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Spinal anaesthesia (or spinal anesthesia), also called spinal analgesia, spinal block or sub-arachnoid block (SAB), is a form of regional anaesthesia involving injection of a local anaesthetic into the subarachnoid space, generally through a fine needle, usually 9 cm long (3.5 inches). For extremely obese patients longer needles are available (12.7 cm / 5 inches). The tip of the spinal needle has a point or small bevel. Recently, pencil point needles have been made available (Whitacre, Sprotte, Gertie Marx & others).
Epidural anesthesia is a technique whereby a local anesthetic drug is injected through a catheter placed into the epidural space. This technique has some similarity to spinal anesthesia, and the two techniques may be easily confused with each other. Differences include:
Bupivacaine (Marcaine) is the local anaesthetic most commonly used, although lidocaine (lignocaine), tetracaine, procaine, ropivacaine, levobupivicaine, prilocaine and cinchocaine may also be used. Commonly opioids are added to improve the block and provide post-operative pain relief, examples include morphine, fentanyl, diamorphine or buprenorphine. Non-opioids like clonidine may also be added to prolong the duration of analgesia (although Clonidine may cause hypotension). In the United Kingdom, since 2004 the National Institute for Clinical Excellence recommends that spinal anaesthesia for Caesarean section is supplemented with intrathecal diamorphine and this combination is now the modal form of anaesthesia for this indication in that country. The peculiar legal status of diamorphine (heroin) means that this cannot easily occur elsewhere.
Baricity refers to the density of a substance compared to the density of human cerebral spinal fluid. Baricity is used in anaesthesia to determine the manner in which a particular drug will spread in the intrathecal space. Usually, the hyperbaric, (for example, hyperbaric bupivacaine) is chosen, as its spread can be effectively and predictably controlled by the Anaesthesiologist or Nurse Anesthetist, by tilting the patient. Hyperbaric solutions are made more dense by adding glucose to the mixture.
Baricity is one factor that determines the spread of a spinal anaesthetic but the effect of adding a solute to a solvent, i.e. dissolution, also has an effect on the spread of the spinal anaesthetic. In tetracaine spinal anaesthesia, it was discovered that the rate of onset of analgesia was faster and the maximum level of analgesia was higher with a 10% glucose solution than with a 5% glucose spinal anaesthetic solution. Also, the amount of ephedrine required was less in the patients who received the 5% glucose solution.  In another study this time with 0.5% bupivacaine the mean maximum extent of sensory block was significantly higher with 8% glucose (T3.6) than with 0.83% glucose (T7.2) or 0.33% glucose (T9.5). Also the rate of onset of sensory block to T12 was fastest with solutions containing 8% glucose. 
Regardless of the anaesthetic 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 anaesthetic used and the properties of the axon. Thin unmyelinated C-fibres associated with pain are blocked first, while thick, heavily myelinated A-alpha motor neurons are blocked moderately. Heavily myelinated, small preganglionic sympathetic fibers are blocked first. 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 mechanoreceptors. 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 anaesthetic the surgery can be performed with the patient wide awake.
Spinal anaesthetics are typically limited to procedures involving most structures below the upper abdomen. To administer a spinal anaesthetic to higher levels may affect the ability to breathe by paralysing 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, injection of spinal anaesthesia higher than the level of L1 can cause damage to the spinal cord, and is therefore usually not done.
The first spinal analgesia was administered in 1885 by James Leonard Corning (1855–1923), a neurologist in New York. He was experimenting with cocaine on the spinal nerves of a dog when he accidentally pierced the dura mater.
The first planned spinal anaesthesia for surgery in man was administered by August Bier (1861–1949) on 16 August 1898, in Kiel, when he injected 3 ml of 0.5% cocaine solution into a 34-year-old labourer. After using it on 6 patients, he and his assistant each injected cocaine into the other's spine. They recommended it for surgeries of legs, but gave it up due to the toxicity of cocaine.
Current usage of this technique is waning in the developed world, with epidural analgesia or combined spinal-epidural anaesthesia emerging as the techniques of choice where the cost of the disposable 'kit' is not an issue.
However spinal analgesia is the mainstay of anaesthesia in countries like India, Pakistan and parts of Africa, excluding the major centres. Thousands of spinal anaesthetics are administered daily in hospitals and nursing homes. At a low cost, a surgery of up to two hours duration can be performed below the umbilicus of the patient.
Spinal anaesthesia is a commonly used anaesthetic technique, both alone and in combination with either sedation or general anaesthesia. Examples of uses include :-
Spinal anaesthesia is the technique of choice for Caesarean Section as it avoids a general anaesthetic and the risk of failed intubation (which is approximately 1 in 250 in pregnant women). It also means the mother is conscious and partner is able to be present at the birth of the child. The post operative analgesia from intrathecal opioids in addition to Non-Steroidal Anti-Inflammatory drugs is also good.
If surgery allows spinal anaesthesia is very useful in patients with severe respiratory disease e.g. COPD as it avoids intubation and ventilation. It may also be useful in patients where anatomical abnormalities may make endotracheal intubation very difficult.
Can be broadly classified as immediate (on the operating table) or late (in the ward or in the P.A.C.U. post-anaesthesia care unit):