Describe a technique for placing a spinal anesthesia

Describe a technique for placing a spinal anesthesia.

After placing the patient in either the lateral decubitus or sitting position, the skin is prepared with antiseptic solution and draped in a sterile fashion. Dural puncture can be performed anywhere along the spinal column, but the risk of spinal cord trauma can be minimized by inserting the spinal needle at a point below which the spinal cord termi-nates. In most patients, the spinal cord ends at the second lumbar vertebral body. Therefore, the spinal needle is inserted just below this, usually at the L₃–L₄ interlaminar space. The iliac crest, which usually lies at the level of the L₄ spinous process, provides an excellent landmark. A small intradermal wheal of local anesthetic is placed at the level of the selected interlaminar space using a 25-gauge needle. A 19-gauge, 1.5-inch introducer needle is placed through the anesthetized skin and into the interspinous ligament. The 25-gauge spinal needle is then inserted through the introducer needle. Twenty-two-gauge spinal needles do not require passage through an introducer needle. The nee-dle’s bevel should be aligned with the longitudinal fibers of the dura in an effort to separate the fibers rather than cut them. It is postulated that lacerating the fibers may retard closure of the dural hole, predisposing patients to headaches. Resistance to needle passage frequently results from encountering bone or calcified ligaments. The coop-erative patient can frequently help differentiate the two. Calcified ligaments do not usually hurt when the needle impinges upon them. Encountering bone is often painful owing to periosteal trauma.

The posterior aspect of the vertebral lamina is curved, so that the inferior portion is more superficial than the superior part. Therefore, if the spinal needle encounters bone superficially, it is probably abutting the inferior aspect of the vertebra above. If the needle encounters bone at a deeper location, then it is probably abutting the supe-rior aspect of the lamina below. The spinal needle is then advanced through the increased resistance of the ligamen-tum flavum, and a loss of resistance is often appreciated as the needle tip enters the epidural space. A characteristic “pop” is obtained as the needle pierces the dura and enters the subarachnoid space. The distance from the skin to the ligamentum flavum is usually between 3.5 and 5.0 cm.

The spinal needle stylet is removed and cerebrospinal fluid (CSF) allowed to escape by gravity drainage or with gentle aspiration. Insertion of the needle too far within the subarachnoid space may place its tip against the vertebral body or intervertebral disc, thereby preventing CSF flow through the needle. This problem is easily corrected by withdrawing the needle slightly and observing the hub for fluid. Flow should be obtained in all four quadrants to ensure proper placement of the bevel within the subarach-noid space. Levy et al. (1985) and Machikanti et al. (1987) have debated the importance of free-flowing CSF as a prognosticator of a successful block. When obtained, CSF should be clear and colorless. Bleeding that does not read-ily stop requires repositioning of the needle. The properly placed needle should be held securely while bracing your hand against the patient’s back. The syringe is attached to the spinal needle hub sufficiently well so that local anes-thetic cannot leak during injection. About 0.2 mL of CSF should be aspirated, observing the resulting change in optical density within the syringe, to confirm that you are still in the subarachnoid space. The local anesthetic is injected, and the syringe aspirated once again to confirm that the needle has not been dislodged from the sub-arachnoid space. The spinal needle is removed at this point. The patient can now be repositioned at any time.

Frequent blood pressure determinations are required to detect hypotension.