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Functional Monitoring

   

Jeff Gadsden, MD. Published by B. Braun Medical Inc.

Advantages

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 result, 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 (0.3-0.5 mA) current.

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.


Serves as a useful functional confirmation of the anatomical image shown on the ultrasound screen (e.g. “is that the lateral or inferior cord?”)


Can help guide the needle if the US image is poor.

Limitations

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).1


The value of the nerve stimulation information may diminish with multiple injections.