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.
A 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.