Epidural - Technique

Technique

Procedures involving injection of any substance into the epidural space require the operator to be technically proficient in order to avoid complications.

The subject may be in the seated, lateral or prone positions. The level of the spine at which the catheter is best placed depends mainly on the site and type of an intended operation or the anatomical origin of pain. The iliac crest is a commonly used anatomical landmark for lumbar epidural injections, as this level roughly corresponds with the fourth lumbar vertebra, which is usually well below the termination of the spinal cord. The Tuohy needle is usually inserted in the midline, between the spinous processes. When using a paramedian approach, the tip of the needle passes along a shelf of vertebral bone called the lamina until just before reaching the ligamentun flavum and the epidural space.

Along with a sudden loss of resistance to pressure on the plunger of the syringe, a slight clicking sensation may be felt by the operator as the tip of the needle breaches the ligamentum flavum and enters the epidural space. Practitioners commonly use air or saline for identifying the epidural space. However, evidence is accumulating that saline is preferable to air, as it associated with a better quality of analgesia and lower incidence of post-dural-puncture headache. In addition to the loss of resistance technique, realtime observation of the advancing needle is becoming more common. This may be done using a portable ultrasound scanner, or with fluoroscopy (moving X-ray pictures).

After placement of the tip of the needle into the epidural space, a catheter is often threaded through the needle. The needle is then withdrawn over the catheter. Generally the catheter is inserted 4–6 cm into the epidural space. The catheter is typically secured to the skin with adhesive tape or dressings to prevent it becoming dislodged.

The catheter is a fine plastic tube, through which anaesthetics may be injected into the epidural space. Many epidural catheters have a blind end but have three or more orifices along the shaft near the distal tip (far end) of the catheter. This not only disperses the injected agents more widely around the catheter, but also reduces the incidence of catheter blockage.

Choice of agents A person receiving an epidural for pain relief may receive local anaesthetic, an opioid, or both. Common local anaesthetics include lidocaine, mepivacaine, bupivacaine, ropivacaine, and chloroprocaine. Common opioids include morphine, fentanyl, sufentanil, and pethidine (known as meperidine in the United States). These are injected in relatively small doses, compared to when they are injected intravenously. Other agents such as clonidine or ketamine are also sometimes used.

Bolus or infusion?

For a short procedure, the anaesthetist may introduce a single dose of medication (the "bolus" technique). This will eventually dissipate. Thereafter, the anaesthetist may repeat the bolus provided the catheter remains undisturbed. For a prolonged effect, a continuous infusion of drugs may be employed. There is some evidence that an automated intermittent bolus technique provides better analgesia than a continuous infusion technique, though the total doses are identical.

Level and intensity of block Typically, the effects of the epidural block are noted below a specific level on the body. This level may be determined by the anaesthetist. A high insertion level may result in sparing of nerve function in the lower spinal nerves. For example, a thoracic epidural may be performed for upper abdominal surgery, but may not have any effect on the perineum (area around the genitals) or pelic organs. Nonetheless, giving very large volumes into the epidural space may spread the block both higher and lower.

The intensity of the block is determined by the concentration of local anaesthetic solution used. For example, 0.1% bupivacaine may provide adequate analgesia for a woman in labour, but would likely be insufficient for surgical anaesthesia. Conversely, 0.5% bupivacaine would provide a more intense block, likely sufficient for surgery.

Removing the catheter

The catheter is usually removed when the subject is able to take oral pain medications. Catheters can safely remain in place for several days with little risk of bacterial infection, particularly if the skin is prepared with a chlorhexidine solution. Subcutaneously tunneled epidural catheters may be left in place for longer periods, with a low risk of infection or other complications.