**11. Human-computer interface for loss of resistance syringe**

With the above developed components, a hardware device has been created consisting of a regular Portex LOR syringe connected to the computer via a serial data transfer device. This allows a regular clinical syringe to be used as part of an interactive system for the epidural simulator development. The syringe was also combined with the haptic device to create a comprehensive human-computer interface. The simulator can measure force applied to the plunger and the resultant pressure of the saline inside the syringe barrel. This interface enables a real clinical syringe to interface with a 3D graphical visualization showing the simulated insertion of the Tuohy epidural needle through the spinal ligaments.

The developed hardware interface makes use of the equipment as developed in Sections 4 & 6 by incorporating custom made hardware with the developed software and the graphical visualization of the needle insertion procedure. The hardware device takes measurements of the forces applied onto the needle and the resultant pressure of the saline inside the barrel of the syringe caused by the pressure from the operators thumb on the plunger. The measurements are sent to the computer by a custom-made hardware interface device (see Sections 4 & 6). The graphical simulation uses these measurements to update the needle in the simulation and calculates the needle position. The graphical software calculates if any collisions have occurred between the needle and any bone structures, plus the resistance of insertion to saline, and the force required for the needle to move forwards through the current ligament.

The developed human-computer interface uses an actual syringe and an epidural Tuohy needle as shown in Figure 17. During insertions, the LOR syringe is normally connected directly onto the Tuohy needle. We have introduced a three-way tap between the needle and syringe. This connects onto a one metre length of saline manometer tubing which runs to a disposable pressure transducer. The transducer converts the pressure of the saline into an electrical signal. The electrical signal is connected into a hardware device which amplifies and sends the pressure reading to the computer. This allows the graphics visualization to

update according the pressure applied by the operator's thumb on the plunger of the syringe. This has the advantage that the user can control the visualization with the same equipment that would be used in-vivo, which is a more natural interface than simply using keyboard or mouse. Additionally, since the saline line separates the hardware device from the needle, the user can move the needle around since it is attached only by the saline line.

Biomedical Engineering in Epidural Anaesthesia Research 407

testing the pin for +12V, and then checking again after 22μS for the same high value. The computer runs the custom designed software which monitors the data as it arrives. Also the values are received by the graphics application which updates the visualisation to match the

This study has demonstrated the development of a human-computer interface based around a clinical Portex LOR syringe connected via a custom made hardware interface device to a computer for use in an epidural simulator. The results show that the device is both fast and accurate enough to be used seamlessly in the simulation. The addition of the Portex LOR syringe with a pressure monitoring device has undoubtedly improved the human-computer interaction. Using the actual medical components in the implementation is beneficial because epiduralists will be familiar with the syringe and use it to interact with the 3D graphics visualization intuitively. The interface could be modified to be bi-directional i.e. the graphics software could send back data to the device which could control a motor to cause forces which affect the physical needle so that the user can feel the forces through the needle as in-vivo.

The presented biomedical engineering ideas have enabled us to develop a simulator with a combination of engineering, computing and clinical technologies as discussed in previous sections above. Data from the developed measurement devices have been used to configure a realistic force feedback epidural simulator [25]. Numerous improvements have been identified that could enhance existing epidural simulators. Manikin models are generally static and only able to represent one or two patient variations, such as normal and obese. An advanced simulator would be able to simulate insertions on a variety of body mass indices because excess fat deposition has the potential to generate very different changes in patient

The developed system offers a virtual reality based epidural simulator (Figure 19) incorporating a 3D graphically modelled spine complete with skin, fat and tissue layers, supraspinous, interspinous ligaments and ligamentum flavum. In the current prototype, a Novint Falcon haptic device is used in combination with a Portex LOR syringe connected as a human-computer interface via a custom made electronic serial interface. As the haptic stylus is moved, the needle follows on the screen in 3D in real time. When pressure is applied to the plunger by the operator's thumb, this is displayed in the graphic model. As the needle is advanced through the tissues, the forces are generated by the haptic device to reconstruct the feelings of needle insertion through each tissue layer. The forces of the needle insertion are based on the recorded forces measured during the clinical trial, and this data based approach is more accurate than previous simulators which have used a user

Novel aspects of our epidural simulator include stereo graphics, modelled vertebrae, spine flexibility, patient variation, haptic force feedback based on measured needle insertion data, custom made syringe interface. The simulated needle can be inserted at any spinal position

pressure applied on the physical syringe.

**12. Creation of a novel epidural simulator** 

evaluation approach to configure the forces.

characteristics.

**Figure 17.** The syringe connected to the computer as an input device.

The hardware device runs at 8MHz. Data is transmitted from the hardware device to the computer using the serial RS232 port. The serial bit rate is running at 22000 bits per second. The serial data transfer protocol uses -12V DC as a positive bit and +12V DC as a negative bit. The serial transfer cycle starts with a negative start bit, followed by 8 data bits sent consecutively and finished with a positive stop bit. As shown in Figure 18, the following start bit can then occur either immediately or after a pause of arbitrary length.

**Figure 18.** Binary serial data transfer protocol.

The 8 data bits are received and interpreted as binary and converted into a decimal number from 0 to 255 for use in the software. The decimal value represents the pressure of the saline between 0 to 70 kPa, which is 0 to 550 mmHg. The 256 possible values give an accuracy resolution to within +/- 0.14 kPa. This can be easily increased to 1024 with 10 bits data transfer which will then provide accuracy of within +/- 0.03kPa. The speed could also increase beyond the current 22000 bits per second but it does not seem necessary since no delay is noticed between pressing the plunger and seeing the results on screen. Currently at 22000 bits per second the time delay between bits is 45μS so the start bit is identified by testing the pin for +12V, and then checking again after 22μS for the same high value. The computer runs the custom designed software which monitors the data as it arrives. Also the values are received by the graphics application which updates the visualisation to match the pressure applied on the physical syringe.

This study has demonstrated the development of a human-computer interface based around a clinical Portex LOR syringe connected via a custom made hardware interface device to a computer for use in an epidural simulator. The results show that the device is both fast and accurate enough to be used seamlessly in the simulation. The addition of the Portex LOR syringe with a pressure monitoring device has undoubtedly improved the human-computer interaction. Using the actual medical components in the implementation is beneficial because epiduralists will be familiar with the syringe and use it to interact with the 3D graphics visualization intuitively. The interface could be modified to be bi-directional i.e. the graphics software could send back data to the device which could control a motor to cause forces which affect the physical needle so that the user can feel the forces through the needle as in-vivo.
