Needle-nerve Contact Research Articles

Using the BBraun BSmartTM Pressure Manometer to Prevent Unsafe Injection Pressures During Simulated Peripheral Nerve Blockade: A Pilot Study

Background: Peripheral nerve injury during regional anaesthesia may result from accidental intraneural placement of the needle, or forceful needle nerve contact. Intraneural injections are associated with increased resistance to injection, typically >15 psi. The BBraun BSmart™ is an inline mechanical manometer, offering a visual display of injection pressures. Objective: The primary objective of this study was to determine if using the BBraun BSmartTM manometer successfully prevents 90% of anaesthetists and anaesthetic assistants from injecting at pressures > 15 psi during simulated nerve block. Methods: This was a prospective observational study involving anaesthetists and anaesthetic assistants. Two 20 ml injections were performed by each participant, once when the BBraun BSmartTM manometer was obscured from view, and once with the manometer visible. A PendoTech PressureMATTMS recorded injection pressures. Results: 39 participants completed the study, with a total of 78 injections recorded. During the study, 32 peak pressures during the 78 procedures were recorded above the recommended upper limit of 15 psi, 41% of the total injections. The peak pressure rose above 15 psi in 24/39 (62%) injections when the BBraun Bsmart™ manometer was obscured, but only in 8/39 (21%) injections when the manometer was visible. Conclusion: The BBraun Bsmart™ manometer did not successfully prevent 90% of anaesthetists or anaesthetic assistants from injecting at unsafe pressures. However, using the BBraun BSmart™ did reduce the number of unsafe injection pressures generated by participants. When utilised in conjunction with PNS and ultrasound guidance, this may offer additional safety during peripheral nerve blockade.

Flow dynamics of ultrasound-guided lumbar plexus block in adults.

Background and Aims:The outcomes of plexus and peripheral nerve blocks depend on needle-nerve contact and the spread of local anesthetic (LA) around the plexus or nerve. Needle-nerve distance and spread of LA could be visualized during US-guided lumbar plexus block (LPB).Material and Methods:After Institutional Ethics Committee approval and after obtaining informed consent, 24 American Society of Anesthesiologists’-physical status I–III patients who underwent surgical fixation of fractures of proximal femur were enrolled. Spinal anesthesia was a primary anesthetic in all patients. At the end of the surgery, all patients received US and neurostimulation-aided LPB at the third lumbar nerve root (LNr). The primary aim was to determine the spread of LA in the lumbar plexus area with the relation of the needle tip and LNr contact. The secondary aim was to understand block efficacy in terms of pain scores monitored at regular intervals and 100 mg intravenous tramadol was administered as a rescue analgesic if VAS >4.Results:In all 24 patients, we observed an oval and antegrade LA spread after lumbar plexus was identified with neurostimulation at L3. With the needle closer to intervertebral foramina (IVF), a retrograde spread was visualized. Only 2/24 patients received rescue analgesia in the first 24 h.Conclusion:The type of spread after the US-guided LPB could predict block success of block and a possible epidural spread.

Combination of real-time needle-tip pressure sensing and minimal intensity stimulation limits unintentional intraneural injection during an ultrasound-guided peripheral nerve block procedure: A randomized, parallel group, controlled trial

Study objectiveUltrasound guidance does not eliminate the risk of intraneural injection, which must be avoided during PNB. Combining ultrasound guidance (USG), nerve stimulation (NS), and injection pressure monitoring is advocated to prevent nerve injury during PNB. We hypothesized that combining patient-tailored dynamic NS and real-time pressure sensing (RTPS) could reduce the incidence of intraneural injection and nerve puncture during USG PNB compared with a traditional fixed thresholds (Control) procedure. DesignRandomized, prospective study. SettingOperating room. PatientsOne hundred ASA physical status I to III patients undergoing orthopedic surgery. InterventionsPatient anesthetized using axillary, sciatic or femoral USG PNB were randomized to the PresStim group (Dynamic RTPS and NS set at 1.5 mA then decreased; n = 50) or Control group (fixed thresholds for in-line pressure mechanical manometer and NS at 0.2 mA; n = 50). MeasurementsProcedural ultrasound images and videos were recorded, stored and reviewed in random order by two experts in ultrasound-guided PNB blinded to the group. They noted the needle-to-nerve relationship and intraneural injection for all blocked nerves. Main resultsOne hundred and twenty-three USG PNBs were performed (56 axillary brachial plexus blocks, 40 femoral nerve blocks and 27 sciatic popliteal nerve blocks); 235 blocked nerves and videos were recorded and analyzed (PresStim, 118; Control, 117). Less paresthesia was noted in the PresStim group (12.7%) compared with the Control group (18.8%). The risk of intraneural injection was significantly higher in the Control group (odds ratio [OR], 17.1; 95% confidence interval [CI], 2.2–135, P = 0.007). The risk of nerve puncture (OR, 22.7; 95% CI, 2.9–175, p = 0.003) and needle-nerve contact (OR, 4.7; 95% CI, 2.4–9.5, p < 0.001) was significantly higher in the Control group than the PresStim group. ConclusionsUnder the conditions of the study, dynamic triple monitoring combining RTPS, NS and USG decreases intraneural injection and unintentional needle-nerve contact and puncture during a PNB procedure.

Needle insertion forces and fluid injection pressures during targeting of nerves in a soft embalmed cadaver model.

Forceful needle-nerve contact and high subepineural pressures and are recognised causes of nerve damage. Pressure and force measurements are necessary to inform the mechanisms of nerve injury, build virtual simulator environments and provide operator feedback during simulation training. However, the range of pressures and forces encountered at tissue layers during targeted needle insertion and fluid injection are not known. We built a needle that recorded in-line pressure during fluid injection and continuously measured force at the needle tip. Two anaesthetists were randomised to insert a 21g block needle at 48 nerve sites on both sides of 3 soft embalmed Thiel cadavers. Our objective was to measure pressure and force during the course of targeted nerve injection at epimysium, in perineural tissue, on epineurium and during subepineural injection. At each interface, we infused a 0.5ml bolus of embalming solution at a rate of 12mlmin-1 and recorded the pressure response. Force was measured continuously in the background throughout the procedure. Pressure was greater at epineurium and within subepineurium than perineural tissue, geometric ratio (95% CI) 4.7 (3.0-7.3) kPa and 3.8 (2.5-5.7) kPa, respectively, both P < 0.0001. Force on nerve contact and on nerve penetration was greater than force in perineural tissue, geometric ratios (95% CI) 3.0 (1.9-4.7) N and 3.6 (2.2-7.5) N, respectively, both P < 0.0001. On nerve contact, 1 in 6 insertions were ≥ 5N CONCLUSIONS: Despite valid infusion pressures, anaesthetists exerted excessive force on nerves.

The role of peripheral nerve stimulation in the era of ultrasound-guided regional anaesthesia.

With the widespread use of ultrasound for localising nerves during peripheral nerve blockade, the value of electrical nerve stimulation of evoked motor responses has been questioned. Studies continue to show that, compared with nerve stimulation, ultrasound guidance alone leads to: significantly improved block success; decreased need for rescue analgesia; decreased procedural pain; and lower rates of vascular puncture. Nerve stimulation combined with ultrasound does also not appear to improve block success rates, apart from those blocks where the nerves are challenging to view, such as the obturator nerve. The role of nerve stimulation has changed in the last 15years from a technique to locate nerves to that of an adjunct to ultrasound. Nerve stimulation can serve as a monitor against needle-nerve contact and may be useful in avoiding nerves that are in the needle trajectory during specific ultrasound guided techniques. Nerve stimulation is also a useful adjunct in teaching novices ultrasound-guided regional anaesthesia, especially when the position and or appearance of nerves may be variable. In this review, the changing role of nerve stimulation in contemporary regional anaesthetic practice is presented and discussed.

Ultrasound-guided Needle nerve contact positions detection in Regional Anaesthesia using Needle tip force and Opening injection pressure

Ultrasound imaging guidance has been widely used in peripheral nerve block to guide the anaesthetist to insert the needle and observe injection fluid diffusion. However, needle-nerve contact(NNC) positions visualization is difficult to achieve because of requirement of high image solution and expert. To better guide the anaesthetist to predict the distance from needle to nerve and reduce the needle trauma, we attempted to use Needle tip force(NTF) and Opening injection pressure (OIP) to identify the NNC positions under ultrasound guidance. The relationship between OIP and NNC, NTF and NNC, NTF and OIP is unclear now, and we conducted a prospective, observational study to define the relationship. In this study, we designed a multi-function block needle and built a real-time NTF and OIP measurement which could trace and record NTF and OIP data in the simulator sciatic nerve. NNC positions were divided into three distinct NNC positions: Pre-NNC, First-NNC, Forceful-NNC using ultrasound guidance image. We statistically analysis NTF, OIP in the three NNC positions, and the results indicate the NTF differentiate in three NNC positions(p<0.001), OIP differentiate as well (p<0.001) and the NTF has a highly positive with the OIP in the same NNC positions with Pearson correlation 0.62.

Diabetic neuropathy increases stimulation threshold during popliteal sciatic nerve block

BackgroundPeripheral nerve stimulation is commonly used for nerve localization in regional anaesthesia, but recommended stimulation currents of 0.3–0.5 mA do not reliably produce motor activity in the absence of intraneural needle placement. As this may be particularly true in patients with diabetic neuropathy, we examined the stimulation threshold in patients with and without diabetes. MethodsPreoperative evaluation included a neurological exam and electroneurography. During ultrasound-guided popliteal sciatic nerve block, we measured the current required to produce motor activity for the tibial and common peroneal nerve in diabetic and non-diabetic patients. Proximity to the nerve was evaluated post-hoc using ultrasound imaging. ResultsAverage stimulation currents did not differ between diabetic (n+55) and non-diabetic patients (n+52). Although the planned number of patients was not reached, the power goal for the mean stimulation current was met. Subjects with diminished pressure perception showed increased thresholds for the common peroneal nerve (median 1.30 vs. 0.57 mA in subjects with normal perception, P+0.042), as did subjects with decreased pain sensation (1.60 vs. 0.50 mA in subjects with normal sensation, P+0.038). Slowed ulnar nerve conduction velocity predicted elevated mean stimulation current (r+−0.35, P+0.002). Finally, 15 diabetic patients required more than 0.5 mA to evoke a motor response, despite intraneural needle placement (n+4), or required currents ≥2 mA despite needle-nerve contact, vs three such patients (1 intraneural, 2 with ≥2 mA) among non-diabetic patients (P+0.003). ConclusionsThese findings suggest that stimulation thresholds of 0.3–0.5 mA may not reliably determine close needle-nerve contact during popliteal sciatic nerve block, particularly in patients with diabetic neuropathy. Clinical trial registrationNCT01488474

High Opening Injection Pressure Is Associated With Needle-Nerve and Needle-Fascia Contact During Femoral Nerve Block.

High opening injection pressures (OIPs) have been shown to predict sustained needle tip contact with the roots of the brachial plexus. Such roots have a uniquely high ratio of fascicular versus connective tissue. It is unknown if this relationship is preserved during multifascicular nerve blockade. We hypothesized that OIP can predict needle-nerve contact during femoral nerve block, as well as detect needle contact with the fascia iliaca. Twenty adults scheduled for femoral block were recruited. Using ultrasound, a 22-gauge needle was sequentially placed in 4 locations: indenting the fascia iliaca, advanced through the fascia iliaca while lateral to the nerve, slightly indenting the femoral nerve, and withdrawn from the nerve 1 mm. At each location, the OIP required to initiate an injection of 1 mL D5W (5% dextrose in water) at 10 mL/min was recorded. Blinded investigators performed evaluations and aborted injections when an OIP of 15 psi was reached. Opening injection pressure was 15 psi or greater for 90% and 100% of cases when the needle indented the femoral nerve and fascia iliaca, respectively. Opening injection pressure was less than 15 psi for all 20 patients when the needle was withdrawn 1 mm from the nerve as well as at the subfascial position (McNemar χ2 P < 0.001). Opening injection pressure greater than 15 psi was associated with a block needle tip position slightly indenting the epineurium of the femoral nerve (90%) and the fascia iliaca (100%). Needle tip positions not indenting these structures were associated with OIP of less than 15 psi (100%).

Risk evaluation for needle-nerve contact related to electrical nerve stimulation in a porcine model.

For performing peripheral nerve blocks, the risk for needle-nerve contact (NNC) as an adverse event for low and high current thresholds has not yet been clearly defined when nerve stimulator guided techniques were applied. This experimental study aimed to investigate the risk of NNCs when applying stimulation currents between 0.1 and 2mA (in increments of 0.1mA). Brachial plexus nerves were exposed surgically in five anesthetized pigs. An insulated needle connected to a nerve stimulator was placed at 10mm distance to the targeted nerves. Current intensity was adjusted according to randomization (0.1-2.0mA, increments of 0.1mA). The needle tip was advanced toward the nerve until a motor response was elicited. The frequencies of NNCs for each of the adjusted current intensities were noted. Risks of NNCs were calculated according to the analysis of 95% confidence intervals (95% CI). We executed 2000 needle placements with stimulation currents ranging from 0.1 to 2.0mA. The risks for NNC at low current intensities were 0.85 (95% CI, 0.77-0.91) for 0.3mA and 0.5 (95% CI, 0.4-0.6) for 0.5mA, respectively. For high current intensities, risks of 0.13 (95% CI, 0.07-0.21) for 0.9mA and 0.01 (95% CI, 0-0.05) for 1.1mA were calculated. In this experimental animal study, a relevant reduction in the risk of NNCs could be observed when high current intensities (0.9-1.1mA) were applied compared to low current intensities (0.3-0.5mA).

Opening Injection Pressure Consistently Detects Needle–Nerve Contact during Ultrasound-guided Interscalene Brachial Plexus Block

Needle trauma may cause neuropathy after nerve blockade. Even without injection, nerve injury can result from forceful needle-nerve contact (NNC). High opening injection pressures (OIPs) have been associated with intrafascicular needle tip placement and nerve damage; however, the relationship between OIP and NNC is unclear. The authors conducted a prospective, observational study to define this relationship. Sixteen patients scheduled for shoulder surgery under interscalene block were enrolled if they had clear ultrasound images of the brachial plexus roots. A 22-gauge stimulating needle was inserted within 1 mm of the root, and 1-ml D5W injected at 10 ml/min by using an automated pump. OIP was monitored using an in-line pressure manometer and injections aborted if 15 psi or greater. The needle was advanced to displace the nerve slightly (NNC), and the procedure repeated. Occurrence of evoked motor response and paresthesia were recorded. Fifteen patients had at least one clearly visible root. OIP at 1 mm distance from the nerve was less than 15 psi (mean peak pressure 8.2 ± 2.4 psi) and the 1-ml injection could be completed in all but two cases (3%). In contrast, OIP during NNC was 15 psi or greater (mean peak pressure 20.9 ± 3.7 psi) in 35 of 36 injections. Aborting the injection when OIP reached 15 psi prevented commencement of injection in all cases of NNC except one. High OIP (≥15 psi) consistently detected NNC, suggesting that injection pressure monitoring may be useful in preventing injection against nerve roots during interscalene block.

Minimal Current Intensity to Elicit an Evoked Motor Response Cannot Discern Between Needle-Nerve Contact and Intraneural Needle Insertion

The ability of an evoked motor response (EMR) with nerve stimulation to detect intraneural needle placement reliably at low current intensity has recently been challenged. In this study, we hypothesized that current intensity is higher in needle-nerve contact than in intraneural needle placement. Brachial plexus nerves were exposed surgically in 6 anesthetized pigs. An insulated needle connected to a nerve stimulator was placed either with 1 mm distance to the nerve (control position), adjacent to nerve epineurium (needle-nerve contact position), or inside the nerve (intraneural position). Three pulse duration settings were applied in random fashion (0.1, 0.3, or 1.0 milliseconds) at each needle position. Starting at 0.0 mA, electrical current was increased until a minimal threshold current resulting in a specific EMR was observed. Fifty threshold current measurements were scheduled for each needle position-pulse duration setting. Four hundred-fifty threshold currents in 50 peripheral nerves were measured. Threshold current intensities (mA) to elicit EMR showed small differences between the needle-nerve contact position [median (25th-75th percentiles); 0.1 milliseconds: 0.12 (0.08-0.18) mA; 0.3 milliseconds: 0.10 (0.06-0.12) mA; 1.0 milliseconds: 0.06 (0.04-0.10) mA] and the intraneural position (0.1 milliseconds: 0.12 [0.10-0.16] mA; 0.3 milliseconds: 0.08 [0.06-0.10] mA; 1.0 milliseconds: 0.06 [0.06-0.08] mA) that are neither statistically significant nor clinically relevant. Regardless of the pulse duration that was applied, the 98.33% confidence interval revealed a difference of at most 0.02 mA. However, threshold current intensities to elicit EMR were lower for the needle-nerve contact position than for the control position (0.1 milliseconds: 0.28 [0.26-0.32] mA; 0.3 milliseconds: 0.20 [0.16-0.22] mA; 1.0 milliseconds: 0.12 [0.10-0.14] mA). The confidence interval for differences suggests minimal current intensity to elicit a motor response that cannot reliably discern between a needle-nerve contact from intraneural needle placement. In addition, an EMR at threshold currents <0.2 mA (irrespective of the applied pulse duration) indicates intraneural needle placement or needle-nerve contact.

The Maximum Effective Needle-to-Nerve Distance for Ultrasound-Guided Interscalene Block

One of the most fundamental, 1 yet controversial, tenets of regional anesthesia practice has been the adage “no paresthesia, no anesthesia.” Implicit to this concept is the requirement for direct needle-nerve contact to achieve a successful block. The advent of ultrasound (US) guidance for peripheral nerve blockade (PNB) has enabled providers to position the needle tip purposefully as close as possible to, and even inside, the target nerve. Consequently, much of the contemporary regional anesthesia literature has focused on the question “How close is too close?” while investigators challenge the safety limits of US-guided PNB. Regrettably, the risk of nerve injury persists despite US guidance and is underscored by reports of new functional deficits after interscalene brachial plexus block (ISB) performed under US guidance by experienced providers. Given that mechanical needle-nerve trauma is an important mechanism of peripheral nerve injury, providers are cautioned to avoid intentional intraneural injection or needle-nerve contact during US-guided PNB.8,14,15 Potentially hazardous needleto-nerve proximity may be especially relevant during US-guided ISB, where inadvertent injection beneath the epineurium may be as high as 50%. Subepineural, and particularly intrafascicular, injection of local anesthetic may increase the risk of nerve injury.17 Neural elements of the interscalene brachial plexus are predominantly comprised of axonal tissue and may be especially

Incidence of intraneural needle insertion in ultrasound-guided femoral nerve block: A comparison between the out-of-plane versus the in-plane approaches

BackgroundThe optimal method of ultrasound-guided femoral nerve block (in-plane vs. out-of-plane) has not been established. We tested the hypothesis that the incidence of needle-nerve contact may be higher with out-of-plane than with in-plane needle insertion. MethodsForty-four patients with hip fracture (American Society of Anaesthesiologists physical status I–III) were randomized to receive the femoral block with an out-of-plane approach (needle inserted at a 45–60° angle 1cm caudal to the midpoint of the ultrasound probe just above the femoral nerve) or with an in-plane technique (needle inserted 0.2–0.4cm from the side of the probe lateral to the femoral nerve). Data collected included depth of needle insertion, response to nerve electric stimulation, and distribution of the injected volume in relation to the nerve (anterior vs. posterior, the latter assuming needle-nerve contact). The sensory block onset was tested at 20min and block recovery and any neurologic symptoms were evaluated at 24h. ResultsThe incidence of needle-nerve contact was significantly higher with the out-of-plane approach (14/22 patients [64%]) than with the in-plane approach (2/22 patients [9%]) (p<0.001) (OR=17.5, 95% CI: 4–79). The rate of paraesthesia on crossing the fascia iliaca was similar in the two groups. All blocks uneventfully regressed; and no patient developed neurologic symptoms. ConclusionsUnder the conditions of our study, needle-nerve contact during femoral nerve block occurs frequently with the out-of-plane approach. An in-plane approach results in an equally effective femoral block and less incidence of needle-nerve contact.

Forced Needle Advancement During Needle-Nerve Contact in a Porcine Model

In this study, we determined whether needle advancement during needle-nerve contact (forced needle-nerve contact) is associated with a higher risk of nerve injury compared with needle-nerve contact without needle advancement (nonforced needle-nerve contact). In 8 anesthetized pigs, the brachial plexus nerves underwent forced (0.15 Newton) or nonforced (0.0 Newton) needle-nerve contact without nerve penetration. The grade of nerve injury was histologically assessed using an objective score ranging from 0 (no injury) to 4 (severe injury). Sixty-nine nerves, including controls, were examined. Histology revealed a significant difference between forced and nonforced needle-nerve contact (median [interquartile range] 3 [2-4] vs 2 [1-2]; P = 0.004). Myelin damage and intraneural hematoma occurred only after forced needle-nerve contact. The severity of structural nerve injury after needle-nerve contact was directly related to force exposure via needle advancement.

Peripheral Nerve Blockade

Peripheral Nerve Blockade

Histological Consequences of Needle-Nerve Contact following Nerve Stimulation in a Pig Model

Background. Nerve stimulation can facilitate correct needle placement in peripheral regional anesthesia. The aim of this study was to determine whether the high threshold current is associated with reduced nerve injury due to fewer needle-nerve contacts compared with low current. Methods. In anaesthetized pigs, thirty-two nerves of the brachial plexus underwent needle placement at low (0.2 mA) or high current (1.0 mA). The occurrence of needle-nerve contact was recorded. After 48 hours, the nerves were analyzed for occurrence of histological changes. Nerve injury was scored ranging from 0 (no injury) to 4 (severe injury). Results. The frequency of needle-nerve contact was 94% at low compared to 6% at high current. The score was significantly higher at low (median [interquartile range] 2.0 [1.0-2.0]) compared to high current (0.0 [0.0-1.0] P = .001). Conclusions. Inflammatory responses were directly related to needle-nerve contacts. Hence, posttraumatic inflammation may be diminished using higher current for nerve localization.

Histological Analysis After Peripheral Nerve Puncture with Pencil-Point or Tuohy Needletip

Continuous peripheral nerve blocks typically are performed with a "through-the-needle technique" and require needles with an inner diameter allowing catheter placement. In case of direct needle-nerve contact, the pencil-point needletip is currently considered less traumatic than are other needle configurations. In this study we determined whether nerve puncture with pencil-point needles is associated with fewer nerve injuries in comparison with Tuohy needles. In 6 anesthetized pigs the brachial plexus were exposed bilaterally. Up to 8 nerves underwent nerve puncture with a pencil-point or a Tuohy needle. After 48 hours, the nerves were resected during anesthesia. The specimens were processed for visual examination and the detection of inflammatory cells, myelin damage, and intraneural hematoma. The grade of nerve injury was assessed using an objective score ranging from 0 (no injury) to 4 (severe injury). Fifty-eight nerves, including controls, were examined. According to the applied injury score, there was no significant difference between the pencil-point needle group [median (interquartile range) 3 (3-4)] and the Tuohy needle group [3 (3-4) P = 0.97]. The occurrence of posttraumatic regional inflammation, myelin damage, and intraneural hematoma was similarly high in both groups. Regardless of the needletip configuration applied for nerve puncture, pencil-point and Tuohy needletips may both lead to comparable magnitude of posttraumatic inflammation and considerable structural changes within the nerve. No significant differences were found comparing pencil-point with Tuohy tip-configured needles.

Regional anaesthesia: awake or asleep?

We read with interest Feely et al.’s survey and were surprised by their finding, that the vast majority of anaesthetists performed both upper, and particularly lower limb blocks after induction of general anaesthesia [1]. Furthermore, for certain blocks over half of those who believed it was safer to perform the block prior to general anaesthesia admitted that this was not their actual practice. It would have been intriguing to explore why this was the case. Presumably, one reason was to avoid patient discomfort. Whilst we consider it important to minimise patient discomfort during peripheral nerve blockade (PNB), this can be achieved without resorting to general anaesthesia. We perform over 2500 regional anaesthetics annually at the Toronto Western Hospital, none of them under general anaesthesia. Instead we sedate the patients as required with titrated doses of intravenous midazolam. Occasionally we will supplement this with fentanyl or propofol if the block is particularly painful or the patient is particularly anxious. We avoid over-sedation so as to be able to detect significant pain, paraesthesia or early signs of local anaesthetic toxicity during block performance. Unlike the majority of the respondents, we routinely use ultrasound to localise nerves. Compared to neurostimulation, ultrasound can reduce performance time, needle passes and procedure related pain [2, 3]. The principal risks in PNB are inadvertent intraneural and intravascular injection. None of the techniques in current practice, including ultrasound, can wholly prevent these complications [4–7]. With respect to nerve injury, although pain and paraesthesia are insensitive indicators of needle-nerve contact [8, 9], their positive predictive value is close to 100%. Hence we continue to regard them as useful warning symptoms in the awake patient, along with visual inspection on ultrasound for intraneural injection. The use of a peripheral nerve stimulator is no guarantee of safety; there is good laboratory and clinical evidence that current thresholds are poor predictors of needle-nerve proximity or even transfixion [8, 10, 11]. Regarding intravascular injection, there is evidence that early recognition and treatment of systemic toxicity with intravenous lipid emulsion is essential to good outcome [12, 13]. One of the early signs of significant local anaesthetic systemic toxicity is a decrease in conscious level; this will not be apparent if the patient is already anaesthetised. The authors of the survey conclude that central neuraxial blocks should not be performed on anaesthetised patients. In the absence of evidence to the contrary, we believe that this practice should be extended to include PNB for the reasons outlined above.

Peripheral nerve stimulation

Peripheral nerve stimulation is the gold standard method of nerve identification in Argentina. In this article we focus on the basic principles needed to understand and perform regional blocks with the aid of a peripheral nerve stimulator, avoiding complications. Based on our experimental investigation in animals, we recommend permanently checking the equipment to always perform the blocks in an awake or slightly sedated patient and to inject the AL solution when a motor response GII is obtained with an intensity of ≤0.5 mA, a pulse duration of 100 μsec and a frequency of 2 Hz. MR at that point must disappear if intensity is decreased to 0.2 mA. Disparition of MR after local anesthetic injection is due to electrophysiologic changes of injectates and not to the physical displacement of the nerve. Paresthesia are the only protection that patients have against needle nerve contact or penetration.

Venipuncture-induced causalgia: anatomic relations of upper extremity superficial veins and nerves, and clinical considerations.

In 1994, 11 patients with injury to upper extremity cutaneous nerves after routine venipuncture were reported. All developed causalgia (Complex Regional Pain Syndrome, Type 2). Nerve injury appeared secondary to direct trauma via "inappropriate" needle or bolused material entry into the plane of the nerves beneath the veins, or nerves overlying the veins. However, in 3 of 13 additional patients, the venipunctures were properly performed and atraumatic. To explore the anatomic relationships of superficial veins and cutaneous nerves with regard to the role of direct nerve trauma during venipuncture in the development of causalgia, the 14 upper extremities of seven randomly chosen cadavers were dissected at three common venipuncture sites. In addition, the clinical features of all 24 patients are presented. Major branches of cutaneous nerves were superficial to and overlay veins in six extremities. In multiple instances, nerves and veins were intertwined, requiring detailed dissection to separate them. In the classic situation, nerves were immediately as deep as veins, often with no fascial separation. Anatomical relationships between upper extremity superficial veins and cutaneous nerves are so intimate that needle-nerve contact during venipuncture is common. Because venipuncture-induced nerve injuries are rare, factors other than direct nerve contact appear necessary for the chronic pain syndrome to occur.