Epidural Anaesthesia (page 2)

Issue 13 (2001) Article 10: Page 2 of 4

Go to page: 1 2 3 4

Epidural Anaesthesia (Continued)

Technique of Epidural Anaesthesia

Preparation

An epidural must be performed in a work area that is equipped for airway management and resuscitation. Facilities for monitoring blood pressure and heart rate must be available. It is advisable to obtain informed consent prior to performing an epidural in the same way as before any other invasive procedure. The patient should be informed of the possible risks and complications associated with epidurals (see below). A formal pre-anaesthetic assessment should be carried out, and this should be no less rigorous than one carried out prior to general anaesthesia. Special attention should be given to the patient's cardiovascular status, with the emphasis on valvular lesions or other conditions that might impair the ability to increase cardiac output in response to the vasodilatation that inevitably follows sympathetic blockade. The back should be examined and any lesions or abnormalities noted. Laboratory assessment of the patient's coagulation status is necessary where there is any doubt regarding coagulopathy or anticoagulation therapy. INR (or prothrombin time), APTT and absolute platelet count should be within the normal range. Where there is doubt about platelet function in the presence of a normal platelet count, a haematologist's advice should be sought.

Prior to performing the block, all equipment should be checked. Intravenous access, preferably with a large bore cannula (e.g. 16G), is mandatory before the block is sited. The skin should be prepared with alcohol or iodine-containing sterilising solution. The back should be draped in a sterile fashion, and the operator should take full sterile precautions, including gown, mask and gloves.

Equipment

Modern epidural kits are usually disposable and packed in a sterile fashion. All equipment and drugs used should be sterile, and drugs should be preservative free.

The epidural needle is typically 16-18G, 8cm long with surface markings at 1cm intervals, and has a blunt bevel with a 15-30 degree curve at the tip. The most commonly used version of this needle is the Tuohy needle, and the tip is referred to as the Huber tip. Most commercially available needles have the Tuohy/Huber configuration and have wings attached at the junction of the needle shaft with the hub, which allow better control of the needle as it is advanced. The original winged needle was called the Weiss needle (figure 2).

Traditionally, a glass syringe with a plunger, which slides very easily, has been used to identify the epidural space. Newer, commercially available disposable epidural packs contain a plastic syringe with a plunger that has very low resistance. Normal syringes should not be used because their greater resistance may make identification of the epidural space more difficult. Epidural catheters are designed to pass through the lumen of the needle and are made of a durable but flexible plastic, and have either a single end-hole or a number of side holes at the distal end (figure 3). A filter is attached via Luer-Lok to a connector, which, when tightened, grips the proximal end of the catheter, and serves to prevent the inadvertent injection of particulate matter into the epidural space, and also acts as a bacterial filter. These filters are also usually included in disposable epidural packs.

Techniques to identify the epidural space

The epidural space is entered by the tip of the needle after it passes through the ligamentum flavum. The space is very narrow and is sometimes called a potential space, as the dura and the ligamentum flavum are usually closely adjacent. The space therefore has to be identified as the bevel of the needle exits the ligamentum flavum, as the dura will be penetrated shortly after if the needle is advanced any further. To identify this point, several techniques have been developed over the years, but currently most practitioners use a syringe to identify a loss of resistance when pressure is applied to the plunger. Some use saline in the syringe, and others use air. The two techniques are broadly similar, with some subtle differences in the way the syringe is advanced and the epidural space entered. Other techniques to identify the epidural space have been used in the past, e.g. the "hanging drop technique". With this technique, a drop of saline is placed at the hub of the needle and the needle (without syringe) is advanced. The epidural space is identified when the drop is "sucked" into the needle by the negative atmospheric pressure in the epidural space (equivalent to the intrapleural pressure). This technique is rarely used today.

The block can be performed with the patient either in the sitting or lateral decubitus position. The patient should be encouraged to adopt a curled up position, as this tends to open the spaces between the spinous processes and facilitates the identification of the intervertebral spaces. After the back has been prepared with sterile solution and draped in sterile fashion, the desired level is selected (see below).

Midline approach (figure 4)

Using local anaesthetic raise a subcutaneous wheal at the midpoint between two adjacent vertebrae. Inflitrate deeper in the midline and paraspinously to anaesthetise the posterior structures. At the planned puncture site make a small hole in the skin using a 19G needle.
Insert epidural needle ito the skin at this point, and advance through the supraspinous ligament, with the needle pointing in a slightly cephalad direction. Then advance the needle into the interspinous ligament, which is encountered at a depth of 2-3 cm.until distinct sensation of increased resistance is felt as the needle passes into the ligamentum flavum (most people pass the needle through the interspinous ligament and into the ligamentum flavum before attaching the LOR syringe)

At this point, remove the needle stylet and attach the syringe to the hub of the needle. If loss of resistance to saline is to be used fill the syringe with 5-10ml of normal saline. Hold the syringe in the right hand (for a right handed operator) with the thumb on the plunger. The left hand grips the wing of the needle between thumb and forefinger, while the dorsum of the left hand rests against the back. The left hand acts to steady the needle and to serve as a "brake" to prevent the needle from advancing in an uncontrolled way. Using the thumb of the right hand to exert constant pressure on the plunger advance the needle through the interspinous ligament and then into the ligamentum flavum. While the tip of the needle is in the interspinous ligament there may be some loss of saline into the tissues as the tissue is not particularly dense, but there is usually significant resistance to pressure on the plunger. Occasionally, this false loss of resistance may cause some difficulty with placing an epidural. Once the needle enters the ligamentum flavum, there is usually a distinctive sensation of increased resistance, as this is a dense ligament with a leathery consistency. With continuous pressure on the plunger, advance the needle slowly until its tip exits the ligamentum flavum and the saline is easily injected into the epidural space, and the needle stops advancing.
Remove the syringe and thread the catheter gently via the needle into the epidural space. The catheter has markings showing the distance from its tip, and should be advanced to 15-18cm at the hub of the needle, to ensure that a sufficient length of catheter has entered the epidural space. Remove the needle carefully, ensuring that the catheter is not drawn back with it. The markings on the needle will show the depth of the needle from the skin to the epidural space, and this distance will help determine the depth to which the catheter should be inserted at the skin. For example, if the needle entered the epidural space at a depth of 5cm, the catheter should be withdrawn so that the 10cm mark is at the skin, thus leaving approximately 5cm of the catheter inside the epidural space, which is an appropriate length.
The technique when using loss of resistance to air is slightly different. With 5-10ml of air in the syringe, attach it to the hub of the needle once it has entered the interspinous ligament. Grip both wings of the needle between the thumb and forefinger of both hands. The plunger is gently pressed, and if there is resistance ("bounce"), the needle is very carefully advanced, with the dorsum of both hands resting against the back to provide stability. After 2-3mm, the plunger is again gently pressed, and this procedure is repeated as the needle is carefully advanced through the tissues. The distinctive increase in resistance when the needle enters the ligamentum flavum is felt, and the process is continued in 2mm increments. There is usually a distinctive "click" when the needle enters the epidural space, and provided great care is taken, and the needle only advanced in 2mm increments, the needle should stop before it reaches the dura. At this point air can be injected into the epidural space very easily. The syringe is removed and the catheter threaded as above.

Paramedian approach

Epidurals can be sited at any level along the lumbar and thoracic spine, enabling its use in procedures ranging from thoracic surgery to lower limb procedures. Due to the downward angulation of the spinous processes of the thoracic vertebrae, particularly in the mid-thoracic region, the needle has to be directed much more cephalad. to proceed through the ligamentous tissue and into the epidural space (figure 5). The ligaments in this area are also less dense and a false loss of resistance is not uncommon. Because of the oblique arrangement of the spinous processes, the needle has to travel a longer distance before reaching the ligamentum flavum, and there is less space between the spinous processes. It is therefore much more common to encounter bony resistance during the placement of thoracic epidurals. For this reason, many practitioners prefer to use a paramedian approach in this region.

Insert the needle, not in the midline in the space between the spinous processes, but 1-2cm lateral to the spinous process of the more cephalad vertebra.
Advance the needle; perpendicular to the skin until the lamina or pedicle is encountered, and then redirect it approx 30° cephalad and 15° medially in an attempt to "walk the needle" off the lamina, at which point the needle should be in close proximity to the ligamentum flavum. The needle is then advanced further using a loss of resistance technique (figure 6).

Thoracic epidurals are technically more difficult to perform than lumbar epidurals, and should be attempted only once a practitioner is experienced and confident in the performance of lumbar epidural blocks.

Problem solving during performance of an epidural

Bony resistance everywhere - try flexing more or changing position. If still unsuccessful, try paramedian approach (if using midline approach).
Unable to thread catheter- try rotating the needle slightly so that the bevel changes direction. Most commercial epidural packs contain a catheter stabiliser, which attaches to the hub of the needle and may make feeding the catheter easier. If still unsuccessful, the needle is unlikely to be in the epidural space. Do not pull back the catheter through the needle as the tip may be cut off.
Fluid through needle - if using saline, wait a few seconds to see if it stops flowing. If not, dural puncture is likely. Resite epidural at a different level. If fluid stops flowing, continue as before, but give small doses of local anaesthetic incrementally and observe carefully for signs of subarachnoid block.
Fluid through catheter - as above
Pain on insertion of the catheter - a brief sensation of "electric shock" on insertion of the catheter is not unusual, but if it persists, the needle or catheter may be up against a nerve root and should be withdrawn and resited.
Blood in catheter. This indicates that the catheter has entered an epidural vein. Withdraw catheter by 1-2cm provided this will leave at least 2-3cm in the space and flush through with saline. Aspirate again to see if blood is still flowing through catheter. If blood has stopped, the catheter may be used, but with great care, making sure at all times that 1) catheter is aspirated prior to any subsequent doses of local anaesthetic 2) all doses are given in small increments 3) the patient is carefully monitored for any early signs of local anaesthetic toxicity.

(Continued ...)