Introduction
Ultrasound (US)-guided peripheral nerve blockade is gaining popularity among anesthesiologists. US guidance transformed peripheral nerve blocks from blind procedures relying on anatomic landmarks and indirect methods to localize different nerves to procedures in which the anesthesia provider is able to visualize the target nerve or nerves and the surrounding structures, introduce the block needle toward the target, and observe the local anesthetic being injected and surrounding the nerve in real time. The interest in US-guided regional anesthesia (USGRA) generated an increasing number of randomized controlled trials (RCTs) addressing the question of whether US leads to better patient outcomes compared with traditional techniques. The outcome variables relevant to the individual blocks were time to performance of nerve blocks, number of attempts, patient discomfort during performance of the nerve blocks, local anesthetic volume, predictable quality of the sensory and motor block, safety profile, learning curve, and the success of the blocks.
Despite the excitement about US, skeptics argue that the lack of an evidence-based foundation makes it difficult to adopt US-guided peripheral nerve blocks as a standard of care. Historically, anesthesiologists performed peripheral nerve blocks by eliciting paresthesia on needle contact with a nerve. By adding nerve-stimulating devices, we have been able to more precisely locate peripheral nerves based on muscle twitch patterns in response to peripheral nerve stimulation. The improved technology of US now allows us to visualize the target peripheral nerves and surrounding structures in real time as we perform these blocks.
Basic Physics: Sound waves are high-energy waves generated by passing an alternating electric current through piezoelectric crystals. The sound waves will travel through different body tissues and will be reflected off the tissue interface and returned to the transducer. The transducer transforms the echo (mechanical energy) into an electric signal, which is processed and displayed as an image on the screen. Different tissues have different affinities for sound waves. Sound waves will be reflected, absorbed, or scattered at different tissue interfaces to variable degrees. Acoustic impedance is the resistance of a tissue to the passage of US. The higher the degree of impedance mismatch between adjacent tissues, the greater the amount of reflection of the sound waves. A transducer that sends a high-frequency signal will generate higher resolution images with well-defined details, with the sacrifice of decreased penetration (depth). Newer US machines, however, can produce higher resolution with relatively deeper penetrations.
Technique
The transducer frequencies used for peripheral nerve blocks range from 3 MHz to 15 MHz. The appropriate probe is chosen based on the depth of the nerve to be blocked and the resolution required. Superficial structures are best visualized with the use of a high-frequency linear probe. To visualize deep neural structures, we recommend using a curved array probe. A preblock scan should be performed to identify the nerve and, perhaps more important, the surrounding structures such as bone, muscle, and vascular structures. To optimize the view, the anesthesiologist adjusts the transducer by sliding it along the skin, rotating it, and tilting it. Almost all peripheral nerves to be blocked are visualized in the short axis view. After sterile preparation of the skin with an alcohol-based solution, the US probes is covered with a sterile sleeve with a conducting US gel inside the sleeve. The block needle is advanced to the target nerve or nerves without making direct contact. The needle is either kept in the US plane and seen in its entirety during the block or advanced to its target out of the US plane and only the tip is visualized. The most common error committed by novices during training on USGRA is losing sight of the needle tip or losing site of the whole needle as it advances toward the target. With the needle, nerve, and surrounding structures in view, a catheter can be placed for continuous perineural infusion or local anesthetic can be injected through the needle to surround the nerve. The local anesthetic should be seen completely surrounding the nerve ; however, if the nerve is directly injected, it will appear swollen.
Evidence
Overall, data support the use of US guidance as a safe adjunct to nerve stimulation techniques or as a complete replacement for nerve stimulation. The most difficult question to answer is whether US guidance improves success rates and decreases the number of complications.
Studies have used various criteria to demonstrate higher success rates and shorter onset time for US guidance compared with conventional techniques. Orebaugh and colleagues conducted a retrospective chart review of more than 5000 cases and concluded that US might offer the potential advantage of decreasing adverse outcomes. In addition, RCTs have demonstrated several benefits of US over nerve stimulation or other landmark techniques.
Procedure Attempts, Times, and Comfort
Time to perform the block, number of attempts, and patient comfort during the procedure are important quality measures when evaluating peripheral nerve blocks. When compared with traditional nerve localization techniques, US was associated with less time to perform the block and fewer needle passes needed to perform the block. Definition of time to perform the block was not consistent among all the RCTs. Although these US techniques are approximately 2 to 6 minutes faster than landmark or nerve stimulation techniques, they do not account for prescanning and preparation of the US machine and probe, which could lengthen the procedural time. Three RCTs involving ankle blocks favored a landmark approach. Fewer needle passes have been reported for US-guided sciatic nerve blocks. These findings indicate that patient comfort is likely improved because the needle is in contact with the patient for a shorter period of time. In fact, children expressed lower pain scores during block performance with US compared with nerve stimulation. The use of US was also associated with a significantly lower incidence of paresthesia compared with landmark techniques during performance of brachial plexus blocks. In patients with fractures of the extremities, painful muscle contractions that occur with nerve stimulation can be avoided with a US-guided nerve block.
Block Onset Time
Block onset time is defined as the time interval from completion of injection of the local anesthetic and removal of the needle to a complete sensory block. In several randomized trials, onset times were shorter for US-guided blocks than for blocks placed with conventional techniques by approximately 2 to 12 minutes. More specifically, shorter onset times have been documented for brachial plexus blocks in children and for femoral (3-in-1) blocks in adults. Casati and colleagues demonstrated a faster onset of sensory axillary brachial plexus block but found no difference in onset of motor block or in overall preparation time for surgery. Similarly, sensory block onset was faster for supraclavicular brachial plexus blocks, but the rate of motor block onset was unchanged. Onset times were also faster for interscalene and axillary brachial plexus blocks with US compared with nerve stimulation. In other RCTs, there was no statistical difference between US and landmark techniques. The close proximity of the needle tip to the target nerve and surrounding the target nerve or nerves with local anesthetic may explain the shorter onset times. The clinical significance of the shorter onset time and the shorter time to perform the block can be argued. The value of decreasing this time is variable, depending on the setting of each individual practice.
Local Anesthetic Volume
US may also provide the means to reduce the dose of local anesthetic necessary to achieve endpoints in a nerve block. For example, a lower volume of local anesthetic was required to encircle sciatic nerves (one-half volume) and femoral nerves (one-third volume) in children with the use of US guidance compared with the set dose used for nerve stimulation. The US-guided group achieved successful blocks that also lasted longer than the nerve stimulation group. For ilioinguinal–iliohypogastric nerve blocks, children needed less local anesthetic with US than with the conventional “facial click” techniques (0.19 mL/kg versus 0.3 mL/kg). The US group of children also had better quality blocks on the basis of a physical examination of sensory and motor block distribution. A US-guided group that received 20 mL of local anesthetic for a femoral nerve block experienced a higher quality block than a nerve stimulation group that received 30 mL of local anesthetic. Casati and colleagues used the up-and-down staircase method to determine the amount of anesthetic required to achieve a sensory and a motor femoral nerve block. The minimum effective volume of 0.5% ropivacaine was 15 mL in the US-guided group and 26 mL in the nerve stimulation group. US guidance likely reduces local anesthetic dosing because reliable visualization of the local anesthetic spreading around a nerve is possible to confirm the block. A lower total dose of local anesthetic may be a means of reducing the incidence of systemic local anesthetic toxicity.
Block Quality and Success
Most RCTs define the quality of a block as a complete sensory block in the area supplied by the target nerve or nerves. Criteria to define success rates for nerve blocks vary depending on the purpose of a block. Success of blocks placed for postoperative analgesia may be measured by opioid consumption or distribution of sensory analgesia. If the purpose of the block is surgical anesthesia, block quality may be assessed through the need for supplemental analgesia or the conversion rate to general anesthesia. Smaller intraoperative and postoperative analgesic doses were required in children receiving ilioinguinal–iliohypogastric nerve blocks with US placement than with conventional facial click technique.
Clinical studies favor US over conventional techniques for improved block quality as assessed by physical examination measurements. Several randomized studies have demonstrated reduced sensitivity to painful stimuli after US-guided femoral (3 : 1) blocks in adults and infraclavicular brachial plexus blocks in children. US-guided interscalene and axillary brachial plexus blocks also produced more complete sensory and motor blocks. A higher incidence of complete sciatic nerve block and better tolerance of a tourniquet have been found with US-guided blocks than with nerve stimulation. Success rates are also higher in US-guided axillary brachial plexus blocks than with the transarterial approach. Half of the failures in the transarterial approach result from an inability to locate the axillary artery, and the remainder are caused by inadequate intraoperative analgesia. In a quality study, Chan and colleagues randomly assigned three groups to receive axillary brachial plexus blocks with the use of US guidance, nerve stimulation, or dual techniques (US and nerve stimulation). US guidance with or without nerve stimulation was superior to nerve stimulation alone, and adding a nerve stimulator to the US technique did not provide any additional benefit. However, another study involving supraclavicular brachial plexus blocks did not report improved success or a reduced conversion rate to general anesthesia with US guidance and nerve stimulation techniques compared with nerve stimulation alone. In a recent meta-analysis, a significant increase was seen in the overall success rate for blocks performed with the use of a US-guided technique versus all non-US techniques. These results are similar to those found in other systematic reviews and meta-analyses. A meta-analysis of 13 RCTs found that peripheral nerve blocks performed with US guidance were more likely to be successful. Another systematic review of 16 RCTs found that the use of US for upper and lower extremity nerve blocks was associated with a better quality of block.
Avoiding Adverse Outcomes
US has the potential to avoid complications of mechanical nerve injury, intravascular injection, and adverse effects. However, adverse outcomes with USGRA are still reported. Of note, the ability of US to prevent potential complications is very operator dependent. Sites et al showed that failure to visualize the needle during advancement occurred in up to 43% of the procedures performed by novice trainees. US resulted in a statistically significant decrease in the rate of paresthesia during block placement and unintended vascular puncture when compared with traditional nerve localization techniques. Liu and colleagues did not show a statistically significant difference in adverse neurologic outcomes between US-guided interscalene blocks and the landmark technique. US may reduce the incidence of intraneural injection of local anesthetic. Most experts agree that intraneural injection is associated with postoperative neurologic dysfunction and should be avoided. Before US technology, the only indicators for intraneural needle placement were very painful paresthesia, high injection pressures, or very low nerve stimulation current necessary to achieve a twitch. In an ultrasonographic study in pigs, Chan and colleagues produced clear image differences between a perineural injection and a direct injection of a nerve in the axillary brachial plexus. According to these images, an intraneural injection is easily detected with US. The histologic examination of the nerves injected revealed infiltration of the injectate within the epineurium or perineurium. On the other hand, Bigeleisen found that intraneural injection of low volumes of local anesthetic during US-guided axillary blocks did not cause neurologic dysfunction. The relationship between neurologic dysfunction and intraneural injection is still unclear, but US imaging can show when a nerve is being injected and may help to avoid injecting high volumes of local anesthetic directly into a nerve. The absence of neurologic complications reported in a recent small series, however, should not be mistakenly interpreted to justify the indiscriminate practice of intraneural injection in all peripheral nerve blockade models ( Table 55-1 ).
| Study (Year) | Site (n) | Study Design | Intervention | Control | Outcomes |
|---|---|---|---|---|---|
| Thomas et al (2011) | Interscalene brachial plexus (41) | Prospective RCT | Ultrasound guided | Nerve stimulation | Faster onset; improved performance in training environment |
| Geiser et al (2011) | Infraclavicular brachial plexus (56) | Prospective RCT, single-blinded | Ultrasound guided | Nerve stimulation | Significantly higher success rates and shorter times of onset |
| Zencirci (2011) | Axillar brachial plexus (60) | Prospective RCT | Ultrasound guided | Nerve stimulation | The motor blockade was more intense in US group |
| Orebaugh et al (2009) | Peripheral noncatheter nerves (2146 versus 3290) | Retrospective chart review | Ultrasound guided | Nerve stimulation | US offers potential advantages of decreasing adverse outcomes, such as seizures and nerve injuries |
| Liu et al (2009) | Interscalene brachial plexus (230) | Prospective RCT, single-blinded | Ultrasound guided | Nerve stimulation | No differences in block failures, patient satisfaction, or incidence and severity of postoperative neurologic symptoms |
| Ponde and Diwan (2009) | Infraclavicular brachial plexus (50) | Prospective RCT | Ultrasound guided | Nerve stimulation | US improves success rate |
| Gurkan et al (2008) | Infraclavicular brachial plexus (80) | Prospective RCT | Ultrasound guided | Nerve stimulation | No statistical difference |
| Perlas et al (2008) | Popliteal fossa sciatic nerve (74) | Prospective RCT | Ultrasound guided | Nerve stimulation | US resulted in higher rate of success and faster onset block |
| Kapral et al (2008) | Interscalene brachial plexus (160) | Prospective RCT | Ultrasound guided | Nerve stimulation | US improved success rate |
| Yu et al (2007) | Axillary brachial plexus (80) | Prospective RCT | Ultrasound guided | Nerve stimulation | US resulted in higher success rate, faster onset, shorter manipulation time, and lower accidental blood vessel puncture |
| Chan et al (2007) | Axillary brachial plexus (188) | Double-blinded RCT | Ultrasound guided with or without nerve stimulation | Nerve stimulation | Improved incidence of complete sensory block |
| Casati et al (2007) | Femoral nerve (60) | Up-and-down staircase method for minimum effective volume | Ultrasound guided | Nerve stimulation | Reduced minimum effective anesthetic volume |
| Dingemans et al (2007) | Infraclavicular brachial plexus (73) | Prospective RCT | Ultrasound guided | Ultrasound guided and nerve stimulation | Faster onset |
| Casati et al (2007) | Axillary brachial plexus (60) | Prospective RCT | Ultrasound guided | Nerve stimulation | Faster onset |
| Domingo-Triado et al (2007) | Sciatic nerve (61) | Prospective RCT | Ultrasound guided | Nerve stimulation | Improved quality of sensory block; improved tourniquet tolerance; reduced attempts |
| Oberndorfer et al (2007) | Pediatric femoral and sciatic (46) | Prospective RCT | Ultrasound guided | Nerve stimulation | Reduced volume of local anesthetic and longer duration of analgesia |
| Sites et al (2006) | Axillary brachial plexus (56) | Prospective RCT | Ultrasound guided | Perivascular technique | Reduced conversion to general anesthesia; reduced performance time |
| Willschke et al (2005) | Pediatric ilioinguinal–iliohypogastric (100) | Prospective RCT | Ultrasound guided | Facial click | Lower local anesthetic volume; lower additional analgesic requirements |
| Marhofer et al (2004) | Infraclavicular brachial plexus (40) | Prospective RCT | Ultrasound guided | Nerve stimulation | Shorter onset time, lower pain scores during performance, longer sensory block, better sensory and motor block quality |
| Williams et al (2003) | Supraclavicular brachial plexus (80) | Prospective RCT | Ultrasound and nerve stimulation | Nerve stimulation | Shorter block performance time; better block distribution |
| Marhofer et al (1998) | 3 : 1 femoral nerve block (60) | Prospective RCT | Ultrasound guided | Nerve stimulation at different volumes | Reduced onset time; improved quality of sensory block |
| Marhofer et al (1997) | 3 : 1 femoral nerve block (40) | Prospective RCT | Ultrasound guided | Nerve stimulation | Reduced onset time; improved quality of sensory block |
| Macaire et al (2008) | Wrist blocks (60) | Prospective RCT | Ultrasound guided | Nerve stimulation | Less time to perform the block; similar success rate |
| Mariano et al (2009) | Popliteal–sciatic perineural catheter insertion (40) | Prospective RCT | Ultrasound guided | Nerve stimulation | Placement of popliteal–sciatic perineural catheters takes less time and produces less procedure-related discomfort when using US guidance compared with ES |
| Mariano et al (2009) | Infraclavicular brachial plexus perineural catheter insertion (40) | Prospective RCT | Ultrasound guided | Nerve stimulation | Placement of infraclavicular perineural catheters takes less time, is more often successful, and results in fewer inadvertent vascular punctures when using US guidance compared with ES |
| Mariano et al (2009) | Femoral perineural catheter insertion (40) | Prospective RCT | Ultrasound guided | Nerve stimulation | Placement of femoral perineural catheters took less time with US guidance compared with ES. US guidance produced less procedure-related pain and prevented inadvertent vascular puncture |
| Fredrickson et al (2009) | Interscalene catheter placement | Prospective RCT | Ultrasound guidance | Nerve stimulation | Interscalene catheters placed with US demonstrated improved effectiveness during the first 24 hr compared with those placed with NS. These catheters were also placed with less needling and a very small reduction in procedure-related pain |
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