Recent evidence has emerged to suggest that the use of multiple monitors during nerve blockade—particularly ultrasonography, nerve stimulation and injection pressure monitoring—provides the optimal environment with which to avoid nerve injury.1
The use of synergistic, complementary monitors provides a greater level of “situational awareness” to help guide more informed decisions on needle placement and injection. The strength of each monitor can be leveraged to cover the limitations of another. The nerve block procedure can be documented with clear, objective values that can be communicated in the future.
The incidence of complications from general anesthesia has been reduced significantly in recent decades, largely due to advances in cardiorespiratory monitoring.2 The use of objective monitors such as pulse oximetry and capnography allow practitioners to quickly identify changing physiologic parameters and intervene quickly and effectively. The practice of regional anesthesia has traditionally lacked a set of similar objective monitors that aid the practitioner in preventing injury.
Peripheral nerve blockade is a widely practiced technique used to provide pain control during and after surgical procedures. While nerve blocks are very effective and generally considered safe, there remain some risks associated with their performance.3
Nerve injury and resulting postoperative neurologic dysfunction (PONS) is a feared complication of peripheral nerve blockade. Estimates of its incidence vary from as little as 0.18% to as much as 16%, depending on the site the block was performed on and the technique.3, 4-6 The majority of neurologic symptoms following nerve blockade are transient and last days to weeks, but some are permanent and can result in profound disability.
Not only does the patient suffer the physical and emotional burden of a prolonged or permanent injury, there are other costs associated with these adverse events. These include the cost of prolonged length of stay and/or neurologic testing, the emotional and financial strain of missed work, and the negative impact on the patient’s day-to-day quality of life. Many patients with long-term nerve injuries following a nerve block eventually seek compensation through the legal system, incurring further expenses for both the patient and for the caregivers and facility.
Evidence Supporting ISM
The debate about whether Ultrasound is superior to Nerve Stimulation is counterproductive. Each monitor has its set of advantages and limitations.
Ultrasound can monitor needle advancement and the spread of local anesthetic in real-time.
It is possible to see the needle in real-time and accurately guide the needle towards the target.
Ultrasound offers the ability to demonstrate local anesthetic spread on the screen image: If corresponding tissue expansion isn’t seen when injection begins and the needle tip is not where it is thought to be, the provider should immediately halt injection and relocate the tip of the needle.
Multiple injection techniques that were difficult or indeed dangerous to do in the era of nerve stimulation alone can now be performed, as the nerves can be deposited at various points under US guidance.
Ultrasound may reduce the likelihood of systemic toxicity by allowing providers to use less local anesthetic. Several studies have been published showing large reductions in the volume required to obtain an equivalent block to standard nerve-stimulation techniques.19-21
Adjacent structures of importance can be seen and avoided: Examples include blood vessels (both large and small), pleura, and other nerve structures in the vicinity of the target structure. A useful adjunct to the visualization of structures on the ultrasound screen is the ability to measure the distance from skin-to-target using electronic calipers. This coupled with needles that have depth markings etched on the side, confers a safety advantage by warning the provider of a “stop-distance”, or a depth at which he/she should cease advancement, reassess the needle visualization, and perhaps withdraw and start again.
The ability of ultrasound to prevent nerve injury is likely not fool proof.
Observing the needle tip in relation to the nerve is user-dependent, and the safety and precision can often be impeded by unfavorable echogenic characteristics of the tissue/needle interface.22
With current ultrasound technology, it is difficult to prevent or detect early enough an intraneural injection occurring on the ultrasound monitor. Krediet et al. performed sciatic blocks in cadavers, some of which were deliberately intraneural.23 Video clips of these blocks were then shown to both experts and novices in ultrasound guided regional anesthesia. The experts missed 16% of the intraneural injections, while the novices missed 35%.
Once injection has begun, even a miniscule amount of local anesthetic (e.g. 0.1ml) can produce neurological injury if intra-fascicular.24 However, the data indicates that the sensitivity of ultrasound to detect intraneural injection of 0.5 ml is only approximately 75%.23 Relying on the visual confirmation of tissue expansion may result in fasicular injury before expansion is detected by ultrasound. It is, in other words, probably too late.
The role of Nerve Stimulation is evolving in the era of Injection Safety Monitoring.
An absence of a motor response can be used to confirm needle tip placement prior to the first injection.
In attempts to get “close, but not too close” to the nerve to have the best block results, needles will occasionally but inevitably contact the epineurium or enter the substance of the nerve. It is for this reason that a reliable electrical monitor of needle tip position is a useful safety instrument.
There is a growing body of evidence to suggest that the presence of a motor response at a very low current (i.e. <0.2 mA) is associated with a very intimate needle tip placement (i.e. inside or directly contacting the epineurium). If a motor twitch is elicited at currents <0.2 mA, a prudent approach is to gently withdraw the needle until the motor response disappears and then attempt to re-elicit the twitch at the more appropriate current (0.3-0.5 mA).
- Voelckel et al. reported that when local anesthetic was injected at currents between 0.3-0.5 mA for sciatic nerve blocks in pigs, the resulting nerve tissue showed no signs of an inflammatory process, whereas injections at <0.2 mA resulted in lymphocytic and granulocytic infiltration in 50% of the nerves.25
- Tsai et al. performed a similar study investigating the effect of distance to the nerve on current required. While a range of currents were recorded for a variety of distances, the only instances in which the motor response was obtained at <0.2 mA was when the needle tip was intraneural.26
- Bigeleisen et al. conducted a study on 55 patients scheduled for upper limb surgery who received ultrasound-guided supraclavicular brachial plexus blocks.27 These authors set out to determine the minimum current threshold for motor response both inside and outside the first trunk encountered. The median minimum stimulation threshold was 0.60 mA outside the nerve and 0.3 mA inside the nerve. Importantly, stimulation currents of 0.2 mA or less were not observed outside the nerve, whereas 36% of patients experienced a twitch at currents <0.2 mA while the needle was intraneural.
Nerve stimulation serves as a useful functional confirmation of the anatomical image shown on the ultrasound screen (e.g. “is that the lateral or inferior cord?”)
It can help guide the needle if the US image is poor.
Stimulation is specific, but not sensitive.
Nerve stimulation does not appear to be able to discern between intraneural needle tip placement and direct contact with the epineurium. Wiesmann and colleagues demonstrated in a pig model that the minimal current threshold to elicit a motor response was similar for both needle-nerve contact and intraneural needle tip positions, irrespective of the pulse duration.28 This is consistent with data from a study done in patients undergoing interscalene block—needle tips positioned such that they were “gently indenting” the epineurium of the C5, C6 and C7 nerve roots elicited a motor response with a minimal current intensity of 0.2-0.3 mA (±0.3 mA).
The value of the nerve stimulation information may diminish with multiple injections.
Objective Injection Pressure Monitoring
Opening injection pressure monitoring reliably detects abnormal resistance to injection, regardless of who performs the injection or the number of needle repositions.
Use of other monitoring devices vary - Ultrasound is user, technique, equipment and anatomy-dependent; and nerve stimulation is specific, but relatively insensitive.
When injection pressure matters, an objective and quantifiable method of monitoring and documenting injection pressure is superior to the subjective syringe-hand-feel technique. Resistance to injection is a part of the standard documentation procedure during nerve blockade. The documentation typically refers to whether the resistance to injection was “normal” or “high” and the course of action if it was abnormal. In the past, documentation of the resistance was merely subjective and relied on the “learned feel” and experience of the provider.
Studies of experienced practitioners blinded to the injection pressure and asked to perform a mock injection using standard equipment reveals wide variations in applied pressure, some grossly exceeding the established thresholds for safety.29 Similarly, providers perform poorly when asked to distinguish between intraneural injection and injection into other tissues such as muscle or tendon in an animal model.30 It is therefore important to use an objective and quantifiable method of gauging injection pressure.
Monitoring a clinically relevant and objective pressure in real-time can help prevent an injection into a fascicle before it occurs. In a study of intraneural injections in canine sciatic nerves, a slow injection of local anesthetic while the neede tip was placed intra-fascicular was associated with an immediate and substantial rise in the pressure in the syringe-tubing-needle system (>20 psi), followed by return to low pressure (<5 psi) as the fascicle ruptured and local anesthetic leaked in the neighbouring tissue. In contrast, perineural and extrafascicular injections were associated with low opening injection pressure <15 psi.31 Moreover, those limbs in which the nerves were exposed to high injection pressures developed clinical signs of nerve injury as well as histological evidence (inflammation, disruption of the nerve architecture).
The implication is that injection into a low compliance compartment, such as within fascicles is associated with a high opening injection pressure, which if not stopped can either directly damage axons and/ or rupture the tough protective barrier-perrineurium leading to a nerve injury.
Objective injection pressure monitoring can help prevent injections against the epineurium and also reliably detects needle-nerve contact. In a clinical study of patients undergoing interscalene brachial plexus blocks using an automated injection pump, the observation of high (>15 psi) pressures prior to initiation of flow predicted the apposition of needle tip against the nerve trunk in 97% of cases as confirmed by ultrasound.1In other words, flow could not be initiated below the threshold level of 15 psi in the vast majority of instances where the needle was in contact with the nerve, and the injection was aborted. This is important, as initiation of flow against the epineurium may result in an intraneural injection or nerve inflamation, and consequent nerve damage.
It’s important to know what injection pressure provides and the limitations - and although highly sensitive, injection pressure is not specific.
Opening injection pressure can not distinguish between intrafascicular needle position or needle-nerve contact and other causes, such as needle obstruction by blood clot, tissue fascia, tendon, etc. Regardless, perineural injection normally requires low opening pressure (<15 psi) since it occurs in the loose connective tissue around nerves. Therefore, presence of high injection pressure (>15 psi) is rarely normal and should be avoided.
The accuracy of the dynamic injection, once injection has begun is impacted by numerous factors. It is important for practioners to understand that injection pressure monitoring is most clinically relevant for monitoring opening pressures, before the injection has begun.
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23.Krediet AC, Moayeri N, Bleys RLAW, Groen GJ: Intraneural or Extraneural: Diagnostic Accuracy of Ultrasound Assessment for Localizing Low-Volume Injection. Reg Anesth Pain Med 2014; 39:409-15
24.Selander D, Dhunér KG, Lundborg G: Peripheral nerve injury due to injection needles used for regional anesthesia. An experimental study of the acute effects of needle point trauma. Acta Anaesthesiol Scand 1977; 21:182–8
25.Voelckel WG, Klima G, Krismer AC, Haslinger C, Stadlbauer KH, Wenzel V, Goedecke A von: Signs of inflammation after sciatic nerve block in pigs. Anesth. Analg 2005; 101:1844–6
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27.Bigeleisen PE, Moayeri N, Groen GJ: Extraneural versus intraneural stimulation thresholds during ultrasound-guided supraclavicular block. Anesthesiology 2009; 110:1235–43
28.Wiesmann T, Bornträger A, Vassiliou T, Hadzic A, Wulf H, Müller H-H, Steinfeldt T: Minimal Current Intensity to Elicit an Evoked Motor Response Cannot Discern Between Needle-Nerve Contact and Intraneural Needle Insertion: Anesthesia & Analgesia 2014; 118:681–6
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30.Theron PS, Mackay Z, Gonzalez JG, Donaldson N, Blanco R: An animal model of “syringe feel” during peripheral nerve block. Reg Anesth Pain Med 2009; 34:330–2
31.Hadzic A, Dilberovic F, Shah S, Kulenovic A, Kapur E, Zaciragic A, Cosovic E, Vuckovic I, Divanovic K-A, Mornjakovic Z, Thys DM, Santos AC: Combination of intraneural injection and high injection pressure leads to fascicular injury and neurologic deficits in dogs. Reg Anesth Pain Med 2004; 29:417–23