• Jeff Gadsden, Malikah Latmore, D Matt Levine, and Allegra Robinson.
• From the *Department of Anesthesiology, Duke University, Durham, NC; †Department of Anesthesiology, St Luke's-Roosevelt Hospital, New York, NY; ‡Wellington Regional Hospital, Wellington, New Zealand; and §New York University, New York, NY.
• Reg Anesth Pain Med. 2016 Jan 1; 41 (1): 50-5.

Background And ObjectivesHigh opening injection pressures (OIPs) have been shown to predict sustained needle tip contact with the roots of the brachial plexus. Such roots have a uniquely high ratio of fascicular versus connective tissue. It is unknown if this relationship is preserved during multifascicular nerve blockade. We hypothesized that OIP can predict needle-nerve contact during femoral nerve block, as well as detect needle contact with the fascia iliaca.MethodsTwenty adults scheduled for femoral block were recruited. Using ultrasound, a 22-gauge needle was sequentially placed in 4 locations: indenting the fascia iliaca, advanced through the fascia iliaca while lateral to the nerve, slightly indenting the femoral nerve, and withdrawn from the nerve 1 mm. At each location, the OIP required to initiate an injection of 1 mL D5W (5% dextrose in water) at 10 mL/min was recorded. Blinded investigators performed evaluations and aborted injections when an OIP of 15 psi was reached.ResultsOpening injection pressure was 15 psi or greater for 90% and 100% of cases when the needle indented the femoral nerve and fascia iliaca, respectively. Opening injection pressure was less than 15 psi for all 20 patients when the needle was withdrawn 1 mm from the nerve as well as at the subfascial position (McNemar χ2 P < 0.001).ConclusionsOpening injection pressure greater than 15 psi was associated with a block needle tip position slightly indenting the epineurium of the femoral nerve (90%) and the fascia iliaca (100%). Needle tip positions not indenting these structures were associated with OIP of less than 15 psi (100%).

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