**2. Epidural procedure and challenges of clinical simulation**

Epidural analgesia and anaesthesia is commonly used as a form of pain relief during childbirth, for the treatment of chronic back pain or as a means to provide anaesthesia or analgesia during specific operations. Monitoring the depth of the needle during an epidural insertion is crucial because once the needle tip enters the epidural space, an epidural catheter is usually sited to a specific length. This enables the intermittent or continuous use of the epidural for anaesthesia or pain relief. If the needle is advanced too far it will puncture the dural sac and cause leakage of cerebrospinal fluid. Post dural puncture headaches may result, which can be extremely disabling for the patient. Other potential risks include nerve damage or bleeding which may very rarely lead to paralysis. If the needle is not within the epidural space, the analgesia or anaesthesia may be ineffective or absent due to incorrect placement of the catheter.

During an epidural insertion, the operator tries to perceive which tissue layer the needle tip is passing through by feeling the resistances on the needle. This is a process known as "haptic" feedback. A simulator can assist the development of this visuospatial awareness of spinal anatomy and 'feel' of the procedure to allow practice prior to attempts on patients. Not only will this serve to enhance patient safety but it also creates a safe and controlled environment in which to learn.

The procedure of inserting an epidural needle into the lumbar spine requires the operator to visualise in their mind a three-dimensional (3D) anatomical image of the bony alignments and the various tissue layers from skin, through to subcutaneous fat, supraspinous and interspinous ligaments, ligamentum flavum and then to the epidural space. Epidural needle insertion is essentially a blind procedure, but utilises a well-known technique referred to as "loss of resistance" (LOR). LOR essentially involves identification of the epidural space by compression of either fluid or air as the epidural needle encounters the various ligaments of the lumbar vertebral column [1]. Initially, the back of the patient is palpated, and using surface landmarks such as the iliac crests, an assessment is made of suitable intervertebral spaces and of midline. For lumbar epidurals, this may be between lumbar vertebra 3 (L3) and lumbar vertebra 4 (L4) for instance. The epidural or Tuohy needle, as it is commonly called, is inserted into the interspinous ligament and a syringe filled with saline is attached to the end of the needle. These LOR syringes are specially manufactured so that there is less friction between the plunger and the inner wall of the LOR syringe. A constant or intermittent force is then applied to the plunger by the operator's thumb as the needle is slowly advanced forward. As the tougher and more fibrous ligamentum flavum is encountered, a higher resistive force to injection is encountered. Once the needle tip traverses the ligamentum flavum, the epidural space is then entered into and saline can be quite easily injected, hence the phenomenon of LOR. It is this haptic perception that informs the operator of needle location within the various tissue layers, obstruction from bone and loss of resistance from potential spaces such as those between the ligaments. Combining this with the creation in one's mind of a three-dimensional image of lumbar spinal anatomy enables successful placement of an epidural catheter.

388 Practical Applications in Biomedical Engineering

provided in section 13.

the design of devices capable of making precise measurements and utilising them in a novel high fidelity epidural simulator. Adequate training on an advanced simulator will help alleviate the risks of epidural failures from inaccurate placement and also reduce potential

This chapter is laid out in various sections to illustrate different aspects of current epidural anaesthesia research. Section 2 describes the actual epidural procedure and its challenges. Section 3 discusses the needle insertion forces in epidurals. Section 4 describes an interspinous pressure measurement device for wireless data collection during needle insertion leading to a porcine trial discussed in Section 5. Section 6 describes an image processing technique for non-contact needle depth measurement that could be used in conjunction with pressure measurement for fully characterising the needle insertion. In Section 7, 3D-modelling of spine with bending and flexing is discussed for flexibility of patient's positions together with heterogeneous volumetric modelling of spinal ligaments in Section 8. Stereo 3D visualisation for depth perception of epidural procedure has been discussed in Section 9. Section 10 applies a haptic force feedback device configured with the measured force data to create an electronic human–computer interface which is described in Section 11. Finally, section 12 brings all these technologies together and demonstrates the complete system that makes up our current epidural simulator prototype with conclusions

morbidity to patients thereby improving the safety of the procedure.

**2. Epidural procedure and challenges of clinical simulation** 

absent due to incorrect placement of the catheter.

environment in which to learn.

Epidural analgesia and anaesthesia is commonly used as a form of pain relief during childbirth, for the treatment of chronic back pain or as a means to provide anaesthesia or analgesia during specific operations. Monitoring the depth of the needle during an epidural insertion is crucial because once the needle tip enters the epidural space, an epidural catheter is usually sited to a specific length. This enables the intermittent or continuous use of the epidural for anaesthesia or pain relief. If the needle is advanced too far it will puncture the dural sac and cause leakage of cerebrospinal fluid. Post dural puncture headaches may result, which can be extremely disabling for the patient. Other potential risks include nerve damage or bleeding which may very rarely lead to paralysis. If the needle is not within the epidural space, the analgesia or anaesthesia may be ineffective or

During an epidural insertion, the operator tries to perceive which tissue layer the needle tip is passing through by feeling the resistances on the needle. This is a process known as "haptic" feedback. A simulator can assist the development of this visuospatial awareness of spinal anatomy and 'feel' of the procedure to allow practice prior to attempts on patients. Not only will this serve to enhance patient safety but it also creates a safe and controlled

The procedure of inserting an epidural needle into the lumbar spine requires the operator to visualise in their mind a three-dimensional (3D) anatomical image of the bony alignments and the various tissue layers from skin, through to subcutaneous fat, supraspinous and The ideal epidural simulator should be capable of replicating the above procedure and aim to recreate as far as possible the in-vivo procedure. A real Tuohy needle could be inserted at any intervertebral space in the lumbar or thoracic region using the midline or paramedian approach [2]. It would contain a force feedback haptic device, with force data originating from measured Tuohy needle insertions from patients. Using measured in-vivo data from patients and integrating this into the epidural simulator software, the resistance would automatically adjust to give patient variation on weight, height and body shape. This could simulate random patients or match measurements from a specific patient. The 3D virtual patient and virtual vertebrae can also be adjusted in size and shape to match measurements from actual patients. As the needle advances, the resultant force should represent each tissue layer and a LOR on reaching the epidural space. Once the epidural space is reached, saline would be released. During the entire insertion, a 3D virtual spine could be displayed on the monitor showing the trajectory of the needle in real time. The manikin could bend forwards to mimic spinal flexion to increase spacing between the vertebrae or alternatively bend backwards (extension) to simulate increased difficulty in locating the interspinous space for training purposes.

Variation in patient size, height, weight and other characteristics should be possible based upon actual patient measurements. Currently, most simulators have only two or three options such as obese, elderly and normal [3-6] which is perhaps not enough to encapsulate reality and could therefore be improved. Simulators could have unlimited patient variation by including parameters such as height, weight, body shape, age, obesity which could be adjustable. Ideally, the settings should match measurements from real patient data. The adjustments can be programmed to occur automatically based on basic patient data, so that the user does not have to manually configure all the settings. The simulator could then recreate a virtual model of a particular patient. Clinicians planning on performing the epidural can practice beforehand on a virtual model of the patient thereby reducing the learning curve during the procedure on the patient. The four common patient positions adopted during epidural insertion are sitting, sitting with lumbar flexion, lateral decubitus and lateral decubitus with lumbar flexion. These four common positions at least should be modelled in an epidural simulator to give a greater level of realism than static epidural simulators. Ideally, variable spine flexibility could be achieved by modelling 3D flexible spine vertebrae and extended to other positions to simulate difficult spinal anatomy. This may allow simulation of spinal conditions such as curvatures and rotations caused by kyphosis and scoliosis. These conditions cause difficulties in placing the needle due to unusually positioned landmarks. Also the accuracy of the forces in epidural simulators is a topic of recent discussion [7-9], so it is important that the forces required to insert a needle during simulation match those achieved in reality. Skills learned during this simulation can then be transferred to the actual clinical environment.

Biomedical Engineering in Epidural Anaesthesia Research 391

the anaesthetist during insertion, and this combination of all forces is what simulators need

A sterile wireless measurement device was developed to record the resultant pressure of the saline inside the syringe during an epidural needle insertion. This measurement device is used to enable data collection to quantify the pressure during the epidural procedure.

Our novel pressure measurement device has wireless functionality and by using entirely sterile components allows in-vivo trials to be conducted with patients. A wireless data transmitter is

The design aims to minimise changes to the standard epidural set up. A small sterile threeway tap (BD ConnectaTM) is connected between the Tuohy needle and syringe (Figure 3). The tap is connected to the pressure transducer via a one metre length of saline-filled sterile manometer tubing. The transducer's electrical plug is connected by a short electrical cable to our wireless transmitter. At the remote site, a wireless receiver is connected via Universal

The UTAH Medical Deltran disposable transducer is used for the pressure measurement sensor. These transducers are commonly used in hospitals to monitor systemic blood pressure and central venous pressure. Transducers produce a small electrical signal based on the pressure of the liquid inside the manometer tubing. Disposable transducers are designed to have accuracy of +/- 3% with the average output of 100.03 +/- 0.55 mm Hg and

Quantifying the pressure will enable accurate configuration of an epidural simulator.

utilized to minimize the equipment and disruption in the hospital room (Figure 2).

**Figure 2.** Remotely monitored wireless epidural pressure measurement system.

the worst cases being 98.53 and 101.36 when 100 mm Hg was applied [10].

Serial Bus (USB) to the computer.

to re-create to simulate the feeling on insertion.

**Figure 1.** Several forces involved with needle insertion

**4. Pressure measurement for realistic epidural simulation** 

## **3. Modelling the needle insertion forces**

Epidural insertion consists of a complicated interaction of many forces, needle position and intrinsic properties of the epidural equipment: a) Each tissue has various viscosity, elasticity, density and frictional properties. b) Bubbles of air in saline can compress. c) The method of insertion can vary depending upon needle inclination angle, paramedian angle, speed of insertion and twisting of the needle. d) Properties of the needle can vary, including the angle of the tip, tip type - side tipped or two-plane symmetric, needle gauge from 15-20 and width of the metallic walls in hollow needles vary. e) Plunger resistance is caused by friction on the inner syringe walls. f) The flow of saline is restricted by the funnel narrow opening of the syringe at LOR. g) The needle orifice can plug with tissue obstructing saline release.

Theoretically, a model can partition reaction force down into its individual constituents. The thumb applies force onto the plunger of the syringe and this force interacting with the frictional and resistive tissue forces contributes to the 'resultant pressure', see Figure 1. This pressure cannot escape so it causes the needle to push forwards. This causes the 'reaction force' which is equal and opposite to the applied force and comprised of several factors: a) The cutting force required for the needle tip to pierce the tissue. b) Friction caused by needle shaft resistance on the tissue. c) Static friction to get the stationary needle moving. d) Side compression force is caused by the surrounding tissues. e) Torque is caused by twisting of the needle. f) All of these forces, resistances and torque vary according to depth and tissue stiffness.

It is not feasible to measure all of these forces individually in-vivo and it would not make sense to measure the exact proportions of each force that make up the reaction force. In practice, it may be sufficient to measure the resultant pressure of the saline instead. Measuring resultant pressure provides a combination of all reaction forces, which is felt by the anaesthetist during insertion, and this combination of all forces is what simulators need to re-create to simulate the feeling on insertion.

**Figure 1.** Several forces involved with needle insertion

390 Practical Applications in Biomedical Engineering

then be transferred to the actual clinical environment.

**3. Modelling the needle insertion forces** 

stiffness.

adjustments can be programmed to occur automatically based on basic patient data, so that the user does not have to manually configure all the settings. The simulator could then recreate a virtual model of a particular patient. Clinicians planning on performing the epidural can practice beforehand on a virtual model of the patient thereby reducing the learning curve during the procedure on the patient. The four common patient positions adopted during epidural insertion are sitting, sitting with lumbar flexion, lateral decubitus and lateral decubitus with lumbar flexion. These four common positions at least should be modelled in an epidural simulator to give a greater level of realism than static epidural simulators. Ideally, variable spine flexibility could be achieved by modelling 3D flexible spine vertebrae and extended to other positions to simulate difficult spinal anatomy. This may allow simulation of spinal conditions such as curvatures and rotations caused by kyphosis and scoliosis. These conditions cause difficulties in placing the needle due to unusually positioned landmarks. Also the accuracy of the forces in epidural simulators is a topic of recent discussion [7-9], so it is important that the forces required to insert a needle during simulation match those achieved in reality. Skills learned during this simulation can

Epidural insertion consists of a complicated interaction of many forces, needle position and intrinsic properties of the epidural equipment: a) Each tissue has various viscosity, elasticity, density and frictional properties. b) Bubbles of air in saline can compress. c) The method of insertion can vary depending upon needle inclination angle, paramedian angle, speed of insertion and twisting of the needle. d) Properties of the needle can vary, including the angle of the tip, tip type - side tipped or two-plane symmetric, needle gauge from 15-20 and width of the metallic walls in hollow needles vary. e) Plunger resistance is caused by friction on the inner syringe walls. f) The flow of saline is restricted by the funnel narrow opening of the syringe at LOR. g) The needle orifice can plug with tissue obstructing saline release.

Theoretically, a model can partition reaction force down into its individual constituents. The thumb applies force onto the plunger of the syringe and this force interacting with the frictional and resistive tissue forces contributes to the 'resultant pressure', see Figure 1. This pressure cannot escape so it causes the needle to push forwards. This causes the 'reaction force' which is equal and opposite to the applied force and comprised of several factors: a) The cutting force required for the needle tip to pierce the tissue. b) Friction caused by needle shaft resistance on the tissue. c) Static friction to get the stationary needle moving. d) Side compression force is caused by the surrounding tissues. e) Torque is caused by twisting of the needle. f) All of these forces, resistances and torque vary according to depth and tissue

It is not feasible to measure all of these forces individually in-vivo and it would not make sense to measure the exact proportions of each force that make up the reaction force. In practice, it may be sufficient to measure the resultant pressure of the saline instead. Measuring resultant pressure provides a combination of all reaction forces, which is felt by
