Activating an Epidural
The quantity (volume and concentration) of
local anesthetic needed for epidural anesthesia is larger than that needed for
spinal anesthesia. Toxic side effects are likely if a “full epidural dose” is
injected intrathecally or intravascularly. Safeguards against toxic epidural
side effects include test and incremental dosing. These safeguards apply
whether the injection is through the needle or an epidural catheter.
test dose is designed to detect both sub-arachnoid and intravascular injection. The classictest dose combines local anesthetic and epinephrine, typically
3 mL of 1.5% lidocaine with 1:200,000 epi-nephrine (0.005 mg/mL). The 45 mg of
lidocaine, if injected intrathecally, will produce spinal anesthesia that
should be rapidly apparent. Some clinicians have suggested the use of lower
doses of local anesthetic, as an unintended injection of 45 mg of intrathecal
lidocaine can be difficult to manage in areas such as labor rooms. The 15 mcg
dose of epinephrine, if injected intravascularly, should produce a noticeable
increase in heart rate (20% or more), with or with-out hypertension.
Unfortunately, epinephrine as a marker of intravenous injection is not ideal.
False positives (a uterine contraction causing pain or an increase in heart
rate coincident to test dosing) and false negatives (bradycardia and exaggerated
hyper-tension in response to epinephrine in patients tak-ing β-blockers) can occur. Simply aspirating prior
to injection is insufficient to avoid inadvertent intrave-nous injection; most
experienced practitioners have encountered false-negative aspirations through
both a needle and a catheter.
Incremental dosing is a very effective method
of avoiding serious complications. If aspiration is nega-tive, a fraction of
the total intended local anesthetic dose is injected, typically 5 mL. This dose
should be large enough for mild symptoms of intravascular injection to occur,
but small enough to avoid seizure or cardiovascular compromise. This is
particularly important for labor epidurals that are to be used for cesarean
section. If the initial labor epidural bolus was delivered through the needle,
and the catheter was then inserted, it may be erroneously assumed that the
catheter is well positioned because the patient is still comfortable from the
initial bolus. If the catheter was inserted into a blood vessel, or after
initial successful placement, has since migrated intravascularly, systemic
toxicity will likely result if the full anesthetic dose is injected. Catheters
can migrate intrathecally or intravascularly from an initially correct epidural
position at any time after placement. Some cases of “catheter migration” may
represent delayed recognition of an improperly positioned catheter.
If a clinician uses an initial test dose, is
diligent about aspirating prior to each injection, and always uses incremental
dosing, major systemic toxic side effects and/or total spinal anesthesia from
accidental intrathecal injections will be rare. Rescue lipid emul-sion (20%
Intralipid 1.5 mL/kg) should be available whenever epidural blocks are
performed, in the event of local anesthetic toxicity.