Epidural anaesthesia

From Ganfyd

Epidural anaesthesia (sometimes shortened for convenience to "epidural") is a form of regional anaesthesia involving injection of drugs through a catheter placed into the epidural space. The injection can cause both a loss of sensation (anaesthesia) and a loss of pain (analgesia), by blocking the transmission of pain signals through nerves in or near the spinal cord.

The epidural space (sometimes called the extradural space or peridural space) is a part of the spine inside the spinal canal separated from the spinal cord and its surrounding cerebrospinal fluid by the dura mater.

Epidural anesthesia

Indications

Injecting medication into the epidural space is primarily performed for analgesia. This may be performed using a number of different techniques and for a variety of reasons. Additionally, some of the side-effects of epidural analgesia may be beneficial in some circumstances (e.g. vasodilation may be beneficial if the patient has peripheral vascular disease). When a catheter is placed into the epidural space (see below), the effects of the analgesia may be prolonged for several days, if required. Epidurals may be used:

• For analgesia alone, where surgery is not contemplated. An epidural for pain relief (e.g. in childbirth) is unlikely to cause loss of muscle power, but is not usually sufficient for surgery.
• As an adjunct to general anaesthesia. The anaesthetist may use epidural analgesia in addition to general anaesthesia. This may reduce the patient's requirement for opioid analgesics. This is suitable for a wide variety of surgery, for example gynaecological surgery (e.g. hysterectomy), orthopaedic surgery (e.g. hip arthroplasty), general surgery (e.g. laparotomy) and vascular surgery (e.g. open aortic aneurysm repair). See also caudal epidural, below.
• As a sole technique for surgical anaesthesia. Some operations, most frequently Caesarean section, may be performed using an epidural anaesthetic as the sole technique. Typically the patient remains awake during the operation. The dose required for anaesthesia is much higher than that required for analgesia.
• For post-operative analgesia, in either of the two situations above. Analgesics are given into the epidural space for a few days after surgery, provided a catheter has been inserted. Through the use of a patient-controlled analgesia (PCA) infusion pump, a patient may be given the ability to control their epidural analgesia.
• For the treatment of back pain. Injection of analgesics and steroids into the epidural space may improve some forms of back pain. See below.
• For the treatment of chronic pain or palliation of symptoms in terminal care, usually in the short or medium term.

A patient getting a modern epidural for pain relief generally receives a combination of local anesthetics and opioids. Common local anesthetics include lidocaine, bupivacaine, and ropivacaine. Common opioids are morphine, diamorphine,and fentanyl; less frequently sufentanil and pethidine (known as meperidine in the U.S.). These are then injected in relatively small doses. Occasionally other agents may be used, such as clonidine or ketamine.

Because of the nature of epidurals, they are most suitable for analgesia for the abdomen, pelvis or legs. They are much less suitable for analgesia for the chest, neck, or arms and are not possible for the head.

Technique

Epidural anaesthesia is performed by a trained anaesthetist under aseptic conditions with the patient either in the (preferred) sitting position or lying on the side. The patient is asked to arch his back, i.e. to push the small of his back out as if to assume the "posture of an angry cat". The anaesthetist palpates the patient's back and identifies a suitable interspace between adjacent spinous processes of the vertebrae. 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, e.g. the pain during labour and childbirth.

Tuffier's line is the line between the posterior superior iliac crests and corresponds to the interspace between L3 and L4. This is used as the landmark for epidural insertion for labour analgesia (but using this landmark identifies the incorrect space 25% of the time).

Using a strict aseptic technique a small volume of local anaesthetic, such as 1% lidocaine, is injected into the skin and interspinous ligament. A 16 or 18 gauge Tuohy needle is then inserted into the interspinous ligament and a "loss of resistance to injection" technique is used to identify the epidural space. This technique works because the interspinous ligament is extremely dense, and injection into it is almost impossible. The anaesthetist advances the Tuohy needle slowly, attempting to inject through it every millimetre or so. Typically a "pop" is felt as the ligamentum flavum is breached. The epidural space contains only loose tissue and veins, which means that injection into it is very easy. The sensation of the "pop" followed by ease of injection is a strong indicator that the tip of the needle is in the epidural space.

Traditionally anaesthetists have used either air or saline for identifying the epidural space, depending on personal preference. However, evidence is accumulating that saline may result in more rapid and satisfactory quality of analgesia^[1].

After placement of the tip of the Tuohy needle into the epidural space a catheter is threaded through the needle. The needle is then removed. Generally the catheter is then withdrawn slightly so that 4-6 cm remains in the epidural space.

Most commonly, the anaesthetist conducting an epidural places the catheter in the mid-lumbar region, although occasionally a catheter is placed in the thoracic (chest) or cervical (neck) region. In adults, the spinal cord terminates at the first lumbar vertebra, below which lies the cauda equina ("horse's tail"). Hence lumbar epidurals carry a very low risk of injuring the spinal cord.

A common solution for epidural infusion in childbirth or for post-operative analgesia is 0.1 percent bupivacaine and 2 μg/mL of fentanyl. This solution is infused at a rate between 4 and 14 mL/hour, following a loading dose to initiate the block.

Typically, the effects of the epidural are noted below a specific dermatome. This level (the "block height") is chosen by the anaesthetist. The level is usually 3-4 dermatomes higher than the point of insertion. A very high insertion level may result in sparing of very low dermatomes. For example, a thoracic epidural may be performed for upper abdominal surgery, but may not have any effect on the perineum. However, giving very large volumes into the epidural space may spread the block both higher and lower.

In some unusual instances, it may not be required to insert a catheter into the epidural space, e.g. for steroid injections; see below. The anaesthetist may inject medication into the epidural space through the needle, then remove the needle.

Combined spinal-epidurals

For some procedures, the anaesthetist may choose to combine the rapid onset and reliable, dense block of a spinal anaesthetic with the post-operative analgesic effects of an epidural. This is called combined spinal and epidural anaesthesia (CSE).

The anaesthetist may insert the spinal anaesthetic at one level, and the epidural at an adjacent level. Alternatively, after locating the epidural space with the Tuohy needle, a spinal needle may be inserted through the Tuohy needle into the subarachnoid space. The spinal dose is then given, the spinal needle withdrawn, and the epidural catheter inserted as normal. This method, known as the "needle-through-needle" technique, may be associated with a slightly higher risk of placing the catheter into the subarachnoid space.

Caudal epidurals

The epidural space may be entered through the sacrococcygeal membrane, using a standard 21G needle. Injecting a volume of local anaesthetic here provides good analgesia of the perineum and genital areas. This is typically a single-injection technique and a catheter is not normally placed. This is known as a caudal epidural or "caudal".

The caudal epidural is an effective and safe analgesic technique in children undergoing pelvic or perineal surgery. It is usually combined with general anaesthesia.

Side effects

In addition to blocking the nerves which carry pain, local anaesthetic drugs in the epidural space will block other types of nerves as well, in a dose-dependent manner. Depending on the drug and dose used, the effects may last only a few minutes or up to several hours. This results in three main effects:

• Loss of other modalities of sensation (including touch, and proprioception)
• Loss of muscle power
• Loss of function of the sympathetic nervous system, which maintains blood pressure

Pain nerves (c-fibres) are most sensitive to the effects of the epidural. This means that a good epidural can provide analgesia without affecting muscle power or other types of sensation. The larger the dose used, the more likely it is that the side-effects will be problematic.

Very large doses of epidural anaesthetic can cause paralysis of the intercostal muscles and diaphragm, and complete loss of sympathetic function, even to the heart itself, causing a profound drop in heart rate and blood pressure. This requires emergency treatment, and usually general anaesthesia with intubation. This happens because the block height is too high ("high block") and the epidural is blocking the heart's own sympathetic nerves (the "cardiac accelerator fibres" from T1-3), as well as the phrenic nerves, which supply the diaphragm.

It is considered safe practice for all patients with epidurals to be confined to bed to prevent the risk of falls.

The loss of the sensation of needing to urinate may require the placement of a urinary catheter for the duration of the epidural.

Opioid drugs in the epidural space are safe as well as effective. However, very large doses may cause troublesome itch, and rarely, delayed respiratory depression.

Complications of epidural use

These include:

• Block failure (about 1 in 20). Partial failure may still give satisfactory pain relief. However, if pain relief is inadequate, another epidural may have to be performed.

For patients on low molecular weight heparin anti-coagulation, many departments advise timing the removal of the catheter to reduce the increased risk of epidural haemtoma, e.g. avoidance of insertion or removal within 4 hours of administration of anti-coagulation. There several case reports of epidural haematomas associated with neuraxial anaesthesia, but it is less clear if the anti-coagulation is causal.[2]

• Bloody tap (about 1 in 30-50). It is easy to injure an epidural vein with the needle. In patients who have normal blood clotting, it is extremely rare (e.g. 1 in 100,000) for problems to develop. However, in a patient who has a coagulopathy, the patient may be at risk of epidural haematoma. If blood comes back down the needle, the anaesthetist will normally site the epidural at another level.
• Accidental dural puncture (about 1 in 100 insertions). The epidural space in the adult lumbar spine is only 3-5mm deep, which means it is comparatively easy to cross it and accidentally puncture the dura (and arachnoid) with the needle. This may cause cerebrospinal fluid (CSF) to leak out into the epidural space, which may in turn cause a post dural puncture headache (PDPH). This can be severe and last several days, and in some cases weeks or months. It is caused by a reduction in CSF pressure and is characterised by exacerbation when the patient raises their head above the lying position. If severe it may be successfully treated with a "blood patch" (a small amount of the patient's own blood given into the epidural space via another epidural needle). Most cases resolve spontaneously with time.
• Catheter misplaced into a vein (uncommon, less than 1 in 300). Occasionally the catheter may be misplaced into an epidural vein, which results in all the anaesthetic being injected intravenously, where it can be toxic in large doses. This also results in block failure.
• High block, as described above (uncommon, less than 1 in 500).
• Catheter misplaced into the subarachnoid space (rare, less than 1 in 1000). If the catheter is accidentally misplaced into the subarachnoid space (e.g. after an unrecognised accidental dural puncture), normally cerebrospinal fluid can be freely aspirated from the catheter (which would usually prompt the anaesthetist to withdraw the catheter and resite it elsewhere). If, however, this is not recognised, large doses of anaesthetic may be delivered directly into the cerebrospinal fluid. This may result in a high block, or, more rarely, a total spinal, where anaesthetic is delivered directly to the brainstem, causing unconsciousness and sometimes seizures.
• Significant damage to a single nerve (very rare, less than 1:10,000).
• Epidural abscess formation (very rare, about 1 in 50,000-75,000). The risk increases greatly with catheters which are left in place longer than 72 hours.
• Paraplegia (extremely rare, less than 1:100,000).
• Death (extremely rare, less than 1:100,000).

Other concerns

Back pain is commonly reported by postpartum women, and an epidural is often blamed. However, it is now known that the majority of postpartum back pain is caused by abnormal posture and pushing during parturition, and the presence of an epidural is incidental.

There is an indisputible association between epidural analgesia in labour and the likelihood of requiring an instrumental delivery. Critics argue that epidurals cause women to require needless interventions at delivery. However, most anaesthetists agree that there is a subset of women who have a problem which is likely to lead to a longer and more painful labour, and that therefore these women are more likely to request epidural analgesia.

More recently, concerns have been raised that epidural fentanyl seems to be associated with difficulty in initiating and maintaining breastfeeding.

Contraindications

These are circumstances in which epidurals should not be used:

• Patient refusal
• Coagulopathy or anticoagulant medication (e.g. warfarin)
• Infection near the point of insertion
• Infection in the bloodstream which may "seed" onto the catheter
• Hypovolaemia (low circulating blood volume)

Cautions

There are circumstances where the risks of an epidural are higher than normal and must be weighed against the benefits. These circumstances include:

• Anatomical abnormalities, such as spina bifida, meningomyelocele or scoliosis
• Previous spinal surgery (which may cause an acquired tethered spinal cord)
• Certain problems of the central nervous system, including multiple sclerosis or syringomyelia
• Certain heart-valve problems (such as aortic stenosis)

Epidural steroid injection

An epidural injection, or epidural steroid injection, is used to help reduce pain caused by a prolapsed intervertebral disc, degenerative spinal disease, or spinal stenosis. These spinal disorders often affect the cervical and lumbar regions of the spine. Pain may be accompanied by numbness or tingling radiating into the arms or legs.

The medicine used in the injection is a combination of a local anesthetic (such as bupivacaine) and a steroid (such as triamcinolone).

Other reading

• Roberts C, Tracy S, Peat B,Rates for obstetric intervention among private and public patients in Australia: population based descriptive study, British Medical Journal (BMJ), v321:p137, 15 July 2000

• Jun Zhang, Michael K. Yancey, Mark A. Klebanoff, Jenifer Schwarz and Dina Schweitzer, Does epidural analgesia prolong labor and increase risk of cesarean delivery? A natural experiment, American Journal of Obstetrics and Gynecology, Volume 185, Issue 1, July 2001, Pages 128-134.[1]

• Barbara L. Leighton and Stephen H. Halpern, The effects of epidural analgesia on labor, maternal, and neonatal outcomes: A systematic review, American Journal of Obstetrics and Gynecology, Volume 186, Issue 5, Part 2, May 2002, Pages S69-S77. Also available online.

• Boqing Chen and Patrick M. Foye, UMDNJ: New Jersey Medical School, Epidural Steroid Injections: Non-surgical Treatment of Spine Pain, eMedicine: Physical Medicine and Rehabilitation (PM&R), August 2005. Also available online.