Introduction

Since its first description in 1946, continuous peripheral nerve block (CPNB) has evolved from an experimental technique worthy of case reports to a well-validated analgesic technique that is widely accepted by the medical community [1]. CPNB has been shown to improve postoperative analgesia and patient satisfaction and reduce opioid medication use for both upper [2, 3] and lower extremity procedures [4–6]. CPNB insertion techniques have advanced a long way from placement based on paresthesia and fluoroscopic imaging in the 1970s to stimulation-based techniques in the 1980s and, most recently, ultrasound (US) guidance. Despite strong evidence supporting the efficacy of CPNB for postoperative analgesia, data regarding optimum catheter placement techniques are limited.

CPNB techniques are dependent on placement of the catheter tip close enough to the nerve to allow effective and sustained analgesia for painful surgeries [7] with a minimum volume of local anesthetic [8]. One of the most common challenges associated with CPNB involves accurate placement of the catheter tip to provide both an initial anesthetic block and subsequent analgesia. If the catheter tip is not accurately placed, it will result in secondary catheter failure, which is defined as failure to provide pain relief after the initial block resolves. The reported incidence of secondary catheter failure ranges from 20 to 40 % [7]. If the use of CPNB is to remain an integral part of providing state of the art postoperative analgesia, high patient satisfaction, reduced length of stay and improved outcomes, eliminating secondary catheter failure is imperative. This is especially important for ambulatory surgical patients in whom non-functioning catheters may not be replaced unless the patient returns to the medical facility.

Inaccurate catheter tip location may also be associated with less common, but devastating complications such as intrathecal catheter placement leading to death after placement of an interscalene catheter [9] or catheter tip location near the phrenic nerve resulting in respiratory compromise [10, 11].

Secondary analgesic failure may also result from migration or dislodgement of the catheter. Marfoher et al. assessed dislocation rates of interscalene and femoral perineural catheters in volunteers who performed standardized physical exercises at regular intervals and the position of both catheters was examined by US confirmation of the spread of fluid. The maximal time of investigation in each volunteer was 6 h. The study found a 15 % overall dislocation rate of interscalene and femoral nerve catheters (25 % for femoral catheters and 5 % for interscalene catheters) with a highly significant correlation between the time from insertion and rate of catheter dislocation. This high incidence of dislodgement in volunteers and potentially higher in real patients emphasizes the importance of localizing the peripheral nerve catheter even after initial insertion [12].

Another factor affecting the block is single-orifice versus multiple-orifice peripheral nerve catheter design. There is a lack of data suggesting that the number of catheter orifice(s) impact the success of peripheral nerve catheter analgesia and incidence of secondary catheter failure. There is no study comparing peripheral nerve catheters with single orifice versus multiple orifices. This has been studied in epidural catheters where multiple-orifice catheters may have superiority.

There are many different techniques and various types of equipment available to facilitate catheter insertion. Although it is impossible to cover all of the potential combinations of technique and equipment, this review will summarize historical peripheral nerve catheter insertion methods, current methods, and discuss new advances in catheter placement techniques.

This is of increasing importance now as catheters are left in place for prolonged periods of time to provide postoperative analgesia, and patients with peripheral nerve catheters are being discharged home with ambulatory pumps. In response to the challenges of providing safe, effective, and consistently reliable analgesia, research and development in this field is expanding rapidly and physicians are constantly confronted with new information. This review will provide a concise overview of the latest developments in peripheral nerve catheter placement techniques.

Blind technique

In this technique, a needle is positioned close to the plexus/nerve using either nerve stimulation or US guidance. This is followed by local anesthetic injection followed by blind insertion of the catheter, a few centimeters beyond the needle tip. In some cases, the catheter is inserted prior to injecting the local anesthetic in order to try to confirm correct catheter location by producing a successful block after injecting local anesthetic through the catheter. In using this technique, it is possible to produce successful surgical block, but still have inaccurate catheter placement with subsequent secondary block failure in interscalene and femoral nerve blocks [8, 13]. Studies with lower-limb catheters have documented that 60–70 % of the catheters are not positioned close to the nerve using a blind technique [14] and can fail to provide pain relief once the initial block resolves. In a study involving the “three-in-one” block, the authors located the femoral nerve using a stimulating needle and subsequently placed the nerve catheter blindly [14]. The authors then used a pelvic radiograph to determine the location of the catheter tip and showed that the location was unpredictable. Their conclusion was that during a continuous “three-in-one block”, the threaded catheter rarely reached the lumbar plexus. This was confirmed in another study by Ganapathy et al. showing that the location of the catheter tip was located in the lumbar plexus in only 40 % of the patients [6].

Nerve stimulation

Catheters that deliver current to their tips have been developed in an attempt to improve success rates [15, 16]. Stimulating catheters were introduced to allow the operator to optimally position the catheter tip close to the nerve by observing muscular twitches in the desired location during catheter advancement. This should, in theory, reduce the incidence of secondary catheter failure; however, the results of clinical studies examining success with these devices are conflicting. In a study comparing stimulating and non-stimulating catheters for brachial plexus blocks, the use of stimulating catheters did not significantly improve postoperative pain control [17].

In the lower extremity, there seemed to be no significant advantage to the use of stimulating catheters for continuous femoral nerve blocks [18, 19]. In some studies, the presence of muscle contraction with nerve stimulation neither assured a successful surgical block nor successful analgesia with postoperative infusion of local anesthetic [16, 20–24]. The increased cost and need for additional catheter adjustments during placement with the stimulating catheter did not appear to offer any significant advantage over the use of non-stimulating catheters [25].

In a recent meta-analysis comparing the efficacy and safety of US vs. nerve stimulation guidance for peripheral nerve catheter placement, US guidance was associated with a higher success rate and a lower risk for vascular puncture [26].

However, nerve stimulation still has a role in patients with difficult ultrasound visualization, such as with deep nerve blocks, and in identifying specific nerves.

Ultrasound guidance

Ultrasound imaging has transformed the practice of PNB and its growing availability in combination with expanded physician training is likely to ensure even greater use in the future. US can be used as a “stand-alone” method to perform nerve catheter insertion for many patients [27, 28].

In a controlled study of 450 patients, supplemental electrical stimulation through the needle or through the catheter was not superior to US guidance alone in terms of pain score and opioid requirement. Additionally, the use of US alone was both faster and less expensive [29].

Visualization of the catheter tip as opposed to the shaft of the catheter is often challenging, as the flexible catheters frequently do not remain within the US plane of view and are located distant from the initial needle-tip position [30]. The use of US for catheter insertion has been described using a variety of validated approaches. This includes short-axis/in-plane view, short-axis/out-of-plane and long-axis/in-plane views [31–33].

The angle between the long axis of the placement needle and nerve is critical for perineural catheter insertion because catheters tend to exit the needle and traverse past the nerve with most of the commonly used catheters for peripheral nerve blocks, especially in short-axis in-plane approach. This may not be the case with flexible epidural catheters, as these are flexible and tend to stay close to the needle tip, but the drawback is that it is sometimes difficult to thread flexible catheters past the tip of the placement needle [31]. The utilization of flexible catheters for deep peripheral nerve blocks may improve the accuracy of catheter placement because depth makes it harder to monitor advancement of the catheter with US. However, this has not been evaluated.

Long-axis US imaging of the nerves and advancement of perineural catheters (long-axis/in-plane view) under direct vision may, in theory, decrease the secondary catheter failure rate. A preliminary report of four patients was published using an 18-gauge epidural Tuohy needle that was inserted tangentially to the nerve with correct needle tip position confirmed visually by small-volume injections of local anesthetic. A rigid epidural catheter was subsequently inserted under long-axis imaging and advanced into the desired perineural position. Local anesthetic was then injected through the catheter and spread was confirmed with both long-axis and short-axis scan [34]. Technically, this approach is more challenging due to difficulty in keeping the nerve, needle, and catheter within the ultrasound plane. It is especially challenging for the brachial plexus, as its elements are not straight; and it has multiple nerves as targets.

Mariano et al. compared long- and short-axis imaging techniques for in-plane US-guided femoral perineural catheter insertion, the long-axis, and in-plane views resulted in slightly faster onset of sensory anesthesia. However, placement takes longer and is not associated with improvement in other secondary outcome measures such as pain relief and muscle weakness reported on postoperative day one, and reduced procedure-related complications [32].

In another study comparing the catheter parallel to the nerve with the catheter perpendicular to the nerve approaches, the authors demonstrated a similar quality of analgesia after total knee arthroplasty. However, the catheter perpendicular to the nerve technique shortened the time of catheter insertion [33].

Hypothetically, a greater catheter tip-to-nerve distance leads to a higher secondary catheter failure rate. Although it has been shown that there was no difference in the quality of postoperative analgesia between inserting the catheter 0–1 and 5–6 cm past the needle tip, there was a trend towards more catheter dislodgements in the minimal-insertion group [35]. Few data have been reported on the optimal distance of the catheter past the needle tip.

Ultrasound markers of catheter tip location

Tissue movement

Pushing the catheter through the needle leads to tissue movement. Unfortunately, tissue motion may be transmitted well beyond the catheter tip as well as along the catheter shaft, making it difficult to definitively locate the catheter tip. In addition, the practice of moving the catheter in and out of the needle may increase the risk of cutting or shearing the catheter on the sharp needle tip.

Hydrolocation/hydrodissection

This involves rapid injection of a small amount of fluid (0.5–1 ml) to confirm catheter-tip position by both tissue movement and the appearance of a small anechoic “pocket” [36]. Some experts suggested flushing the catheter with normal saline to enhance its echogenicity, the clinical applicability and evidence of this recommendation is not known yet.

Microbubble injection

Air bubbles are highly echogenic and may serve as US contrast agent. Microbubble injection has been used to confirm catheter tip location in CPNB [37]. The visualization of the hyperechoic air artifact closes to the target nerve aid in localizing the catheter tip location. The use of microbubbles has not been evaluated extensively. Mariano et al. tested the hypothesis that microbubble injection predicts accurate catheter location using a bovine tendon representing a target nerve inserted between the fascial planes of two adjacent porcine muscle groups model. The use of the microbubble test improved the expert clinician’s assessment of catheter tip position compared to the blind approach. However, there was no difference when compared to direct visualization of the catheter without air injection [38]. The potential disadvantage of injecting air or microbubbles into the soft tissue is deterioration of US image quality, and distorts the anatomy because air bubbles cause acoustic shadowing artifact that obscures the target area. Another potential disadvantage of injecting air or microbubbles is that any further adjustment of the catheter can be challenging as air can persist for more than 2 min.

Another potential concern is intravascular injection of air if the catheter was inadvertently inserted, or subsequently migrated, into a blood vessel.

Combined techniques (US-guided and nerve stimulation)

Does the use of combined techniques (US-guided and nerve stimulation) improve the localization and decrease secondary catheter failure? Dhir et al. compared the efficacy of continuous infraclavicular brachial plexus blocks using non-stimulating catheters, stimulating catheters, and US-guided catheter placement with nerve stimulation assistance. The authors concluded that US guidance with nerve stimulation assistance significantly improved primary success and reduced secondary catheter failure in continuous infraclavicular brachial plexus blocks [23]. Other investigators studied the combined technique with conflicting results [39–41].

Farag et al. demonstrated the supplemental electrical stimulation through the needle or the catheter was not superior to US guidance alone in terms of pain score and opioid requirement [29].

New techniques in ultrasound guidance

Three- and four-dimensional ultrasound

New developments are also emerging in the field of three-dimensional (3D) US as a modality to improve catheter tip localization. There are multiple case reports published using the US to localize the catheter tip location [42–44].

Advantages of 3D US may include the ability to monitor correct needle placement and spread of local anesthetic and better identify adjacent unwanted targets that cannot be identified with two-dimensional ultrasound 2D US alone and optimization of catheter location (Fig. 1). Current (2D) images only provide a “doughnut” sign of spread of local anesthesia in contrast to 3D imaging, which provides 360-degree images for the distribution of the local anesthesia. This can be obtained with multiple planes without movement of the 3D US probe. It may enhance predictability and safety aspects of peripheral nerve blocks.

Fig. 1
figure 1

Three-dimensional ultrasound showing oblique view of the infraclavicular area with catheter inferior to the posterior cord (copyright© 2010 Clendenen et al.)

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Radiologists have employed 4D US-guided nerve tracking with success; four-dimensional ultrasound refers to moving 3D US, (with time being the 4th dimension) [45, 46]. Studies have demonstrated the feasibility of 4D US-guided peripheral nerve blocks and thoracic paravertebral blocks [44, 47, 48]. Four-dimensional US offers distinct advantages, allowing for simultaneous visualization of multiple planes of view, thereby permitting longitudinal, cross-sectional, and coronal images without probe adjustment [48].

Well-designed trials are needed to evaluate the applicability and practicality of this new imaging modality. The high cost and cumbersome size of the transducer and the noisy machines complicate its routine use for regional anesthesia. In addition, the increased complexity of learning 3D and 4D US may not be practical in routine clinical practice until further improvements occur in the technology.

Pumping maneuver/color ultrasound

Pumping maneuver achieved by repeatedly advancing and retracting the guide wire within the catheter to create motion that is detected by the color US. This pumping effect can induce movement within the detected frequency, which can be detected by either velocity imaging or power imaging mode, which are both available in most conventional US machines. Moreover, the pumping effect transiently increases the echogenicity of the catheter by formation of micro-bubbles around and inside both the catheter tip and catheter shaft. This technique has been used to assess catheter placement and patency and has been reported in the radiology literature [49]. The “pumping” technique has also been used to localize the needle, the stylet advanced and retracted in and out of a stationary needle shaft, which increases echogenicity of the needle shaft and tip and facilitates needle localization with color [49, 50]. This technique is limited because catheter tip identification using the color signal is only present while the device is being manipulated manually.

This led to the development of vibration devices in interventional cardiology, and there are several systems that improve the detection of vibration of interventional devices using two-dimensional and three-dimensional US systems and color (Fig. 2) [51–53].

Fig. 2
figure 2

Color images of the cardiac electrophysiological (EP) catheter. a Vibrating motor off. b Vibrating motor on (copyright© 2008 Kalyan et al.)

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The first report in regional anesthesia introduced in 2007 by Klein et al. is the piezoelectric vibrating needle and catheter [54]. By modifying the needle and catheter with an attached piezoelectric generator, both vibrate at a preset 1 to 8-kHz frequency. The vibrations are transmitted down the shaft of the needle or via the metal stylet causing the tip to vibrate and allowing detection with color. Although the studies are interesting, they were done in a cadaver, where the tissue resistance is different from living humans. A higher level of complexity is encountered in living subjects that include pulsatile vascular structures and many other sources of potential artifacts. In addition, the piezoelectric generator will increase the cost of the procedure.

Recently, Elsharkawy et al. studied the technique in cadaver experiments and a patient trial [55]. The authors used a catheter whose guidewire extends to the tip and moved in and out to create a pumping effect. This pumping effect detected by either color. They also introduced another concept—using M-mode ultrasonography. M-mode cursor was placed over the presumed location of the catheter, and two different patterns were observed on the screen: the motionless image of the tissues before the pumping technique or injection of medication shown as the “horizontal waves”, and the image demonstrating a granular pattern distal (deeper) to the catheter location during the pumping technique and during injection of a local anesthetic (Fig. 3).

Fig. 3
figure 3

Popliteal approach for sciatic nerve block. Catheter location deep to the sciatic nerve at 3 cm from the skin surface and the changes in the M-mode at also 3 cm from the skin when the M-mode was applied to the expected location of the catheter. SN sciatic nerve (Copyright© 2014 Cleveland Clinic)

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The main limitation of pumping techniques is that it can be difficult to make a distinction between the movement of the shaft of the catheter and the tip of the catheter. In addition, there are many other concerns to accurately detect the signal.

It is difficult to control many variables, for example: velocity of the manual pumping, the angle of insonation, and surroundings tissue resistance. As it is very difficult to control these variables, further research is needed in this area.

Newly developed catheters

Echogenic technology

Echogenic technology was developed to improve needle visualization and documented that it improved needle visibility independent of experience level, and allowed steeper insertion angles of the needle [56, 57].

The ultrasonographic visibility of the catheter is more challenging because the wavelengths used in medical ultrasonic imaging (200–500 μm), are several times smaller than the catheter’s dimensions. Catheter technology has improved in recent years for other indications and designs optimized for regional anesthesia have been tried. There are many patents and inventions describing flexible catheters with modifications at the distal portions to increase the catheter tip echogenicity, for example: coiled membrane, spring, and an uneven surface [58].

Takatani et al. investigated the (US) visibility of catheters in a pork phantom and recommend selecting a catheter with a structure that enhances the echogenicity at the center of catheter [59].

One of the potential concerns is that if echogenic catheters remain for more than 72 h it might lead to adhesions, as its outer surface is uneven. The adhesions are not studied around echogenic catheters but have been described to develop after 7 days around stimulating catheters with coiled steel in a rat model [60]. This has also been described in case reports [61, 62].

Self-coiling catheters

When using US guidance, nerves are often visualized in short axis, while the needle and catheter are advanced in-plane and thus perpendicular to the nerve course. Catheters are often stiff and designed to avoid kinking, which can potentially lead to inaccurate catheter placement beyond the nerve [31, 63].

Luyet et al. designed a catheter that coils up as soon as it is advanced beyond the needle tip (Fig. 4). This allows the catheter tip to remain close to the initial needle-tip position, even when a perpendicular approach to the nerve has been chosen [64]. The pigtail catheter coils after introduction beyond the needle tip and therefore remains close to the needle tip position (Fig. 4). The authors tested the new catheter in a cadaveric imaging study. The sciatic nerves of both legs were tracked and visualized with ultrasound. To detect the catheter location, computer tomographic imaging was done. Thereafter, 5 ml of magnetic resonance imaging (MRI) contrast dye was injected through the catheters, and MRI assessed the spread of contrast dye. A total of 40 catheter placements were performed showing direct contact of the catheter with the nerve in 37 of the 40 placements.

Fig. 4
figure 4

Self-coiling catheter set containing the 17-gauge Tuohy needle, guidewire, and the catheter with markings (copyright© 2014 Cleveland Clinic)

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To reduce the risk of wrapping around nerves using coiled catheters, the authors used only one coil at the tip. The authors reported four catheters hooking around the nerve but there was no resistance felt during withdrawal of the catheters [64]. The authors concluded that the self-coiling catheter could be introduced through a needle placed under ultrasound guidance with a low risk of catheter misplacement away from the targeted nerve.

Paravertebral catheters can be used for analgesia after a wide variety of surgeries. However, there are inconsistent results, as the final positions of the catheters were often within the anterior part of the paravertebral space [65]. “Luyet et al.” evaluated the placement of 60 paravertebral pigtail self-coiling catheters in cadavers under US guidance [66]. No catheter was misplaced into the epidural, pleural, or prevertebral spaces (Fig. 5).

Fig. 5
figure 5

A reconstructed 3D CT image showing needles and catheters placed in the paravertebral spaces. The catheter coils remain close to the needle tip and lie close to the intervertebral foramen (copyright© 2012 Luyet et al.)

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The results of both studies [64, 66] should be interpreted with caution, because they are confined to the sciatic nerve and paravertebral block and to the study settings with cadavers with different tissue resistance than living humans. These findings must be tested in other anatomical locations, in humans, and cost–benefit studies versus conventional catheters before introducing the coiled catheter into routine clinical practice.

Catheter over needle (CON)

The traditional catheter-through-needle insertion technique can potentially lead to over-feeding of the catheter beyond the needle tip. This can lead to unpredictable catheter tip placement.

In addition to unpredictable catheter tip location, this can lead to local anesthetic leakage. An explanation for the leakage is that the diameter of the catheter (e.g., 20 G) is smaller than the needle (e.g., 18 G) used for initial skin puncture. This mismatch may contribute to leakage, secondary catheter failure, and dislodgement.

New catheter designs (CON), similar to intravenous catheter designs where the catheter is fitted over the needle, result in a smaller diameter for the skin puncture than the catheter diameter. This leads to a tight fit at the skin insertion site reducing the risk of leakage and secondary failure [67–69]. The CON technique is relatively simple and relies on two major components: an outer catheter sheath and a flexible, kink-resistant inner catheter. When the tip is in the desirable location, the needle is withdrawn and the inner catheter, whose length is similar to that of the needle, is inserted into the outer catheter. Thus, the inner catheter literally replaces the needle without the need for overfeeding and enables the inner catheter tip to be in the desired position and potentially decreases the risk of having the catheter tip far from the target nerve. Further research is needed to confirm the cost–benefit analysis of CON technique versus conventional catheter through needle technique.

Conclusions

Tremendous progress has been made in continuous regional anesthesia in recent years. Catheter migration and difficulty in localization however continues to be a problem. Accurate location of the catheter tip appears vital to ensuring a high-quality block and reducing the risk of secondary failure and complications. Continuing improvement in peripheral nerve catheter development technologies and additional prospective trials could aid accurate catheter placement and making perioperative management of patients with peripheral nerve catheter a less-complicated task.