The effects of ultrasound guidance and neurostimulation on the minimum effective anesthetic volume of mepivacaine 1.5% required to block the sciatic nerve using the subgluteal approach.
BACKGROUND
We tested the hypothesis that ultrasound (US) guidance may reduce the minimum effective anesthetic volume (MEAV(50)) of 1.5% mepivacaine required to block the sciatic nerve with a subgluteal approach compared with neurostimulation (NS).


METHODS
After premedication and single-injection femoral nerve block, 60 patients undergoing knee arthroscopy were randomly allocated to receive a sciatic nerve block with either NS (n = 30) or US (n = 30). In the US group, the sciatic nerve was localized between the ischial tuberosity and the greater trochanter. In the NS group, the appropriate muscular response (foot plantar flexion or inversion) was elicited (1.5 mA, 2 Hz, 0.1 ms) and maintained to <or=0.5 mA. The volume of the injected local anesthetic was varied for consecutive patients based on an up-and-down method, according to the response of the previous patient. The initial volume was 12 mL. An independent observer evaluated the occurrence of complete loss of pinprick sensation and motor block: positive or negative responses within 20 min after the injection determined a 2-mL decrease or increase for the next patient, respectively.


RESULTS
The mean MEAV(50) for sciatic nerve block was 12 mL (95% confidence interval [CI], 10-23 mL) in Group US and 19 mL (95% CI, 15-23 mL) in Group NS (P < 0.001). The effective dose in 95% of cases was 14 mL (95% CI, 12-17 mL) in Group US and 29 mL (95% CI, 25-40 mL) in Group NS (P = 0.008).


CONCLUSIONS
US provided a 37% reduction in the MEAV(50) of 1.5% mepivacaine required to block the sciatic nerve compared with NS.
Danelli
Giorgio
G
Department of Anesthesia, Intensive Care and Pain Therapy, University Hospital of Parma, 43100 Parma, PR, Italy. gdanelli@ao.pr.i
Ghisi
Daniela
D
Fanelli
Andrea
A
Ortu
Andrea
A
Moschini
Elisa
E
Berti
Marco
M
Ziegler
Stefanie
S
Fanelli
Guido
G
D000328:Adult; D000779:Anesthetics, Local / Q000008:administration & dosage*; D001182:Arthroscopy / Q000009:adverse effects; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D005260:Female; D005267:Femoral Nerve / Q000187:drug effects; D006801:Humans; D007719:Knee Joint / Q000000981:diagnostic imaging / Q000294:innervation / Q000601:surgery*; D008297:Male; D008619:Mepivacaine / Q000008:administration & dosage*; D008875:Middle Aged; D009407:Nerve Block; D010147:Pain Measurement; D017288:Pain Threshold / Q000187:drug effects; D010149:Postoperative Pain / Q000517:prevention & control; D012584:Sciatic Nerve / Q000000981:diagnostic imaging* / Q000187:drug effects*; D013997:Time Factors; D016896:Treatment Outcome; D018084:Ultrasonography, Interventional
D000779:Anesthetics, Local; D008619:Mepivacaine