Professor of Anaesthesiology Govt Medical College, Trichy
Epidural anaesthesia is a central neuraxial block technique with many applications. The improvement in equipment, drugs and technique have made it a popular and versatile anaesthetic technique, with application in surgery, obstetrics and pain control. Even though both single injection and catheter techniques are used, the indications for single shot epidurals have come down. Its versatility means, it can be used as an anaesthetic, as an analgesic adjuvant to general anaesthesia and for postoperative analgesia in procedures involving the lower limbs, perineum, pelvis, abdomen and thorax. To ensure proper catheterisation of the epidural space, we need to know some basics about the space and the technique.
Anatomy of epidural space:
Epidural space is a potential space that lies between the dura mater and the periosteum lining the inside of the vertebral canal. The anterior and posterior nerve roots with their dural covering pass across this potential space to unite in the intervertebral foramen to form segmental nerves. The anterior border consists of the posterior longitudiual ligament covering the vertebral bodies and the intervertebral discs. Laterally, the epidural space is bordered by the periosteum of the vertebral pedicles and the intervertebral foraminae. Posteriorly, the bordering structures are the periosteum of the anterior surface of the laminae and articular processes and their connecting ligaments, the periosteum of the root of the spines and the interlaminar spaces filled by the ligamentum flavum. The space contains venous plexuses and fatty tissue which is continuous with the fat in the paravertebral space.
Epidural Spinal Anesthesia – Animation by Cal Shipley, M.D.
A good understanding of the anatomy of the epidural space is essential to know about the fate of the local anaesthetic solution injected into that space. There may be anatomical barriers to hinder the movement of the drug or the epidural catheter. The epidural space is widest in the dorsal midline and narrows as it goes laterally behind the laminae. The chance of getting a dural tap is less if the epidural needle is in midline as it has to traverse a little more distance to hitch the dura. Epidural catheter will also get threaded easily if the needle enters the epidural space in the midline.
As the lateral boundary is formed by the intervertebral foraminae, catheter can also leave the space and enter into paravertebral space through them. The presence of complete anatomical septae within the epidural space is very rare. However filmsy adhesion and incomplete septae are not that rare, which may interfere with the drug movement and catheter placement.
Technique of epidural anaesthesia:
Whether to give the epidural in the sitting posture or in lateral decubitus position, is a matter of preference to the patient and anesthesiologist. Sitting posture will be comfortable for the patient as well as for the anaesthetist, as it favours easy identification of spine and midline. The chance of dural tap may be more in sitting posture as the C.S.F pressure in the dural sac in lumbar region will be more in sitting posture. Proper flexion of the back will open up the intervertebral spaces.
Local anaesthetic infiltration: 2ml of 2% lignocaine should be used for liberal infiltration of the skin and subcutaneous tissue at the needle point entry. A skin wheal with a peau-de-orange appearance is necessary to properly anaesthetize the skin. A good local anaesthetic infiltration will make the procedure painless and ensures success.
Projection and puncture:
Identify midline-if off the midline, it will be difficult to locate the epidural space. If the epidural needle is inserted little laterally, chance of bloody tap and paraesthesia occurance is more.
Insert the needle into the ligamentatum flavum. Anatomical structures traversed include skin, subcutaneous tissue, supraspinous ligament and interspinous ligament. If the needle is not placed in the ligamentum flavum, anaesthesiologist may experience false positives with the loss of resistance technique.
Both air and saline have been used to identify the epidural space with loss of resistance technique. Most of the anaesthesiologist world wide use saline filled syringe for space identification.
It is claimed that incidence of false positive identification is less with saline technique. Since the air is compressible,a small degenerative area within the ligamentum flavum will permit one or two ml of air to be injected which may make the operator to think that space is reached. People using air filled syringe claim that the solution dripping back the epidural needle after identification may be confused with a C.S.F tap. But C.S.F tap is so obvious with a 16 or 18G Tuohy needle. The needle orientation to identify the space, even in lumbar region has to be little caphalad. This favors easy catheterization as the catheter need not make a 900 turn into the epidural space.
2-4 ml of local anaesthetic solution can be injected through the needle into the epidural space which pushes the dura away and break the minor adhesion.
Paramedian approach of epidural space is commonly done in thoracic region. People claim that the chance of successful catheterization is more with paramedian technique. This may be true as the angle of sliding of catheter into the space is more obtuse compared to the midline approach.
Hanging drop technique: This is also an established technique to identify the space. The drop of fluid should be placed on the hub, once the needle tip is in ligamentum flavum and pushed further. Some people, after identifying the space with L.O.R technique, used to put a drop of liquid on the hub for further confirmation. This technique is wrong as the negative pressure will get neutralized once the stillette is taken out of the needle. An additional point to remember, the identification of epidural space was reported at 2mm deeper for the hanging drop when compared with LOR, possibly indicating increased risk of dural puncture.
Other method:
A number of technical aids for identification have been described, have none of them have sufficient accuracy and practicability to justify the increased effort and cost of their routine use in adults.
Epidural catheterization technique:
Once the epidural space is identified without doubt, then catheterization to be tried. Assess the depth of the needle from skin to space. In the average Indian adults, it is between 3.5 to 5 cm
There should be free flow of air or saline on injection. Don’t inject too much of saline or air, it may cause patchy block.
The Huber point shall be turned to cephalad side as it favors easy catheterization.
If the catheter doesn’t go in, needle can be rotated carefully after taking the catheter out. Never turn the needle with the catheter in situ
There are two ways of taking the needle out after catheterisation. In the first method, catheter is threaded up to 15cm mark and then needle is withdrawn slowly while pushing the catheter in. In the next method, catheter is threaded up to 20cm mark and then needle is withdrawn casually without the catheter manipulation.
After withdrawal of the needle, 10cm mark to be near the skin of the entry point as it places 5cm length of catheter inside the epidural space.
Ideally 3-5cm of the catheter to be kept inside the epidural space. If it is more than that, the chance of curling or going out of epidural space through the intervertebral foramen is high
A test dose consists of 3 ml of 1.5% preservative free lidocaine with 1:200,000 epinephrine. Forty-five milligrams of lidocaine, if injected intrathecally, will result in a spinal anesthetic. Fifteen micrograms of epinephrine, if injected intravascularly, will result in a 20% or more increase in heart rate. Blood pressure may be elevated or remain the same. False positives may occur with epinephrine. For example, a laboring patient may have a contraction at the same time that the test dose was administered, resulting in a concurrent increase in heart rate or blood pressure. False negatives may occur. For example, a patient may be on a beta blocker which will block/blunt an increase in heart rate.
Aspiration before each injection is helpful, but may not always detect intravascular or subarachnoid placement of a catheter.
Incremental dosing of 5 ml every 5 minutes should be performed. This dose should be enough to cause symptoms of intravascular injection without seizures and/or cardiovascular collapse.
Catheter migration may occur any time. This can lead to an intravascular or intrathecal injection. Aspirate before dosing and dose the epidural incrementally. Be cautious and have a high index of suspicion. Some clinicians will use a test dose with each subsequent injection, along with aspiration and incremental dosing.
UPPER THORACIC EPIDURAL
LOWER THORACIC EPIDURAL
DOUBLE EPIDURAL CATHITERISATION
REVERSE EPIDURAL CATHITERISATION
Problems in epidural catheterization:
Bone is encountered by the needle-reassess the direction of the needle, ensure the needle is in midline and the patient is correctly positioned.
Inability to thread the catheter. Ensure the needle is in the epidural space by placing an additional 3 ml of preservative free normal saline. If there is a loss of resistance, then attempt to insert the catheter. Rotating the needle slightly may help. If the catheter still cannot be inserted, start over.
Fluid returns through the needle. When using preservative free normal saline, a small amount may come back. If it does not stop and continues, then the needle may have transversed the dura. If this is suspected, place the epidural at another level and monitor for the development of a post dural puncture headache. If fluid stops, thread the catheter and administer a test dose to ensure that the needle/catheter did not cross the dura. If the test dose is negative, cautiously dose the epidural.
Blood returns in the catheter or needle. The needle/catheter may have entered into an epidural vein. Remove the needle and/or catheter and start over. Make sure the needle is midline.
Pain (paresthesia) upon insertion of the needle. Remove needle immediately and assess position. Commonly, insertion is not midline and the needle should be repositioned.
Pain (paresthesia) upon insertion of the epidural catheter. It is not unusual to get a brief shock like symptom or sensation during catheter insertion. If it does not stop, remove the catheter. It may be in contact with a nerve root.
Pain with injection. The direction of the catheter cannot be controlled during insertion. The tip may be against a nerve root. Pull the catheter back 1 cm and attempt to inject again. If pain continues, remove the catheter and start over.
Ten Tips to Optimize Your Epidural Technique
Failed epidural:
For those who have studied the epidural space, it may seem amazing that epidurals ever work. The epidural space is filled with fat, connective tissue, and an extensive venous plexus. It behaves less like a collapsed Penrose drain and more like a container filled with sand and variously sized pebbles, such that drugs must traverse a maze of obstacles to reach the nerves. This anatomic labyrinth may result in the initial placement of the catheter tip in an unfavorable microenvironment. There are several factors that may lead to a failed epidural block. These include false loss of resistance, misplaced local anesthetic, unilateral block, segmental sparing, and visceral pain.
False loss of resistance. It is possible to insert a catheter in tissue other than the epidural space. Spinal ligaments may be soft, resulting in a false loss of resistance. Being off the midline, in the paraspinous muscle, may also result in a false sense of loss of resistance.
Misplaced local anesthetic. Local anesthetics may be misplaced in other anatomical areas including the subarachnoid space, subdural space, and intravenously. These complications have been discussed earlier.
Unilateral block. The catheter may have traveled out of the epidural space or pointed laterally. Pulling the catheter back 1-2 cm may move the catheter back into a midline position. Pulling back on the catheter may also move the catheter out of the epidural space. Asymmetric blocks can either be completely unilateral or be manifest as windows in an otherwise complete block, in which one or more dermatomes are spared. Despite apparently proper placement, approximately 5% to 8% of epidural blocks may provide incomplete analgesia of this sort.2 Although the cause of any particular failure is usually unknown, it is generally believed that either an anatomic barrier to free flow of local anesthetic or unfavorable positioning of the tip of the catheter is responsible for asymmetric block.
Multiple and extensive studies evaluating epidural anatomy have been done including injecting cadaveric epidural spaces with resin,14 cadaveric epiduroscopy,15 computed tomography epidurography,16 and anatomic dissection with cryomicrotome section.17 The presence of a dorsal median connective tissue band (DMCTB) in some individuals has also been described but is generally thought to be rare and incomplete when present. This DMCTB has been attributed to an artifact of how the epidural space was studied.17 The existence of unilaterally functioning epidurals is not controversial, and remains a challenge for anesthesiologists.
Radiographic studies have shown that when a catheter is threaded beyond the tip of the needle in the epidural space it rarely stays midline, and it may also be directed caudally rather than the preferred cranial direction.18 Anesthesiologists compensate for this by providing a large volume of a dilute solution of a local anesthetic and opioid in order to ensure spread of the medication to both sides of the epidural space as well as up and down the spine. In addition, studies show that when catheters are threaded no more than 5 cm into the epidural space, fewer insufficient blocks occur.19 Perhaps this is because these catheters are less likely to site laterally or, even worse, to exit the epidural space along the course of a nerve root. When these events occur, the spread of the local anesthetic may be limited to either a specific side or a specific dermatome. This provides additional rationale for partially withdrawing an epidural catheter producing an asymmetric block.
Visceral pain is not a failure of epidural anesthesia. Visceral afferent fibers travel with the vagus nerve and are difficult to block. Intravenous supplementation with analgesics and sedatives may be required to get the patient through “uncomfortable” portions of the surgical procedure. If unable to adequately treat discomfort, then general anesthesia should be induced.
Segmental sparing may occur due to anatomical conditions (septa) within the epidural space. Additional local anesthetic may help alleviate this condition. Sacral sparing may occur due to the larger size of L5-S2. Elevating the head of the bed and adding local anesthetic will increase the concentration of local anesthetic in this area, creating a denser block.
Migration: An epidural that works well initially may not continue to work well throughout the entire labor and delivery. The position of epidural catheters is not static, and catheters may migrate completely out of the epidural space (resulting in cessation of analgesia), laterally (resulting in a unilateral block), or even intrathecally or intravascularly (resulting in toxicity). The migration of epidural catheters is not rare; in large retrospective series, 6.8% of patients with initially adequate blocks subsequently developed insufficient analgesia.2
When the catheter cannot be removed using minimal traction a number of manoeuvres may facilitate removal of the catheter. These include:
maximal flexion of the back (and, more rarely, extension of the back) with the patient in the lateral decubitus position,
rotation of the spine,
returning the patient to the position she was in at the time of insertion (eg. the sitting position with legs extended),
allowing tissues to soften before another attempt is made,
placing the patient in the sitting position with the legs extended or the kneeling position with hands down and back flexed,
filling the catheter with a rapid injection of saline to increase the turgor of the catheter and to lubricate it,
placing the patient prone on a Wilson (Zimmer) convex or the “Sydney Harbour Bridge” laminectomy frame,
complete relaxation using general anaesthesia with muscle relaxant
surgical removal.
Conclusion:
Epidural anaesthesia is more subjective than spinal anaesthesia. There is not a clear cut end point like CSF with spinal. The anatomy of the epidural space lends to a less predictable spread of local anaesthetic. Proper understanding of these factors make the epidural more likely to succeed.
References:
Update in Anaesthesia- Article on epidural anaesthesia by Dr.Lion Vissen, Michigan. U.S.A
Why epidural do not always work – by Dr.Catherine Arendt and Dr.Scott Segal
Failed epidurals: Causes and management – by Dr.J.Hermanides et al.
Identification of the epidural space: stop using the loss of resistance to air technique-M.Van De Velde – Acta Anaesthesia Belgium ,2006-57
