Regional anaesthesia, specifically peripheral nerve blockade (PNB), is an increasingly popular method of providing both anaesthesia and analgesia during the peri-operative period. Custom-designed regional anaesthesia needles are used to perform PNB, an example is shown in Fig. (1). They are manufactured with integrated injecting tubing connected to a hollow needle, to facilitate remote injecting whilst being able to keep the needle still. These echogenic needles are easily visible on ultrasound. When performing PNB, the intention is for the anaesthetist to place the needle tip immediately adjacent to the target nerve, then deposit local anaesthetic alongside the nerve [1]. This is described as a perineural or extraneural injection. The needle should not make contact directly with the nerve, to reduce the risk of peripheral nerve injury (PNI).

Fig. (1). BBraun Stimuplex Ultra regional anaesthesia needle.

The incidence of permanent PNI resulting from regional anaesthesia is fortunately rare, quoted as 2-4 per 10 000 nerve blocks [2]. Whilst transient neurological symptoms are common after PNB, the vast majority resolve with time [2]. Disappointingly, this incidence has not declined in the last decade since the introduction of ultrasound guidance into routine clinical practice [2].

Nerve fascicles are bundles of nerve axons that run together, bound by the perineurium within a peripheral nerve. The perineurium offers the vulnerable fascicles physical and chemical protection. It prevents potentially toxic substances from diffusing into the nerve, maintaining the internal environment of the fascicle [2].

Accidental placement of the needle tip within the nerve during PNB is termed an intraneural injection. An intraneural injection may be sub-classified as intrafascicular or extrafascicular, depending on the exact location of the needle tip within the nerve structure. An intraneural injection may also be innocuous, without subsequent PNI: the delicate fascicles occupy only one-quarter of the cross-sectional area of a peripheral nerve, with the rest comprising of connective tissue [3]. It is possible that a needle tip inserted beneath the perineurium does not contact a fascicle, therefore avoiding mechanical disruption. The image resolution of commercially available ultrasound equipment is not detailed enough to distinguish between intra- and extra-fascicular needle tip placement.

PNB has not been identified as an independent risk factor for peri-operative nerve injury [4]. However, the American Society of Regional Anesthesia (ASRA) committee maintains that deliberate needle-nerve contact (NNC), intraneural needle insertion, and intraneural injection should all be avoided due to the potential risk of PNI [2].

To date, three methods of identifying the exact location of peripheral nerves for PNB have been utilised in clinical practise:

1.5. Commercially Available Inline Manometers

The BBraun BSmart^TM injection pressure manometer (BBraun Melsungen AG, Melsungen, Germany) is a commercially available mechanical pressure monitor. The BBraun BSmart™ is intended to reduce inter-individual variability and facilitate rapid termination of injection when high pressures are detected during PNB. The device is pictured in Fig. (2).

Fig. (2). BBraun BSmart™ pressure manometer.

The single-use device is placed in-line between the syringe and injection tubing, prior to the entire system being primed with injectate. Fluid from the syringe passes through the pressure monitor, into the extension tubing, and into the needle. The manometer has a piston, which is forced upwards during injection, proportionately to the pressure generated during the injection. The piston is colour-coded. Injecting into compliant tissues will cause the piston to rise up to display the white bar on the piston (< 15 psi). Increased resistance results in the piston rising higher to display the yellow bar (15-20 psi). Pressures greater than 20 psi expose the red bar on the piston. This may be due to NNC intraneural injection, or the needle lying against bone, tendons or fascial planes. If this is detected during clinical use, injecting should cease immediately, and the needle repositioned. An increase in pressure detected during PNB injection is not always a result of incorrect needle tip placement. Injection pressure may be elevated as a result of the speed of injection, the dimensions of the needle and the length of integrated injection tubing. [14, 15].

Gadsden et al. performed a prospective observational study in patients undergoing elective arthroscopic shoulder surgery with interscalene brachial plexus PNB [10]. Deliberate NNC consistently generated injection pressures above 15 psi. When the needle tip was greater than 1mm from the nerve root, opening injection pressures were persistently less than 15 psi, with a mean peak pressure of 8.2 psi. Therefore, the authors recommend that opening pressures greater than 15 psi during PNB should result in immediate discontinuation of injection. Similar studies performed on cadavers confirm these pressure limits. Krol et al. confirmed injection pressures of 29.4 ± 9.3 psi during intraneural injections, compared to 7.2 ± 2.5 psi during extraneural injections [16]. Vermeylen et al. found deliberate intraneural injections resulted in opening injection pressures (OIP) of 21.5-25.8 psi. Perineural injections resulted in a lower OIP of 3.8-6.1 psi [17]. To date, there are no cases published that report clinically significant neuropathy in the presence of low injection pressures. Thus, injection pressure monitoring may prove most useful as a negative predictor of PNI [18].

Increasing the speed of injection generates higher flow rates. This leads to an increase in pressure values recorded by inline pressure monitors. However, it is a result of the inherent resistant in the needle and connecting tubing, rather than resistance at the needle tip [15]. Patil et al. recommend that injection flow rates should not exceed 15 ml/min (0.25 ml/s) [11].

Peri-operative PNI is the third most common cause of litigation related to anaesthetic practise [19]. Permanent nerve damage following PNI may result in a devastating loss of function and chronic pain in the affected limb, with a significant reduction in the patient’s quality of life [20].

Studies have validated the BBraun BSmart™ as an accurate digital pressure manometer at identifying high injection pressures [21]. However, the literature has not yet confirmed if the BBraun BSmart™ does indeed successfully prevent anaesthetists or anaesthetic assistants from injecting at unsafe pressures.

The primary objective of this study was to determine if the use of the BBraun BSmart^TM manometer successfully prevents 90% of anaesthetists and anaesthetic assistants injecting at pressures of greater than 15 psi during simulated PNB. Secondary objectives included the measurement of peak injection pressures (psi) generated during simulated PNB, and average flow rates (ml/min) during simulated PNB.

This was a prospective observational study, carried out at a central London teaching hospital.

Inclusion criteria consisted of the following: permanent employees, anaesthetists and anaesthetic assistants, and familiarity with performing injections for PNB. Anaesthetic assistants may be either Operating Department Practitioners (ODPs) or Anaesthetic Nurses (ANs).

Exclusion criteria consisted of the following: refusal to participate in the study, loss of capacity to consent to the study, unfamiliarity with performing injections for PNB, physical inability to depress the syringe plunger, inability to visualize the BBraun BSmart^TM piston.

The first part of the study involved sourcing and purchasing the equipment we required in order to display, and record, the injection pressures generated during simulated PNB.

The company representative from BBraun was contacted. BBraun kindly donated 10 BBraun BSmart^TM pressure monitors to be used in the study. These have a commercial value of approximately £5 each.

A PendoTECH pressure sensor and PendoTech PressureMAT^TMS were purchased from the United States (PendoTECH, Princeton, New Jersey). These two electronic items are designed to be used in conjunction with each other. The pressure sensor is connected simultaneously to the PendoTECH PressureMAT^TMS, and in-line with the syringe and PNB needle extension tubing. The MAT^TMS functions as a pressure monitor, high-pressure alarm, and a transmitter [22]. According to the manufacturer’s website, the portable, lightweight monitor does not need calibration or maintenance [22]. The PressureMAT^TMS has a digital display where the process pressure can be read in either kPa or psi. There is a button to zero tare to atmospheric pressure prior to each measurement. Fig. (3) shows a PendoTECH PressureMAT^TMS displaying a pressure measurement in psi.

The next step of the study involved assembling the equipment and simulating a PNB with the study participants. Two sets of study equipment were assembled.

The first set was used to demonstrate the correct use of the BBraun BSmart^TM manometer in clinical practise: A 20ml syringe was connected to the BBraun BSmart^TM manometer, which in turn was connected to the extension tubing of a 100 mm Pajunk Stimuplex Ultra^® regional anaesthesia needle. The syringe contained water. This set of equipment was not used to gather data during the experiment, only to facilitate candidate’s practise using the BBraun BSmart^TM.

The second set of equipment was used to gather the study data. Regional anaesthesia needles were not required for this stage of the study. A 20 ml syringe was connected to the BBraun BSmart^TM manometer, which in turn was connected to PendoTECH pressure sensor. The PendoTECH pressure sensor cable was in turn connected to the proximal end of extension tubing of a LogiCal^® invasive pressure monitoring kit. This extension tubing was used in place of the extension tubing of a PNB needle: The distal end of the LogiCal^® extension tubing has a three-way tap which the study team could secure in a partially closed position. By creating a constant restriction to flow during injection, we simulated an increased resistance during PNB injection. Fig. (4) shows the assembly of study equipment.

The PendoTECH pressure sensor was connected to the PressureMAT^TMS and coupled to a computer. This allowed digital recordings of the injection pressures (psi) and to record the duration of injection during simulated PNB.

In summary, the equipment assembly was: Syringe - BBraun BSmart^TM - PendoTECH pressure sensor - extension tubing - three-way tap.

Fig. (3). The PendoTECH Pressure MATTMS monitor.

Fig. (4). Assembly of study equipment.

For each candidate, the principal investigator (PI) performed a demonstration of simulated PNB. A 20 ml syringe filled with water was attached to the BBraun BSmart^TM inline pressure monitor, which in turn was connected to the proximal end of extension tubing of a peripheral nerve block needle. The PI flushed the system with water, then performed a simulated injection, demonstrating how the BBraun BSmart^TM monitor’s piston works. This was done by the PI manually occluding the extension tubing to simulate increased resistance. The BBraun BSmart^TM colour coding and pressure readings were explained to each participant. The clinical relevance of these pressures was discussed, notably that injecting should cease at pressures above 15 psi. Following this demonstration, participants were allowed to perform a single practice injection whilst being able to see the BBraun BSmart ^TM inline pressure monitor.

Following this, the study equipment was assembled as described above, with the PressureMAT^TMS digital display obscured from the candidates. The participants were asked to imagine a hypothetical clinical scenario where they were assisting a colleague in performing regional anaesthesia and were responsible for administering the local anaesthetic injection. Participants were asked to inject 20 ml saline into a beaker on two sequential occasions. They were to do this at the rate and pressure that they would normally use in their clinical practice when performing PNB. Candidates were requested to aspirate every 5 ml, as is common in clinical practise to exclude intravascular injection. Thus, injecting 20 ml took place in 4 phases of 5 ml injections.

The first 20 ml injection was performed with the participant unable to see the piston of BBraun BSmart^TM, it was obscured by a piece of cardboard. The PressureMAT^TMS recorded the pressures in psi generated during each phase of the injection, and the total duration of time in seconds over which the injection was performed. The PendoTECH PressureMAT^TMS is programmed to sample process pressures every second.

The second 20 ml injection was then performed with the participant able to see the BBraun BSmart^TM piston. The PressureMAT^TMS again recorded the pressures generated during each phase of the injection, and the time over which the injection was performed.

Forty participants were recruited in total. For one volunteer, the PendoTECH software did not record any pressure-time data, so was excluded from the subsequent analysis.

Two pressure-time curves for each of the 39 candidates were available for analysis. The pressures generated during simulated injection were recorded at a sampling rate of 1 per second (1Hz).

The first pressure-time curve was recorded with the candidate injecting 20 ml saline, whilst unable to see the BBraun BSmart™ manometer. During the second injection of 20ml, the participant was able to visualise the manometer.

The study group consisted of 23 (59%) women and 16 (41%) men. Regarding occupation, there were 26 (67%) anaesthetists and 13 (33%) Operating Department Practitioner (ODPs) or Anaesthetic Nurses (ANs).

Table 1 below displays the number of years of clinical experience for each of the candidates. In the case of ODPs/ANs, this refers to the time since qualifying in this field. For anaesthetists, this is taken as the time since entering the specialty of anaesthesia.

Table 2 below displays the estimated number of PNBs assisted or performed per year for each candidate.

Pressure-time curves were constructed for each candidate. An overview of the curves for all participants is displayed below in Fig. (5). For each candidate, the red line represents the first phase of the study with the BBraun BSmart™ manometer obscured from view. The blue line represents the second 20ml injection, with the BBraun BSmart™ manometer visible to the candidates. In clinical practice, the 20 ml syringe was injected in 5 ml aliquots, resulting in four pressure peaks and four pressure troughs.

During the study, a total of 32 peak pressures during the 78 procedures (BBraun Bsmart™ manometer visible and non-visible) were recorded above the recommended upper limit of 15 psi. This is 41% of the total injections performed by the candidates. Of these 32 episodes, 19 (59%) injections reached pressures above 20 psi. A total of nine (28%) injections generated pressures greater than 25 psi.

The peak pressure rose above 15 psi in 24/39 (62%) injections when the candidates were unable to see the BBraun Bsmart™ manometer but only in 8/39 (21%) injections when the manometer was visible (p = 0.0004 Fisher’s exact test). With a null hypothesis that the BBraun BSmart™ device would not prevent 90% of subjects injecting at pressure over 15 psi, an exact binomial test was performed, which gave a one-sided p-value of 0.037. This makes the observed data consistent with the null hypothesis. In this study, we set out to evaluate if the BBraun Bsmart™ does indeed successfully prevent 90% of anaesthetists or anaesthetic assistants from injecting at unsafe pressures. These results, therefore, demonstrate a negative finding for the primary outcome of our study.

A total of four pressures greater than 30psi were recorded, generated by three study participants. Three of four (75%) of these took place when the BBraun BSmart™ was hidden from the candidates. One candidate reached a peak pressure of 38.8 psi during the first injection, and a peak pressure of 32.8 psi while completing the second.

Using the BBraun BSmart™ did not result in a universal reduction of peak injection pressures from the first to the second injection, although this was a more common finding. For ten candidates, the peak injection pressure was higher when the BBraun BSmart™ was visible to them during the second phase of the study.

Fig. (5). Pressure-time curves for all candidates.

The use of the BBraun BSmart™ inline pressure manometer during simulated PNB injection did not prevent 90% of anaesthetists and anaesthetic assistants from injecting at pressures above 15 psi.

This result was unexpected. We had anticipated that the BBraun BSmart™ would be simple to use and result in 100% of simulated PNB injections being performed within a pressure range considered to be safe (<15 psi). The colour markings on the piston of the BBraun BSmart™ provide immediate visual feedback during simulated PNB and eliminate the subjective nature of “syringe feel.” In our study, despite the use of the BBraun BSmart™, 21% of participants injected at pressures > 15 psi during the second phase of the experiment. However, while this does not achieve our 90% aim, it is still a marked improvement on the first set of injections performed by the participants. During the first phase, 62% injections were above 15 psi. These results are similar to those obtained during a benchtop study by Claudio et al. [12]. When this group evaluated the accuracy of “syringe feel” during simulated PNB, they found 70% of subjects injected at pressures > 20 psi.

It is worth contemplating the reasons why using the BBraun BSmart™ did not successfully modify the behaviour of all anaesthetists and anaesthetic assistants. The possibility of the device being too complex to understand does exist. However, it is a simple piece of equipment. In addition, the PI explained how the manometer worked, demonstrated it to the candidates, and then gave the candidates the opportunity to use it prior to performing the study. A second possibility is that the manometer is slow to respond to increases in pressure, so there is delay before the candidates recognise the increase in pressure and respond. This is unlikely as it is a free-moving piston and is in-line with the syringe. Thirdly, it may be a result of the artificial nature of the study and the brief given to the participants. As a result of informing them we were calibrating the equipment, rather than explaining the true intention of the study, participants may have been less cautious when injecting than in a clinical setting.

Patil et al. noted that pressure monitors placed upstream from the needle, such as the BBraun BSmart™, may produce false-positive readings during higher flow rates of injection [23]. These false-positive readings are caused by the flow characteristics of the injectate changing from laminar to turbulent flow during rapid injection, or the inherent resistance to flow from the needle shaft [23]. Their study recommends restricting flow rates to ≤15 ml/min to reduce the likelihood of false-positive pressure readings due to proximal factors. This is especially applicable when using custom-designed PNB needles such as the Stimuplex Ultra^® in our study (BBraun Melsungen AG, Melsungen, Germany). These needles have integrated plastic extension tubing, which further increases the resistance of the system.

In our study, there was little difference in the average flow rates between the two phases of injecting. During both phases, over three-quarters of candidates exceeded the recommended flow rate of 15 ml/min. This is greater than the proportion in the study by Patil et al., in which fifty percent of their cohort were noted to inject at rates above the recommended 15ml/min or 0.25 ml/s [11]. This is another unexpected finding. We had assumed that the use of the BBraun BSmart™ would result in participants injecting with greater caution, at a slower rate.

While there are concerns regarding too high flows during PNB, it is equally possible that a missed intraneural injection may result from injecting at a much slower speed, where there is a very slow rise in pressure.

Using the BBraun BSmart^TM pressure, the manometer did not fully prevent unsafe injection pressures being generated during simulated PNB. However, 79% of anaesthetists and anaesthetic assistants injected only at safe pressures when able to view the BBraun BSmart™ pressure manometer. When utilised in conjunction with PNS and ultrasound guidance, this may offer additional safety to the patient.

Measurement of injection pressures may be particularly useful when the image obtained on ultrasound is suboptimal, or as an adjunct when trainees are performing PNB [10]. Additionally, it may also be of some reassurance to anaesthetists to have injection pressures objectively monitored when not self-administering the local anaesthetic [11].

We recommend incorporating the BBraun BSmart™ manometer as part of routine clinical care when performing PNB to reduce the rare yet devastating occurrence of PNI. Perhaps in using a combination of imperfect methods, we will be able to offer the greatest safety during PNB: “Safety is a mix of proper training, reliable monitors, good judgement, and plain old common sense [25].”