Submitted by Stacey Kast RN,BSN
Spinal anesthesia, also called spinal analgesia, sub-arachnoid block (SAB) or intrathecal, is a form of regional anesthesia involving an injection of a local anesthetic into the cerebral spinal fluid with a fine needle. A spinal block is a type of short anesthetic treatment that may completely reduce sensation from the point of injection in the spine, at just above the hips, down to the feet. People use spinal blocks as a means of reducing pain for chronic medical conditions or lower back injuries.
Bupivacaine (Marcaine) is the local anesthetic most commonly used; (lidocaine), tetracaine, procaine, ropivacaine, levobupivicaine and cinchocaine are also available. Sometimes a vasoconstrictor such as epinephrine is added to the local anesthetic to prolong its duration. Of late, many anesthesiologists are preferring to add opioids like morphine, fentanyl or buprenorphine, or non-opioids like clonidine, to the local anesthetic used in spinal, to give a smoother 'effect' and to provide prolonged pain relief once the action of the 'spinal' has worn off.
Regardless of the anesthetic agent (drug) used, the desired effect is to block the transmission of afferent nerve signals from peripheral nociceptors. Sensory signals from the site are blocked, thereby eliminating pain. The degree of neuronal blockade depends on the amount and concentration of local anesthetic used, and the properties of the axon. Thin unmylenated C-fibres associated with pain are blocked first, while thick, heavily mylenated A-alpha motor neurons are blocked last. The desired result is total numbness of the area. A pressure sensation is permissible and often occurs due to incomplete blockade of the thicker A-beta mechnorecptors. This allows surgical procedures to be performed with no painful sensation to the person undergoing the procedure.
Some sedation is sometimes provided to help the patient relax and pass the time during the procedure, but with a successful spinal anesthetic the surgery can be performed with the patient wide awake. Spinal anesthetics are limited to procedures involving most structures below the upper abdomen. To administer a spinal anesthetic to higher levels may affect the ability to breathe by paralyzing the intercostal respiratory muscles, or even the diaphragm in extreme cases (called a "high spinal", or a "total spinal", with which consciousness is lost), as well as the body's ability to control the heart rate via the cardiac accelerator fibres. Also, administration of spinal anesthesia higher than the level of L1 can cause damage to the spinal cord, and is therefore usually not done.
Figure 1: Spinal Space
Can be broadly classified as immediate (on the operating table) or late (in the ward or in the P.A.C.U. post-anesthesia care unit):
*As for risk factors, spinal block and epidural share some features. In both procedures there is risk for infection and chance that the dura will pierced, which can result in development of headaches that may come and go for months. Generally, greater potential problems result from an epidural including allergic reaction, though this may also occur to any anesthetic in a spinal block, back troubles, damage to nerves, and occasionally fever.
The term epidural is often short for epidural anesthesia, a form of regional anesthesia involving injection of drugs through a catheter placed into the epidural space. The injection can cause both a loss of sensation (anesthesia) and a loss of pain (analgesia), by blocking the transmission of signals through nerves in or near the spinal cord. The epidural space is the space inside the bony spinal canal but outside the membrane called the dura mater (sometimes called the "dura"). In contact with the inner surface of the dura is another membrane called the arachnoid mater ("arachnoid"). The arachnoid encompasses the cerebrospinal fluid that surrounds the spinal cord.
A patient receiving an epidural for pain relief typically receives a combination of local anesthetics and opioids. This combination works better than either type of drug used alone. Common local anesthetics include lidocaine, bupivacaine, ropivacaine, and chloroprocaine. Common opioids include morphine, fentanyl, sufentanil, and meperidine. These are injected in relatively small doses. Occasionally other agents may be used, such as clonidine or ketamine. For a short procedure, the anesthetist may introduce a single dose of medication (the "bolus" technique). This will eventually wear off. Thereafter, the anesthetist may repeat the bolus provided the catheter remains undisturbed. For a prolonged effect, a continuous infusion of drugs may be employed.
v A common solution for epidural infusion in childbirth or for post-operative analgesia is 0.2% ropivacaine or 0.125% bupivacaine, with 2 μg/mL of fentanyl added. This solution is infused at a rate between 4 and 14 mL/hour, following a loading dose to initiate the nerve block.
Figure 2: Epidural Space
Block height and intensity:
Typically, the effects of the epidural are noted below a specific level on the body (dermatome). This level (the "block height") is chosen by the anesthetist. 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 (area around the genitals) or bladder. 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 anesthetic drugs used. For example, 15 ml 0.1% bupivacaine may provide good analgesia for a woman in labor, but would likely be insufficient for surgery. Conversely, 15 ml of 0.5% bupivacaine would provide a more intense block, likely sufficient for surgery. Since the volume used in each case is the same, the spread of drug, and hence the block height, is likely to be similar.
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 a continuous infusion can be maintained for several days, if needed. Epidural analgesia may be used:
The epidural space is more difficult and risky to access as one ascends the spine, so epidural techniques are most suitable for analgesia for the chest, abdomen, pelvis or legs. They are (usually) much less suitable for analgesia for the neck, or arms and are not possible for the head (since sensory innervation for the head arises directly from the brain via cranial nerves rather than from the spinal cord via the epidural space.)
For a short procedure, the anesthetist may introduce a single dose of medication (the "bolus" technique). This will eventually wear off. Thereafter, the anesthetist may repeat the bolus provided the catheter remains undisturbed. For a prolonged effect, a continuous infusion of drugs may be employed. A common solution for epidural infusion in childbirth or for post-operative analgesia is 0.2% ropivacaine or 0.125% bupivacaine, with 2 μg/mL of fentanyl added. This solution is infused at a rate between 4 and 14 mL/hour, following a loading dose to initiate the nerve block.
There are circumstances where the risks of an epidural are higher than normal. These
the anesthetic may impair blood supply to the thickened heart muscle.)
Circumstances in which epidurals should not be used:
Epidural analgesia has been demonstrated to have several benefits after surgery. These include:
*Despite these benefits, no survival benefit has been proven for high-risk patients.
In addition to blocking the nerves which carry pain, local anesthetic 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. Epidural typically involves using the opiates fentanyl or sufentanil, with bupivacaine, Fentanyl is a powerful opiate with potency and side effects 80X that of morphine. Sufentanil is another opiate, 5 to 10Xs more potent than Fentanyl. Bupivacaine is markedly toxic, causing excitation: nervousness, tingling around the mouth, tinnitus, tremor, dizziness, blurred vision, or seizures, followed by depression: drowsiness, loss of consciousness, respiratory depression and apnea. Bupivacaine has caused several deaths by cardiac arrest when epidural anesthetic has been accidentally inserted into vein instead of epidural space in the spine. Epidural correctly administered results in three main effects:
Pain nerves 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.
v For example, a laboring woman may have a continuous epidural during labor that in 85% of cases provides good analgesia without impairing her ability to move around in bed. If she requires a Caesarean section, she is given a larger dose of epidural bupivacaine. After a few minutes, she can no longer move her legs, or feel her abdomen. If her blood pressure drops below 80/50 she is given an intravenous bolus of ephedrine or phenylephrine infusion to compensate. During the operation, she feels no pain.
Note: Very large doses of epidural anesthetic can cause paralysis of the intercostal muscles and diaphragm (which are responsible for breathing), and loss of sympathetic function to the heart itself, causing a profound drop in heart rate and blood pressure. This requires emergency treatment, and in severe cases may require airway support. This happens because the epidural is blocking the heart's sympathetic nerves, 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 sensation of needing to urinate is diminished, which often requires the placement of a urinary catheter for the duration of the epidural
· Opioid drugs in the epidural space are relatively safe (as well as effective). However, very large doses may cause troublesome itch, and rarely, delayed respiratory depression.
Complications: * related to epidurals in healthy individuals.
The following factors are associated with no pain relief, or block failure with epidurals:
• history of a previous failure of epidural anesthesia
• cervical dilation of more than 7 cm at insertion
• being a regular opiate user.
Epidural slows down labor significantly- The following are accepted explanations for why this happens:
The principal difference between spinal block and epidural is fairly easily explained. The spinal block is a single shot and a one-time administration of anesthetic medication. The epidural places a direct line into the spine through which medication can be fed. Amounts given can be less or more depending on need and time elements of a procedure. In contrast, the spinal block medicine will work as long as it lasts, which is about one to two hours. Should more medication be required, another block would be necessary. Generally the epidural is favored for any surgical or medical procedures that might exceed a couple of hours in length. Spinal blocks may be preferable as temporary relief or during surgeries that are very short in duration.
Figure 3: Spinal versus Epidural Block
http://www.wikipedia.org/wiki/Spinal-anaesthesia (Modified 17June 2010)
http://en.wikipedia.org/wiki/Epidural (Modified 4 June 2010)
Jonas W, Johansson LM, Nissen E, et al. Effects of Intrapartum Oxytocin Administration and Epidural Analgesia on the Concentration of Plasma Oxytocin and Prolactin, in Response to Suckling During the Second Day Postpartum. Breastfeed Med. 2009;4(2):71-82.