The Relationship Between Needle Position and Injection Pressure During Femoral Nerve Block

Single center, observational study. Patients scheduled for elective lower limb surgery with femoral nerve block will be recruited. Femoral nerve will be identified using a linear ultrasound transducer. A 22 GA 5 cm nerve block needle will be inserted with bevel downward and advanced to the following conditions:

• Needle tip slightly indenting the fascia iliaca lateral to the femoral nerve
• Needle tip advanced through fascia iliaca
• Needle tip slightly indenting the anterior surface of the femoral nerve
• Needle tip withdrawn 1 mm from nerve.

At each of these conditions, 1 ml of dextrose solution will be injected via an automated pump at 10 ml/min and the spread of injectate observed sonographically. Simultaneously, a blinded observer will measure opening injection pressure using both an electronic and a mechanical transducer. If opening pressure reaches 15 psi, this investigator will halt the injection. In addition, minimum threshold current required to elicit a motor response will be recorded for conditions 3 and 4. Patients will be contacted at 7 days and asked about any adverse effects.

Inclusion criteria

• Age 18-65
• ASA physical status I-III
• scheduled for lower limb surgery where femoral block is part of intended analgesic plan

Exclusion criteria

• Contraindication to femoral nerve block (e.g. infection)
• BMI > 35 kg/m2
• inability to communicate postoperative symptoms
• pre-existing neurologic deficits in the operative extremity
• allergy to local anesthetics
• history of opioid dependence