Ans. d (Subarachnoid space). (Ref. Harrisons, Medicine, 18th/735)SPINAL ANESTHESIA# The term "spinal anesthesia" was coined in 1885 by Leonard Corning.Technique of spinal anesthesia:# In spinal anaesthesia the spinal needle is pierced upto subarachnoid space where the anaesthetic agent is injected to produce the anaesthesia.# The selected level should be below LI in an adult and L3 in a child to avoid needle trauma to the spinal cord. As an anatomic landmark, the L3-L4 interspace is located at the line intersecting the top of the iliac crests. Either a midline or paramedian approach can be used.# The anatomic layers passed through include skin, subcutaneous structures, supraspinous ligament, interspinous ligament, ligamentum flavum, dura mater, and arachnoid membrane.# Once the needle tip is believed to be in the subarachnoid space, the stylet is removed to see if CSF appears at the needle hub. With small diameter needles (26 to 29 gauge) this generally requires 5 to 10 seconds, but may require >1 minute in some patients. Gentle aspiration may speed the appearance of CSF. If CSF does not appear, the needle orifice may be obstructed by a nerve root and rotating the needle 90 degrees may result in CSF flow. Alternatively, the needle orifice may not be completely in the subarachnoid space and advancing an additional 1 to 2 mm may result in brisk CSF flow. This is particularly true of pencil-point needles, which have their orifice on the side of the needle shaft proximal to the needle tip.# Finally, failure to obtain CSF suggests that the needle orifice is not in the subarachnoid space and the needle should be reinserted.# Common complications include hypotension, bradycardia, increased sensitivity to sedative medications, nausea and vomiting (possibly secondary to hypotension), postdural puncture headache, nerve injury, total spinal, and hematoma/abscess formation at the site of puncture.# Total spinal anesthesia results from local anesthetic depression of the cervical spinal cord and brain stem. Signs and symptoms include dysphonia, dyspnea, upper extremity weakness, loss of consciousness, pupillary dilation, hypotension, bradycardia, and cardiopulmonary arrest. Eaily recognition is the key to management. Treatment includes securing the airway, mechanical ventilation, volume infusion, and pressor support.# Absolute contraindications include local infection at the puncture site, bacteremia, severe hypovolemia, coagulopathy, severe stenotic valvular disease, infection at the site of the procedure, and intracranial hypertension. Relative contraindications include progressive degenerative (demyelinating) neurologic disease (multiple sclerosis), low back pain, and sepsis.# Intrathecal opioids:- Opioids produce intense visceral analgesia and may prolong sensory blockade without affecting motor or sympathetic function.- Fentanyl and sufentanil have a rapid onset of action and an effective duration greater than 6 hours.- Morphine lasts 6-24 hours.- Side effects include respiratory depression (which may occur late with hydrophilic agents), nausea, vomiting, pruritus, and urinary retention.