This is a research study to determine if the concentration of local anesthetic through a catheter next to the nerves that go to the hip that is undergoing surgery, affects muscle strength and sense of touch experienced after surgery. This study is looking at the varying concentrations of local anesthetic placed through the catheter.

These results will help define the optimal concentration of local anesthetic used for continuous peripheral nerve blocks and help guide future research in this clinically-relevant area.

This investigation will be a randomized, observer-masked, controlled, parallel-arm, human-subjects clinical trial.

Enrollment. Subjects will be patients undergoing hip arthroplasty. Study inclusion will be proposed to eligible patients by the orthopedic surgery or anesthesia services or research coordinator/assistant within four weeks prior to surgery. If a patient desires study participation, written, informed consent will be obtained. Selection for inclusion will not be based on gender, race, or socioeconomic status. The study population of interest includes men and women of all races and socioeconomic status.

Preoperative Management. Prior to surgery but following written, informed consent, subjects will have baseline endpoints measured. Patients will then have a perineural catheter placed on the operative side using standard techniques currently used at UCSD, and previously described. A 15 mL bolus of 2% mepivacaine with epinephrine (5 µg/mL) will be injected through the catheter to demonstrate correct catheter placement and provide intraoperative analgesia. Patients with a misplaced catheter indicated by a lack of sensory changes in the surgical extremity within 15 min will have their catheter replaced or be withdrawn from the study prior to randomization.

The remaining patients will then be randomized to one of two treatment groups: a postoperative ropivacaine concentration of 0.1% or 0.4% ("study infusate"). Randomization will be based on computer-generated codes that will be created and maintained by the Investigational Drug Service. Randomization will be in blocks of four, and stratified by hip arthroplasty procedure (either total or resurfacing). A portable electronic infusion pump with study infusate will be attached to the perineural catheter. The basal rate and patient-controlled bolus volume will depend upon the treatment group (note that the basal rate and bolus volume differ for each concentration, but the total dose of local anesthetic is the same for each):

Ropivacaine Concentration 0.1%: Basal Rate (12mL/h); Basal Dose (12 mg/h); Bolus Volume (4 mL); Bolus Dose (4 mg); Lockout Duration (30 min); Maximum Dose (20 mg/h)

Ropivacaine Concentration 0.4%: Basal Rate (3 mL/h); Basal Dose (12 mg/h); Bolus Volume (1 mL); Bolus Dose (4 mg); Lockout Duration (30 min); Maximum Dose (20 mg/h)

Intraoperative Management. Patients will receive a standardized general anesthetic with an inhaled anesthetic in N2O and O2. These gasses will be titrated for a Bispectral Index of 40-60 in order to provide adequate anesthesia while minimizing postoperative recovery duration. Esmolol and hydralazine will be used to provide hemodynamic stability, and opioids administered if necessary (fentanyl in 25 µg increments). The ropivacaine infusion provided by the Investigational Drug Service will be initiated using an infusion pump attached to the perineural catheter. Just prior to emergence, IV morphine will be titrated for a respiratory rate of 12-14. Upon emergence, patients will be taken to the recovery room and then to the surgical ward.

Postoperative Pain Management. For the duration of the study, all patients will receive the current usual and customary analgesics for hip arthroplasty patients at Hillcrest and Thornton hospitals. Perineural infusions will be administered per standard UCSD routine: continued administration until recommended discontinuation by the surgical service at which time the catheter is removed by the acute pain service.

Outcome Measurements. We have selected measures that have established reliability and validity. Staff blinded to treatment group assignment will perform all measures and assessments. Preoperative measurements (performed prior to perineural catheter placement the day of surgery): strength of the quadriceps femoris, hip adductors, and hip flexors; and sensory level (measurement descriptions below). Postoperative measurements will be performed the day following surgery as close to 8:00-9:00 and 12:30-13:30 as possible (logistics occasionally preclude therapist evaluation at these exact hours).

Inclusion Criteria:

• Primary, unilateral hip arthroplasty
• age greater than or equal to 18 years
• postoperative analgesic plan includes perineural local anesthetic infusion

Exclusion Criteria:

• morbid obesity as defined by a body mass index greater than 40
• chronic high dose opioid use
• history of opioid abuse
• neuro-muscular deficit of the ipsilateral femoral nerve and/or quadriceps muscle
• vulnerable populations i.e., children, pregnant women, or prisoners

Through the day following surgery