Epidural anesthesia: test dose

Which of the following is NOT TRUE of the use of 3milliliters of lidocaine 1.5% with 1:200K epinephrine as a test dose for epidural placement?

A. This dose is equivalent to 45mg of lidocaine, which when injected intrathecally, would result in a dense motor and sensory block within 10 minutes of injection.
B. Epinephrine is for intravascular detection, and the concentration is 10mcg/ml (total dose of 30mcg), resulting in a heart rate increase of 10 beats per minute and systolic pressure increase of 30mmHg if intravascular.
C. The effects of the test dose on detection of intravascular needle or catheter placement are blunted in elderly patients, patients under general anesthesia, and patients on beta-blockade.
D. The use of benzodiazepines for sedation would make signs of systemic local anesthetic such as circumoral numbness or tinnitus less sensitive of a marker for intravascular injection.

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Correct Answer: B

Explanation:

Lidocaine is a reasonably effective local anesthetic to detect intrathecal catheter placement with less risk of systemic toxicity when accidentally injected intravascularly. The concentration of local anesthetic is usually determined by multiplying the % by 10 to determine the concentration in mg/ml. A 45-60mg dose of lidocaine provides effective detection of intrathecal injection within 10 minutes. There have been cases of high spinals associated with this dose. The epinephrine concentration of a 1:200K solution is 5mcg/ml. Therefore, 3 ml of this solution is 15mcg, and should usually result in a heart rate increase of 10bpm and a systolic blood pressure increase of 30mmHg in awake adult patients if given intravascularly. However, this dose is less sensitive in elderly patients, patients under general anesthesia, and patients on beta-blockade.

References:

Tanaka M. “Epidural test dose: isoproterenol is a reliable marker for intravascular injection in anesthetized adults.” Anesth Analg 1996;82:1056-9.

Liu SS, Carpenter RL. “Hemodynamic responses to intravascular injection of epinephrine-containing epidural test doses in adults during general anesthesia.” Anesthesiology 1996;84(1): 81-7

Schoenwald PK, Whalley DG, Schluchter MD, Gottlieb A, Ryckman JV, Bedocx NM. “The hemodynamic responses to an intravenous test dose in vascular surgical patients.” Anesth Analg 1995;80:864-8.

Moore JM, Liu SS, Neal JM. “Premedication with Fentanyl and Midazolam decreases the reliability of intravenous lidocaine test dose.” Anesth Analg 1998;86:1015-7

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