Ultrasound (U-S) guidance is being increasingly used for performing peripheral nerve blocks (1,2). Unlike more traditional peripheral nerve block techniques, in which a sensory paresthesia is elicited or motor nerve stimulation is performed, U-S guidance uses an anatomic rather than a neurophysiologic end point for block performance. Anesthesiologists, however, often continue to use nerve stimulation along with U-S guidance for additional confirmation of nerve identification (2).
With motor nerve stimulation, the most commonly accepted end point for evidence that an insulated, stimulating needle is close enough to the target nerve to result in reliable anesthesia is a motor response at or below 0.5 mA (3). When using both U-S and motor nerve stimulation for performing interscalene block (ISB) it has been noted that, despite sonographic evidence of the needle tip in close proximity to the nerves, motor response cannot be consistently achieved at or below 0.5 mA (4). In this situation, the clinician must decide whether to accept the U-S evidence of needle placement in the interscalene space or to reposition the needle to evoke a motor response before injecting local anesthetic (LA). The objective of this study was to evaluate any differences in block characteristics or success based on whether the current observed to evoke a motor response was above or below 0.5 mA after U-S-guided needle placement in the interscalene space.
Sixty-one patients scheduled for outpatient shoulder surgery under ISB and general anesthesia (GA) at St. Francis Hospital gave written, informed consent to participate in this IRB-approved study. Exclusion criteria included age <18 years or >70 years, pregnancy, neuropathy, neurologic disease, diabetes, or contraindication to ISB. After application of standard monitors, patients were lightly sedated in an induction area and supplemental oxygen was administered by a nasal cannula. The maximum sedation administered was 2 mg midazolam and 100 μg fentanyl, and verbal responsiveness was maintained throughout the block. The nerves in the interscalene space were identified by U-S (Titan®, SonoSite Inc., Bothell, WA), at approximately the level of the cricoid cartilage in the transverse or short-axis view. The footprint of the linear, 5–10 mHz transducer was then outlined by a marking pen, the skin was prepped and draped in a sterile fashion, and the transducer was enclosed in a sterile sleeve. After skin anesthesia, a 50-mm, 22-guage insulated needle (Stim-A2250, B. Braun Medical, Inc., Bethlehem, PA) was advanced under U-S guidance until the tip of the needle was between the two most lateral nerves in the interscalene space (Fig. 1). These nerve structures are most likely the C5 and C6 roots, or the upper trunk and the C7 root/middle trunk (5). The block needle was advanced in the long axis of the transducer (in plane), so that the shaft and tip of the needle were always in view (Fig. 1). Once the tip of the needle was positioned, the needle was immobilized and nerve stimulation was initiated at 0.1 ms pulse width, 2 Hz, (Stimuplex®, B. Braun Medical Inc., Bethlehem, PA). The current was increased slowly until a brachial plexus motor response was noted in the upper extremity (deltoid, bicep, tricep, brachioradialis) and this current was recorded. Because a variety of motor responses have resulted in successful interscalene anesthesia (6,7), the specific motor response elicited was not recorded for group stratification. Without any further needle movement, and after negative aspiration, 30 mL of bupivacaine 0.5% with epinephrine 1:200,000 was injected incrementally over 2–3 min with intermittent negative aspiration.
The end of bupivacaine injection was denoted time zero, and the onset of sensory and motor blockade was assessed at 5, 10, and 15 min by a participating nurse. This nurse was not blinded to current measurement, but was unaware of the 0.5 mA group determinate. A three-point scale was used to assess sensory and motor block in the upper trunk (C5–6) distribution: normal, partial block, or complete block. Complete upper trunk sensory block was defined as absence of pain with 50 Hz, 30 mA tetanic stimulation in the anatomical snuffbox (MiniStim MS-1, Life-Tech Inc., Stafford, TX). The dermatomal innervation of the snuffbox is typically depicted as C6, although C7 may contribute. Complete upper trunk motor block was defined as the inability to flex the elbow against gravity (biceps—C5–6). The time to perform the block (from preblock scan with skin marking, to the end of injection) was also recorded.
Anesthesia was induced with propofol, tracheal intubation was facilitated by rocuronium, and GA was maintained with desflurane, nitrous oxide, oxygen, and fentanyl without specific limitation. The pre- and intraoperative fentanyl dosages were recorded. After emergence and tracheal extubation, patients were reassessed in the postanesthesia care unit (PACU) for upper trunk block quality and pain level. The sensory and motor blocks were reassessed with the same three-point scale used preoperatively, a 10-point numerical pain score was determined, and PACU fentanyl use was recorded as was surgical duration. Surgeons were asked (post hoc) to categorize the surgical procedures as moderate, moderately severe, or severe with respect to level of surgical trauma and expected nociception.
After discharge, nurses contacted the patient on postoperative day 1 and block duration, home analgesic consumption, patient satisfaction, and any complications were recorded. Block duration was defined as the time to first oral narcotic analgesic, or if none was required, the time to resolution of numbness in the arm. The nurse(s) performing the postoperative sensorimotor examination and the phone follow-up call were not blinded to current measurement, but were unaware of the group designation based on current (see below).
Based on the observed current required to elicit a motor response, patients were divided a priori into Group A (current ≤0.5 mA) and Group B (current >0.5 mA). Differences between groups were evaluated using independent samples t-test for data that had at least interval properties, (e.g., age, body mass index) and χ2 test for difference in frequency data (e.g., gender). All relevant comparisons were two-sided and α was set at 0.05. The home analgesic consumption was converted to hydrocodone equivalents and acetaminophen consumption, and these nonparametric data were analyzed using Mann– Whitney U.
Twenty-five patients, or 42%, had motor threshold current ≤0.5 mA (Group A), whereas 35 (58%) had response current above 0.5 mA (Group B) (Fig. 2). One patient was excluded for protocol violation and another patient experienced a transient paresthesia at final needle positioning; current measurement was performed at that site.
No differences were noted in group characteristics, including demographics, preoperative and surgical narcotic use, block performance time, and surgical duration and intensity, as rated by the surgeon (Table 1). There was no difference in the progression of complete upper trunk sensory block between the groups: 96% in Group A and 91.4% in Group B had complete sensory block at 15 min (P = 0.46). However, fewer patients had complete upper trunk motor blockade at 15 min in Group B (62.9%) than in Group A (88%) P = 0.03 (Table 2). All patients had complete upper trunk sensory and motor block in the PACU. No patient was given fentanyl in the PACU because visual analog scale pain scores were 0 in all cases.
Block duration in Group A, mean 17.8 h (range 10.2–26.7 h), was not different from Group B, mean 17.8 h (range 9.0–27.1 h). In addition, the mean hydrocodone equivalents and acetaminophen consumed postoperatively did not differ between groups (Table 2).
U-S needle placement between the two most lateral interscalene nerve structures resulted in successful postoperative upper trunk block regardless of motor response from 0.14 to 1.7 mA. Although the onset of complete upper trunk sensory block was not different between groups, there was a delay in onset of complete upper trunk motor block in patients with current above 0.5 mA. This finding may have been because of the nonhomogenous distribution of motor, sensory, and connective tissue in nerve roots (8,9), such that a needle may have been proximate to one fiber type whereas at some distance from another type (10). Because the current density is inversely related to the distance from needle tip to motor fascicle (11), one might expect a slower onset of motor blockade in patients who required >0.5 mA to evoke a motor response. Other authors have shown that, for both ISB and axillary block, the needle location which produces a sensory paresthesia will not necessarily produce a motor response at a low current and vice versa (12–14). In a study of U-S-guided needle placement and motor response during axillary block, U-S-guided direct nerve contact resulted in motor response above 0.5 mA in 25.5% of cases (15). A higher percentage (58%) of patients in our study had motor response above 0.5 mA, perhaps because the needle was placed between nerve structures and between the fasciae of the anterior and middle scalene muscles rather than directed against a nerve.
A shortcoming of the study is that complete characterization of the onset of block was not performed after 15 min, and the sensory and motor block was only assessed in the C5–6 dermatome and myotome. In a practice setting where patients receive a block for postoperative analgesia and also undergo GA for shoulder surgery, the most important end point was considered to be the adequacy and duration of postoperative upper trunk block, because this may be all that is necessary for shoulder analgesia (16). Also, successful ISB often spares lower trunk anesthesia unpredictably (17). Extrapolation of the results of this study of U-S-guided ISB to production of anesthesia of the entire brachial plexus cannot be made.
An additional shortcoming of the study design is that the PACU nurses recorded outcome data, and they were not blinded to the current determined during block performance. They were not, however, aware of the predetermined group division at 0.5 mA, which should reduce the potential for bias. Finally, the U-S appearance of the spread of LA was not characterized, because needle movement was not allowed after initiating injection; optimizing LA spread by needle movement would have invalidated the current measurement.
In summary, U-S-guided interscalene needle placement without further adjustment for the current amplitude required to elicit a motor response resulted in successful postoperative upper trunk anesthesia in all 60 patients studied. Onset of complete biceps paralysis was delayed in patients with motor response above 0.5 mA, but there was no difference in upper trunk sensory block onset which was complete by 15 min in 93% of patients, and there was no difference in the postoperative success or duration of the ISB.
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© 2007 International Anesthesia Research Society
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