The epidural space is a potential space. The average distance to the epidural space is ~ 5 cm [Segal S et al. Reg Anesth 21: 451, 1996], but the range is up to 3-9 cm. The epidural space contains Batson’s venous plexus, which is dilated during pregnancy and thus more susceptible to needle injury. Tuffier’s line connects the iliac crests and normally crosses the L4 spinous process or the L4-5 interspace. Most epidurals are either placed at L3-4 or L2-3.
Both single and multiorifice catheters are available. Multiorifice catheters were designed to reduce the incidence of failed or unilateral blockade, however the literature on single versus multiorifice is not definitive – Dickson et al. studied 364 total patients and found no difference [Dickson MA Br J Anaesth 79: 297, 1997], whereas Segal et al. studied 872 patients and found that multiorifice catheters lowered the incidence of both unilateral blocks and replaced catheters [Segal S et al. J Clin Anesth 9: 109, 1997]
Traditionally practitioners have been taught to “prehydrate” these patients with volume, although this practice has recently been questioned. A study of 140 patients undergoing spinal anesthesia showed that while 20 cc/kg affect the incidence of hypotension, it did not impact the severity [Rout CC et al. Anesthesiology 79: 262, 1993]
Identification of Midline
In obese patients, the midline may be difficult to locate. Ultrasonography may assist in locating the midline. Alternatively, the needle used to inject local anesthetic can be used as a “seeker” – when in the midline, when the needle is fully advanced it should be difficult to inject local (because the interspinous ligament is very rigid). If local is easy to inject, consider redirecting the needle left or right and attempting to find an area in which it is very difficult to inject the local. In this way, the true midline can be identified.
Sitting versus Lying Down
Note that while placement in the sitting position may be easier for the practitioner, it is more stressful for the patient and her baby. Furthermore, note that the catheter is drawn inward when parturients lie down (especially in obese patients), thus it is recommended that the catheter is not taped until the patient is on her side (otherwise it is possible that the tip of the catheter could be pulled back).
There is considerable controversy regarding the efficacy of the test dose. The purpose of the test dose is to detect intravascular injections as well as subarachnoid injections (the latter of which more frequently occur in obstetric anesthesia closed claims databases). A variety of techniques can be used for test dosing – 15 ucg of epinephrine (subsequently questioned for having a false positive rate as high as 45% [Leighton BL and Norris MC Anesth Analg 76: 1174, 1993]). Others have used and air/Doppler approach to rule out intravascular injection [Leighton BL and Norris MC Anesth Analg 76: 1174, 1993]], and still others advocate the “every dose is a test dose” technique by which the patient is examined for symptoms of intravascular and/or subarachnoid injection after every 3-5 cc administered.