Epidural Anesthesia: Technique Needles
Rigidity and low internal resistance are desirable qualities in needles used for epidural anesthesia. The ski-like bevel of the Tuohy needle promotes greater deflection off its course compared with pencil point needles.1,2 Increasing the size of the Tuohy needle decreases the degree of deflection, thus larger needles (16-18 gauge) are recommended for epidural anesthesia. The low internal resistance of larger needles also better transmits changes in tissue pressure and facilitates the loss of resistance technique. Unnecessarily large needles, however, can potentially cause more patient discomfort during the procedure.
The needle bevel type may influence the incidence of accidental dural puncture. Bromage claims an irreducible incidence of dural puncture of 1.5% with straight bevel needles compared with 0.2% incidence with Tuohy needles with the Huber tip.3 Plunger Syringe
Although disposable plastic syringes designed for the loss of resistance technique have a higher resistance than all-glass syringes, their plungers are less likely to stick in the barrel.3 Sticking contributes to the incidence of accidental dural puncture. Use of an air-filled system with all-glass syringes may help prevent this complication. In an air-filled system, the barrel and plunger of the syringe are first wetted and made freely moving. A rapid tremolo movement of the thumb produces intermittent pressure on the plunger, alternately compressing and releasing the air in the syringe. This monitors for loss of resistance and prevents sticking of the plunger in the barrel.
Positioning
Epidural anesthesia is performed with the patient in either the lateral decubitus or sitting position. In parturients, the choice makes little difference in patient comfort or incidence of complications.4 In the lateral decubitus position, the patient lies with the knees drawn up to the abdomen, the upper arm resting across the chest, the lower arm lying at a right angle to the body, and the head flexed and resting on a small pillow.5 The vertebral column should rest at the edge of the bed and parallel to the table. Flexion of the lumbar spine opens the interspaces. When performing the procedure without an assistant to hold the patient in this position, one can ask the patient to clasp her hands behind her head and flex her back to make the elbows and knees touch.
Sitting Position
In the obese patient, the weight of subcutaneous tissue in the lateral position can pull the skin line marking the middle of the back some distance below the spinous processes.3 The sitting position may facilitate identification of the midline. The patient sits on the edge of the bed with the feet supported by a stool. The head is flexed on the chest and arms folded across the upper abdomen, or supported in front of the chest on a table or Mayo stand. An assistant stands in front of the patient to hold the shoulders level and prevent lateral flexion or rotation of the spine. The distance from the skin to the epidural space is less when epidural puncture is performed in the sitting compared with the lateral decubitus position.6 Outward migration of the catheter may occur when the the catheter is fixed to the skin and the distance to the epidural space is increased by a change from the sitting to the lateral position.
Landmarks
The intercristal line (the line between the highest points of the two iliac crests) runs through the spinous process of L4. The spinous processes identify the midline. Grasping the process transversely with the thumb and forefinger helps determine their width.3 The spinous processes immediately above and below the site of needle puncture are marked. The interspace immediately above or below the L4 process is the usual site for needle insertion. This avoids the termination of the spinal cord at the L1-L2 level.
VERTEBRAL LEVELS: LANDMARKS
spinous process
vertebra prominens
C7
root of the spine of the scapula
T3
inferior angle of the scapula
(arms at sides)
T7
intercristal line
L4
posterior superior iliac spines
S2
Structures Encountered During the Midline Approach
The needle should meet little resistance as it passes through skin and subcutaneous tissue. As it engages the tough supraspinous ligament, the tissue should support the needle perpendicular to the skin. With the loss of resistance technique, the thumb of the dominant hand places continuous pressure against the plunger of the syringe while the needle is advanced through the interspinous ligament with the nondominant hand. The anesthesiologist feels an increase in resistance and then complete obstruction to injection as the needle enters the ligamentum flavum, then a sudden loss of resistance as the needle bevel enters into the epidural space.
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