In the lateral decubitus position, the patient lies with the operative side down when hyperbaric solutions are being used. The patient's shoulders and hips are positioned perpendicular to the bed (preventing rotation of the spine), the knees are drawn up to the chest, the neck is flexed, and the patient is asked to actively curve the back outward (which spreads apart the spinous processes).
Using the iliac crests as landmarks, the L2 to L3, L3 to L4, and L4 to L5 interspaces are identified and the desired interspace chosen.
All antiseptic solutions are neurotoxic, and care must be taken not to contaminate spinal needles and local anesthetics. Chlorhexidine–alcohol antiseptic prevents colonization of percutaneous catheters better than 10% povidone–iodine and is the recommended prep for skin asepsis before regional anesthesia procedures.
Midline Approach
After infiltration of the selected needle insertion site with local anesthetic solution, the needle is advanced (subcutaneous tissue to supraspinous ligament to interspinous ligament to ligamentum flavum to epidural space to dura mater [“pop”] to arachnoid mater) until CSF is obtained (gentle aspiration may be helpful). The spinal meninges are typically at a depth of 4 to 6 cm.
If the patient experiences paresthesia (which should be differentiated from discomfort caused by contacting bone), it is important to immediately stop advancing the needle and determine whether the needle tip has encountered a nerve root in the epidural space or in the subarachnoid space. (The presence of CSF confirms that the needle has encountered a cauda equina nerve root.)
After completing the injection of local anesthetic solution, a small volume of CSF is again aspirated to confirm that the needle tip has remained in the subarachnoid space.
After the block has been placed, strict attention must be directed to the patient's hemodynamic status with blood pressure and heart rate supported as necessary.
The level of anesthesia should be assessed by pinprick or temperature sensation. If the anesthesia is not rising high enough, the table may be tilted to influence spread of a hyperbaric or hypobaric local anesthetic.
The paramedian approach is used when the patient cannot flex the spine or heavily calcified interspinous ligaments. The needle is inserted 1 cm lateral to the desired interspace with advancement toward the midline. (The first significant resistance is the ligamentum flavum as the interspinous ligament is bypassed.)
The lumbosacral approach is a paramedian approach directed at the L5 to S1 interspace.