Minimal threshold for stimulating catheters.

A patient undergoing total knee replacement was fitted with a stimulating femoral catheter for postoperative analgesia. After the catheter was secured to the skin, the minimal stimulatory threshold was verified again and found to be extremely low (0.00-0.01 mA; pulse width, 0.1 ms). However, in spite of this situation, no paresthesia or pain was experienced by the patient. Use of the catheter resulted in satisfactory analgesia with no complications noted during the hospital stay or at follow-up three months later. When the nerve stimulator was sent to our biomedical engineering department to verify the accuracy of its current output, it was found to be functioning adequately. Neurostimulation is a complex phenomenon that is far from being completely understood. On the basis of this single report, we cannot recommend the routine use of a threshold inferior to 0.2 mA for neural electrolocation. However, we do not advocate the requirement of strict numerical thresholds either. Further research is needed to understand the relationship linking stimulatory threshold and distance between needle (or catheter) tip and nerve.

Effect of a continuous peripheral nerve block on the inflammatory response in knee arthroplasty.

BACKGROUND AND OBJECTIVES: Experimental nerve block in animals inhibits the inflammatory response. The purpose of this study was to determine to what extent a 48-hour local anesthetic block of all afferent and efferent nerve fibers of the knee area has an impact on postoperative inflammatory response. METHODS: Twelve patients scheduled for primary total knee arthroplasty received spinal anesthesia, and then were randomly allocated to either patient-controlled analgesia with morphine (n = 6) or a combination of continuous lumbar plexus and sciatic nerve blocks (continuous peripheral nerve block; CPNB) with ropivacaine 0.2% for 48 hours. Blood samples were collected before surgery and at 3, 8, 24, and 48 hours after surgical incision to measure plasma glucose, serum insulin and cortisol, C-reactive protein, interleukin-6, and leukocyte count. Pain visual analog scale at rest and on knee flexion were recorded and complications classified. RESULTS: Visual analog scale was lower in the CPNB group at rest and on knee flexion on postoperative days 1 and 2 (P < .05). There were no differences in circulating levels of glucose, insulin, and cortisol. C-reactive protein and leukocyte count were lower in the CPNB group (P < .05). There was a positive correlation between the peak leukocyte count and the inflammatory markers (P < .03). Three patients in the patient-controlled analgesia group and one in the CPNB group had complications requiring conservative management. CONCLUSIONS: Continuous lumbar plexus and sciatic nerve blocks with ropivacaine contribute to the attenuation of the postoperative inflammatory response.

Comparison of analgesic methods for total knee arthroplasty: metabolic effect of exogenous glucose.

BACKGROUND AND OBJECTIVES: This study was set up to assess whether postoperative suppression of gluconeogenesis by dextrose infusion would be influenced by continuous peripheral nerve block (CPNB) with local anesthetics, in comparison with epidural and with intravenous analgesia. METHODS: Twenty-seven patients, undergoing elective primary total knee arthroplasty for osteoarthritis, were randomly allocated to one of the three groups of 9 patients each: patient controlled analgesia (PCA) with i.v. morphine, epidural with bupivacaine 0.1% and fentanyl 3mug/ml, or continuous femoral and sciatic blocks with ropivacaine 0.2%. Endogenous glucose production, an index of gluconeogenesis, and glucose clearance, an index of whole body glucose uptake, were assessed on the second postoperative day by measuring [6,6-(2)H(2)]glucose kinetics after an overnight fast (fasted state), and during a 3-h period infusion of dextrose at 4 mg/kg/min (fed state). Visual analog scale (VAS) at rest and at knee flexion, use of morphine, mobilization, nutritional intake, and bowel function were also collected. RESULTS: Endogenous glucose production was totally suppressed by 3 h of dextrose infusion in all 3 groups (P < .001) while glucose clearance was unchanged. Blood glucose and insulin increased (P < .001), while glucagon decreased, with the greatest change in the epidural group (P < .05). VAS at rest and at knee flexion was significantly lower in patients receiving epidural and CPNB compared to i.v. morphine (P < .05). Restoration of bowel function, assessed as return of bowel movements, was faster in the CPNB group (P < .05). CONCLUSION: Excellent analgesia was achieved in the epidural and continuous nerve block groups. Postoperative gluconeogenesis was totally suppressed by dextrose infusion independent of the analgesia technique with no change in glucose utilization.