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Any and every injection into a presumed epidural catheter may unexpectedly be administered intravascularly or intrathecally. Local anesthetic should always be administered incrementally with aspiration before each dose!1
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Every Dose Is A Test Dose. Intravascular injection of local anesthetics can produce local anesthetic systemic toxicity (LAST)—a life-threatening condition that may result in seizures, cardiac arrhythmias, and cardiovascular collapse (Chapter 61, “Local Anesthetic Systemic Toxicity”).1 Inadvertent intrathecal injection of large doses of local anesthetics can lead to a high or total spinal anesthesia, which presents as paralysis, loss of consciousness, respiratory arrest, hypotension, bradycardia, and possible cardiac arrest (Chapter 60, “High Spinal”).2 The purpose of the test dose is to identify intravascular or intrathecal catheters using a small, safe dose of epinephrine-containing local anesthetic in order to prevent the consequences of a larger dose in the intravascular or intrathecal injection.3 The classic test dose containing 3 mL of 1.5% lidocaine (45 mg) with 1:200,000 epinephrine (15 µg) was introduced by Moore and Batra in 1981.4
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To identify intravascular and intrathecal catheters
To prevent serious side effects
To minimize false-positive results that lead to unnecessary catheter replacement
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The incidence of unintended intravascular catheter in the obstetric population is 4.9% to 7% with a stiff plastic epidural catheter but may be as low as <1% with a soft, flexible epidural catheter.3,5 The incidence of intravascular injection undetected by aspiration is as high as 25% for single-orifice catheters and about 5% for multiorifice catheters. The incidence of unintended intrathecal catheter is 0.6% to 1.6%.3 The incidence of subarachnoid injection after negative aspiration is between 0.06% and 0.0008%.3
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A positive test dose is defined as:
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Heart rate increase >10 beats per minute at 25 to 30 seconds after the injection of 10 or 15 µg epinephrine.
Observation of both a metallic taste and tinnitus after the injection of at least 100 mg lidocaine.
Warm or heavy sensation in the lower extremities at 3 minutes, or inability to raise legs at 4 to 10 minutes after injection of 30 or 45 mg lidocaine.3
The sensitivity of a test dose is reduced in the parturient in labor or who is on β-blockers.
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The intravenous or epidural injection of epinephrine has the potential to cause vasoconstriction resulting in decreased uteroplacental blood flow, which may lead to fetal bradycardia.6 If the epidural catheter is intravascular, intravenous injection of epinephrine may induce maternal tachyarrhythmias.6 If the epidural catheter is intrathecal, total spinal block and respiratory paralysis are extremely rare but possible with the relatively low dose of lidocaine utilized for a test dose.6,7 Subarachnoid injection of lidocaine may also produce sympathectomy and maternal hypotension leading to ...