Spinal anaesthesia (or spinal anesthesia), also called spinal analgesia^{[citation needed]} 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, some anaesthesiologists prefer spinal needles which are 12.7 cm long (5 inches). The tip of the spinal needle has a point or small bevel. Recently, pencil point needles have been made available (Whitacre, Sprotte, & others).

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仙骨硬膜外麻酔　caudal anesthesia




Epidural anaesthesia is a technique whereby a local anaesthetic drug is injected through a catheter placed into the epidural space.This technique has some similarity to spinal anaesthesia, and the two techniques may be easily confused with each other. Differences include:

• The involved space is larger for an epidural, and subsequently the
  injected dose is larger, being about 10–20 mL in epidural anaesthesia
  compared to 1.5–3.5 mL in a spinal.
  
• In an epidural, an indwelling catheter may be placed that avails for
  additional injections later, while a spinal is almost always a one-shot
  only.
  
• The onset of analgesia is approximately 15–30 minutes in an epidural, while it is approximately 5 minutes in a spinal.
  
• An epidural often does not cause as significant neuromuscular block unless specific local anaesthetics are used which block motor fibres as
  readily as sensory nerve fibres, while a spinal more often does.
  
• An epidural may be given at a cervical, thoracic, or lumbar site, while a spinal must be injected below L2 to avoid piercing the spinal cord.
  

Mechanism

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 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 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.
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