Abstract ID: A4

Abstract Title: Ultrasound-Guided Interscalene Needle Placement Produces Successful Anesthesia Regardless of Motor Stimulation Above or Below 0.5mA

Authors: Sinha S¹, Abrams J², Weller R³
St. Francis Hospital and Medical Center Hartford CT USA¹, St. Francis Hospital and Medical Center Hartford CT USA², Wake Forest University School of Medicine Winston-Salem NC USA³
Poster Type: Either

ABSTRACT BODY

Unlike paresthesia or motor nerve-stimulation approaches to interscalene block (ISB), ultrasound (U-S) guidance employs an anatomic rather than a neurophysiologic endpoint. This IRB-approved study was designed to observe nerve stimulator response after U-S-guided needle placement for ISB. Block success and duration were also measured.
Methods: Sixty-one patients scheduled for ambulatory shoulder surgery under ISB and general anesthesia consented to participate. After minimal sedation, a 50mm insulated needle was positioned by U-S between the 2 most lateral nerve structures in the interscalene groove at the cricoid level (Fig I). Without further needle movement, nerve stimulation was begun (2Hz, 0.1mS) and the lowest current producing upper extremity motor response was recorded; 30ml 0.5% bupivacaine with 1:200,000 epinephrine was then incrementally injected. Complete upper trunk motor and sensory block were respectively defined as paralysis of biceps flexion and absence of pain upon 50mA tetanic electrical stimulus in the anatomic snuffbox. Nurses recorded block performance time, onset and success of block, pain score in PACU, and block duration and analgesic tablet consumption overnight (phone contact). Patients were divided a priori into Group A (current ≤ 0.5mA) and Group B (current >0.5mA). Differences between groups were evaluated using Independent samples t-tests for data that had at least interval properties (i.e., BMI, age) and Chi-square tests for differences in frequency data (i.e., gender). All relevant comparisons were two-sided and alpha was set at 0.05.
Results: One patient was excluded for protocol violation, and both groups were demographically equivalent (Table 1). Only one patient had a paresthesia upon needle placement. All patients had complete sensorimotor block postop, none required narcotics in the PACU, and block duration and home analgesic usage were equivalent. Overall, complete sensory block developed faster than motor, with 57% of patients showing complete sensory block at 5 min; at 15 min, 93% had complete sensory compared to 73% complete motor block. Sensory block onset was equivalent between groups, but incomplete motor block at 15 min was more likely (p=0.04) in group B (Table 2).
Discussion: Others (1,2) have shown a lack of correlation between the physiologic endpoints of paresthesia (sensory) or motor nerve stimulation during regional block. This study has shown an further lack of correlation between motor stimulation and an anatomic (U/S) endpoint for needle placement which results in successful ISB. This finding is not unexpected since nerve roots have non-homogeneous distribution of sensory, motor, and connective tissue elements on cross-section (3). Anesthesiologists choosing to perform ISB as described with both U-S and nerve stimulation guidance need not reposition the needle to produce motor response below 0.5mA since there is no benefit in success or duration of block.
1. Urmey W: Anesthesiology. 2002;96:552-4
2. Choyce A: Reg Anesth Pain Med. 2001;26:100-4
3. Sunderland S: Brain 1945;68(Part 4):243-299

ATTACHED FILES

A4_Weller Table 1 vers 2Final.doc

A4_Weller Table 2 vers2 Final.doc

Reg Anesth Pain Med 2005; 30(3):A4