microcomputers communicate via their input/ output ports. Conventionally, a character is represented as a stream of eight on/off pulses allowing up to 256 (2 8 ) character codes to be denned. Rather than transmit this information as a series of eight pulses one after the other (serial), it can be transmitted down eight lines at once

2 A COMPUTER-CONTROLLED SYRINGE DRIVER 671 SYRINGE DRIVER Type MS 16A S3 Graseby FIG. 1. The Graseby MS16A syringe pump, showing the two rotary controls for adjusting the rate in mm h~'. Computer control is via CNl at the bottom left of the pump (shown in detail in the inset). (parallel). The interface described first converts a serial stream of data to parallel which it then uses, as described later, to function effectively as eight on/off logic switches mimicking the normal control system of the pump and allowing retention of its alarm functions. Hardware Pump modification. The front casing of the pump was removed and a small recess, found behind the company logo at its lower left hand corner, was enlarged and drilled through to take a 10-pin connector (CNl). Thirty-gauge wire was used to link the connector to pins 1-8 and pin 23 of IC1P. The remaining link was made to ground. The pump remains fully functional and can be used in the normal fashion when computer control is not required. All the wiring is internal with the only external sign of modification being CNl. Computer-MS16A interface. A circuit diagram is provided (fig. 2). The serial (RS232) signal is made compatible with the interface logic by IC1 and is then fed into the Universal Asynchronous Receiver Transmitter (UART) at pin 20. The UART converts the eight-bit serial logic signal to produce the corresponding parallel data on pins IC3 determines the data conversion rate (baud rate) which must match the computer's transmission rate. Most microcomputers will only allow the serial interface to work at one speed during bidirectional transmission and reception of nonviewdata signals; consequently, the baud rate used may be determined by the transmission speed of the monitoring equipment used. In figure 2 the system is configured to receive data from a Dinamap 1846 non-invasive arterial pressure monitor at 600 baud. The baud rate can be made switchable by connecting four dual in-line switches to pins 4-7 of IC3 such that the logic value on each pin is adjustable in accordance with the Manufacturer's data sheet. IC3 itself is controlled by the mHz quartz reference crystal. Once the UART has transformed a set of data, its output logic signals remain constant until it is updated. This combination of logic signals is used to control the infusion pump by feeding the oscillating signal derived from IC1P back to pins 1-8 of the same chip. The feedback pattern is determined by the output of two logic chips IC4 and IC5, which each function as four separate

3 672 BRITISH JOURNAL OF ANAESTHESIA TO PINS 1-8 ON IC1P SERIAL INPUT FROM COMPUTER FROM PIN 23 ON IC1P fttt TO PINS 1-8 ON IC1P FIG. 2. Basic circuit diagram for Graseby MS16A/microcomputer interface. For clarity, details have been omitted such as charging and low battery warning circuits. For certain uses handshaking facilities may also be desirable. IC1 = MC1489;IC2 = AY31015;IC3 = AY58116;IC4andIC5 = 4081 ;C1 and C2 = 56pf; Ql = mhz quartz crystal. " AND " circuits. The output of an AND circuit is high if both inputs are high and low in any other circumstance (fig. 3). The primary oscillation is applied to input 1 of each of the eight AND stages and the output of the UART applied to input 2. Consequently, whenever the UART output is high, the output stage of the relevant AND segment oscillates at the same frequency as input 1. INPUT -nnnn. OUTPUT FIG. 3. Diagram of "AND " logic gate. The output is positive only if both the inputs are positive. The oscillating signal from the Graseby pump is applied to pin 1 continuously, but the output will remain negative until input 2 becomes positive. At this time the output will oscillate at the same frequency as input 1. The interface box measures 90 mm x 70 mm x 26 mm and weighs 180 g. Power is derived from four half-aa sized nickel-cadmium rechargeable cells. A charger can be plugged into an external socket at the side of the interface and used to power the unit directly, if required. The pump is connected to the interface box using a 10-core 200-cm length flat cable fitted with insulation displacement connectors at each end. The computer serial output is connected via a 25-way "D " connector wired in the standard way. An on/off switch is avoided by arranging for the power to be automatically supplied to the circuit only if the D connector is in place. The possibility that modification of the pump had any effect on its accuracy or alarms was excluded by testing in the laboratory. To take over control by computer, the interface lead is simply plugged in and the rotary dials on the pump are set to 00 mm tr 1. If the pump is to be used exclusively for computer control or to provide additional safety the rotary dials can be removed and the holes temporarily closed, thus enabling it to be fixed in the 00 mm h" 1 position.

4 A COMPUTER-CONTROLLED SYRINGE DRIVER 673 Software There are two 10-position rotary dials on the front of the pump. Each dial controls the logic level on four of the pins of IC1P. The controlling computer program written in 30 lines of BASIC is designed to mimic these switches. A flowchart can be seen in figure 4. The first step is to convert the desired infusion rate (ml h" 1 ) to the equivalent (mm h" 1 ) of plunger travel for the syringe type and size being used. The resultant number between 00 and 99 is then treated as two separate digits, one corresponding to the tens dial and one corresponding with the units dial. The computer generates a character code corresponding to the digital pattern produced by setting the dial to this number. The two codes are concatenated and the joined code is then transmitted via the computer output port. Input syringe size Input infusion rate (mlh- 1 ) Convert to plunger travel (mm h" 1 ) Convert units to corresponding character code Convert tens to corresponding character code Concatenate Sendcode to output port FIG. 4. Flowchart of the MS16A control program. The BASIC routine can be used within other programs designed for adjusting infusion rates automatically or calculating infusion rates for particular drugs. Safety The purpose of the modification and the interface described is to produce a computercontrolled pump which could be built by a medical electronics department for a fraction of the cost of the closest commercial equivalent. Its small size and low power requirements make it particularly convenient to use. A commercially available computer-controlled pump has been described in which communication between the computer and the pump is bidirectional. This allows both the computer to check for errors in the pump and the pump to check for errors in the output of the computer (Kenny et al., 1985). This near perfect fail-safe system is suitable for use in a high dependency unit. For simplicity, the circuit detailed here is the original used by the author for the administration of drugs such as neuromuscular blockers and analgesics during anaesthesia. As transmission of an incorrect code to the syringe pump will have unpredictable results, this must be prevented by adding safety features to the basic design. The degree of sophistication necessary will depend on the use to which the pump is being put. Before transmission, the code can be checked for errors by the software. Error trapping routines are incorporated in the current software and these not only prevent premature termination of the program (a crash) but also ensure that, if there is a software error, a logical code is sent to the interface. The commonest scources of error experienced when using this sort of equipment in the operating theatre are the result of temporary disturbances in power, such as caused by accidental disconnection of monitoring equipment. If this equipment is connected to the computer it can transmit inappropriate codes which, in the absence of suitable error-trapping routines could cause a "crash" or enter the controlling program. By monitoring the status of pins of the UART (IC2), the interface itself can continuously check for errors related to incorrectly set transmission parameters or parity errors which would indicate corrupted code. A logic " 1" on any of these pins is indicative of an error and can be used to stop the pump or light a warning indicator. The interface is powered by nickel-cadmium

5 674 BRITISH JOURNAL OF ANAESTHESIA cells that can be recharged in situ. The four half-aa sized cells were chosen to be suitable for approximately 4 h continuous use; however, if prolonged periods of use are anticipated, then these should be replaced by cells with a larger capacity or the unit should be powered from the mains, using a suitable charger. If the batteries are allowed to exhaust themselves the interface will not function correctly; therefore, a low power warning circuit is valuable to warn of potential data corruption. To confirm continuity of connection between the computer and the patient, disconnection indicators can be added to the system to terminate automatically the power supply to the pump, or warn the operator that a fault condition exists. An infusion rate monitor can be provided by the computer software to allow for the volume of fluid infused by the pump to be verified. This facility uses information provided by counting the rate of opening and closing of the microswitch inside the pump. Electrical safety is another problem that must be considered when using computer equipment in clinical areas. Most microcomputers are not designed for use in the clinical environment, where there are not only many potential hazards that may harm the machine or cause it to fail, but where the consequences of minor electrical faults could be dangerous. It is possible to isolate the computer from patient-connected equipment optically, using commercially available opto-isolators. An alternative is to use a battery-operated computer. CLINICAL EXPERIENCE The original modification was designed to provide a technique allowing the computer-controlled infusion of analgesic (alfentanil) and neuromuscular blocking (atracurium) drugs. A battery operated computer (Epson HX-20) was chosen to reduce the problems of electrical safety. The HX-20 has a battery life of h, depending on which peripherals are being used. Other features making it ideal for use in the operating theatre are: (1) Compact size (290 mm x 216 mm x 44 mm) and weight (1.7 kg). (2) Integrated liquid crystal display, obviating the need for a large cathode ray tube display. (3) Integrated microprinter, allowing patient data and infusion rates to be automatically recorded. (4) Integrated software-controlled microcassette for storing programs or data logging. (5) Non-volatile memory, to preserve programs when machine is switched off and avoid having to reload. Non-volatile random access memory, to allow storage of data and downloading to large printer or disk at the end of the day. (6) Real time clock and date for accurate data logging. (7) Inexpensive, less than half the cost of a basic ECG monitor. After the early success of the modification, software was developed to allow the use of closed loops. The system has been used for the past 18 months, mainly for projects involving the administration of alfentanil and atracurium. The commonest problems that occurred during the development of the system, except for simple software errors, were related to interface power supply failure which has been corrected using a low battery warning circuit, and errors caused by the reception of erroneous codes from the monitoring equipment which have been corrected using software error trapping routines. All patients were anaesthetized using a balanced anaesthetic technique, with the addition of a small quantity of volatile agent, if required. A computercontrolled infusion was commenced in the operating theatre, using a monitor of neuromuscular tone, in patients receiving infusions of atracurium, or monitors of cardiovascular status in patients in whom alfentanil was being infused. Separate i.v. access was always used for the infusion, to avoid drug interactions and accidental reverse flow into the drip chamber. Whereas the technique has been used successfully during short procedures, it is particularly useful during major surgery. Patients undergoing many types of procedure, including vascular, urological and renal transplantation surgery and from ASA groups 1 to 4, have been successfully anaesthetized for operations lasting up to 4 h. Detailed data from 50 patients undergoing closed-loop drug administration using this system are being prepared for publication. This reflects approximately one-third of the patients anaesthetized in this manner. Up to the present, the system has been used to administer narcotics using cardiovascular signs of lightness of anaesthesia, and neuromuscular blocking drugs by monitoring impairment of neuromuscular transmission. Potentially, however,

6 A COMPUTER-CONTROLLED SYRINGE DRIVER 675 it could help with the control of many other variables. The use of the system for closed-loop administration of drugs is designed to help rather than replace the anaesthetist. Failure of the system with the above two drugs has not been a problem, since it would be demonstrated by inadequate anaesthesia or excessive recovery times. All software currently written allows the operator easily to alter the drug infusion rate or to terminate it, if necessary, and incorporates alarm features that automatically act in appropriate situations. The computer rarely requires external help, except if the monitoring equipment is working incorrectly, such as may be caused by the effect of external pressure on the cuff of a non-invasive arterial pressure monitor. The provision of continuous written records is particularly helpful, especially in association with prolonged anaesthesia. It is intended that further software support will concentrate on performing more statistical analyses on the data collected, such as mean infusion rates, total dose infused and effectiveness of infusion. ACKNOWLEDGEMENTS I would like to thank the Medical Illustration Department of St James' Hospital for their help in the preparation of the illustrations for this paper. REFERENCES Health Equipment Information (1982), 106, 11. Kenny, G. N. C, Rennie, R., Toal, F., and Reid J. A. (1985). Computer control of an Imed 929 infusion pump. J. Med. Eng. Technol., 5, 227. Rithalia, S. V. S., and Tinker, J. (1981). Recent developments in infusion devices. Br. J. Hosp. Med., 25, 69.