Chapter 62. Monitoring, Documentation, and Consent for Regional Anesthesia Procedures
What amount of epinephrine is best used during test dosing to see a reliable increase in systolic blood pressure by more than 15 mm Hg?
C is correct. 10 mcg. Intravenous injection of 10–15 mcg of epinephrine reliably increases the systolic blood pressure greater than 15 mm Hg even in the presence of beta blockade and sedation. This has become the standard dose for detection of intravascular injectate in regional anesthetic procedures.
A and B are incorrect. Though a typical concentration of epinephrine used is 2.5–5 mcg/mL (1:200,000 and 1:400,000, respectively), an amount of 2.5–5 mcg of epinephrine is insufficient to produce a reliable blood pressure increase.
D is incorrect. 50 mcg of epinephrine would certainly result in a hemodynamic response in most patients. However, in the concentrations typically used (2.5–5 mcg/mL) this would mean administering 10–20 mL of local anesthetic to reach a dose of 50 mcg. This essentially negates the purpose of the test dose, which is to minimize the volume accidentally injected into the vasculature. Increasing the concentration of epinephrine in the block solution above 2.5–5 mcg/mL is not recommended as there is a risk of vasoconstriction and vascular compromise to the nerve and adjacent tissues.
When utilizing neurostimulation for placement of a peripheral nerve block, an evoked motor response (EMR) below what value suggests intraneural or needle-nerve contact?
B is correct. Numerous animal and human studies have suggested that 0.2 mA is a current intensity threshold below which intimate needle-nerve contact can be expected. When an evoked motor response is present at less than 0.2 mA, there is a high likelihood of intraneural needle-tip placement and intraneural inflammation after injection.
A is incorrect. Voelckel et al examined sciatic nerve blocks with evoked motor response < 0.2 mA and at 0.3–0.5 mA. A motor response at a current of < 0.2 mA was always either intraneural or very close to the epineurium. This was further supported by Wiesmann et al, who demonstrated similar findings in a brachial plexus model.
C and D are incorrect. The 0.3–0.5 mA cohort in the Voelckel et al study showed no evidence of nerve tissue inflammation.