Opioid Induced Loss of Local Anesthetic Potency in Patients Undergoing Total Knee

3 groups of patients will be included in this prospective study: opioid naïve, those using opioids at the time of surgery (opioid tolerant), and those with a history of opioid use, but who are not currently on opioids (tolerant history). The FDA guidelines will be used to define the opioid tolerant patients as a patient taking, for 1 week or longer, at least 60 mg oral morphine /day; 25 ug transdermal fentanyl/hour; 30 mg oral oxycodone /day; 8 mg oral hydromorphone /day; 25 mg oral oxymorphone/day, or an equianalgesic dose of any other opioid.

Patients will be recruited based on the inclusion and exclusion criteria (see below). Patients eligible for the study will be identified in the population seen by orthopedic surgeons practicing at UPMC Shadyside Hospital and recruited at the pre-surgery clinic by the research coordinator or PI. Study patients will be asked to sign the informed consent form at that time. Pre-operative pain socres, dose and duration of pre-operative opioid consumption will be collected. Knee function (Knee Society score and the functional scores) will be recorded. Data collection will be performed by the research coordinator or PI.

All study patients will receive pre-operative femoral and sciatic nerve block catheters under ultrasound guidance. This is the standard care for postoperative pain control. Once nerve block catheters are placed, research study will be performed to determined the LA potency prior to surgery using the up-down approach, to avoid the confound of post-operative pain. Femoral nerve catheters will be initially dosed with 10 ml, 0.5% ropivacaine. This dose (50 mg) is to produce a complete block in 50% of the patients based on previous studies (7). This is the standard dose of LA provided through femoral nerve catheters.

The following approaches to increase and decrease the dose of ropivacaine are part of research procedures:

After the completion of the initial catheter bolus, sensory and motor blocks will be evaluated every 2 min for the first 10 min after injection, and then every 5 min until 30 min after injection. Sensory block is assessed by evaluating the presence and absence of response to thermal stimulation applied to the central sensory region of the blocked nerve. Thermal stimulation will be delivered with a 5 X 5 mm contact thermode (Medoc, Minneapolis, MN). Two different heat ramps are used to preferentially activate A-delta or C-fiber primary afferents. As previously reported, rises in temperature of < 2 °C/s activates C-fiber afferents, whereas heat ramps > 2 °C/s results in activation of A-delta primary afferents (8). Fast (3.4 °C/s) and slow (1.0 °C/s) heat ramps are used to increase temperatures from a 35 °C holding temperature to a plateau of 50°C. The plateau temperature is maintained for 10 and 5 s for the fast and slow heat ramps, respectively. The sequence of fast and slow heat stimulation is applied with a three-minute interval between trials to avoid sensitization. Nociceptive block will be confirmed with a pin prick test . Motor block is assessed by the MRC (Muscle Function Council) scale (0, no active contraction; 5 the normal power) of quadriceps muscles. Effective blocks are defined as complete loss of a thermal sensation at fast and slow ramp tests with MRC scale =0.