Nerve mapping is a technique whereby superficial peripheral nerves can be traced and located transcutaneously for peripheral nerve blocks during regional anaesthesia procedures. The technique enables the anesthesiologist to determine the site for needle insertion prior to puncturing the skin.

The current density radiates outward from the nerve mapping pen in a spherical form. There is also a decrease in behavioural density as the distance is increased from the source. Axons with a larger diameter exhibit a lower activation threshold than small axons. This results in electrical stimulation activating larger axons first before activating the smaller axons. Looking at the behavioural density, most of the axons will be activated close to the probe, whereas only the larger diameter axons will be activated further away from the probe. There are two options to increase the energy delivered to a nerve without changing the distance from the electrode – increase the current amplitude and/or increase the pulse width.

There is an optimal Pulse Width at which a specific nerve is most excitable. This is called the chronaxie threshold. It is preferable to keep the pulse width as close to this value for the related nerve or nerve plexus, as the Peripheral Nerve Stimulator allows, then increase only the current.

It should be noted that, although there are many published values for chronaxie for various excitable tissues, the range of variability for a given tissue type is quite large. It is generally assumed, however, that nerves can be classified according to their chronaxie thresholds as follows:

From the above table, it would seem reasonable to deduce that the ideal pulse width to facilitate a motor nerve response (A alpha), would be around 100μs. If one sets the nerve stimulator at 100μs and increases the amplitude to 5mA giving a total charge of 500nC one would not get the same muscle response as if the setting is at 500μs and 1mA, also giving a total charge of 500nC. In the second case even though the total charge transferred to the nerve is the same, because of the chronaxie threshold of 100μs for the nerve, much of the energy transferred to the nerve after the 100μs is wasted on the nerve.

This is clearly shown by the graph below. The strength-duration curve (green) indicates the current necessary at the different pulse widths to facilitate a contraction. The energy cost or total charge is shown by the blue curve. It can be seen that the stimulation is the most energy efficient at the chronaxie pulse of +- 80μs width as would be expected. It should be noted how the energy cost increases when pulse width increases.

As a preference, keep the nerve stimulator at a 100μs pulse width and adjust the current. If the nerve stimulator is already set at 20mA and the Nerve Mapping Probe does not elicit any neuromuscular response, increasing the pulse width to 300μs will offer 3 x more charge, however bear in mind that the net effect on the nerve will not constitute a contraction which is 3 times more powerful.

Due to the fact that the surface location of the nerve is pre-determined and thus the optimal entry point for the needle, the technique reduces the need for multiple needle insertions and discomfort to the patient. It also reduces the time to perform the peripheral regional nerve block.

The nerve mapping technique may be used for various approaches to the brachial plexus, as well as the axillary, musculo-cutaneous, ulnar, median and radial nerve blocks of the upper limb; and the femoral, sciatic and popliteal nerve blocks in the lower limb. Surface nerve mapping is particular useful where classic anatomical landmarks are absent or difficult to define, for example in children with contractures (arthrogryposis multiplex congenital; burns) or with major congenital limb defects.

It relates the charge intensity necessary to elicit a neuromuscular response to the distance between the probe and the nerve.

In order for nerve excitation to take place, the energy delivered to the nerve must be high enough to trigger the threshold voltage. The current-distance relationship is governed by Coulomb’s Law:

E = K(Q/r²)  where E is the energy required, K is a constant, Q is the minimum current and r is the distance away from the electrode.

The equation shows what effect the distance has on the energy delivered .: 2 x Distance = ¼ Energy

The equation also shows how you need to increase the current as your distance increases .: 2 x Distance = 4 x Current

The Stimpod peripheral nerve stimulator allows the user to use the needle and probe simultaneously, eliminating the need to change cables or make markings on the patient.

The practical application of nerve mapping as a technique has always been a cumbersome technique. In order to use nerve mapping effectively, the position and angle of the probe must be recorded exactly before inserting the needle. Marking the probe position with a marker has proved unsuccessful and cumbersome, as this does not capture the exact position and angle relating to subcutaneous structures – i.e. bone structure, muscles etc. It is important to have the ability to keep the probe in its original position when inserting the needle. This will ensure exact positioning and angle of the needle.

The Stimpod peripheral nerve stimulator facilitates this procedure by providing a combined Nerve Mapping/Locating cable. The unit will automatically switch between the probe and the needle, depending on which device is in contact with the skin. The Stimpod will guide you through the entire procedure, switching between the probe and the needle as needed whilst keeping an eye out for high impedance and nerve proximity.

It means that the maximum Voltage that the nerve stimulator can deliver is not enough to accommodate for the higher impedance encountered by the circuit.  Depending on the pulse width of the square section of the waveform, the contraction from the stimulated muscle may or may not represent an artefact.

When one stimulates with a good current source, the shape and amplitude of your stimulus pulse will always be as selected, as long as the nerve stimulator can deliver the voltage required to accommodate for the varying circuit impedance. All brands of nerve stimulators are limited in the way they can accommodate varying impedances by their maximum voltage.

The figure above shows a typical current and voltage stimulation response. V (Voltage – channel 2) is measured across the two electrodes connected to a subject’s body. I (Current – channel 1) is measured over a 10Ω resistor connected in series with one of the electrodes. The maximum current as displayed in this picture is 5mA. The maximum voltage necessary to facilitate this is approximately 40V. Even though stimulation was done with a 5mA, 1ms square wave stimulus, the approximately 80μs negative current component is indicative of the reactive impedance of the combined electrode, tissue impedance.

The figure above shows the nerve stimulator at the same settings, however, the impedance of the electrode/ epidermis interface was increased to a level where the nerve stimulator cannot supply enough voltage to facilitate the increased impedance. It is clear that after approximately 140μs the nerve stimulator could not deliver the required voltage. The current immediately dropped to around 4mA. According to the discussion on chronaxie thresholds, though, it is quite likely thou that the second waveform will elicit a very similar response to the first waveform. This is due to the fact that the second waveform is ‘square’ for the first 140μs while the chronaxie of the nerve is 100μs. This means that the drop in current (charge) supplied after 140μs would have a limited effect on the nerve due to the fact that it would’ve been ‘wasted’  in any case.

One approach to offering the user an indication of the expected net stimulus effect would be to average out the total current delivered. This would give the user the impression that the observed response was equivalent to a perfect square wave of 1ms pulse width and 3.7mA amplitude. However, due to the discussion in the paragraph above, it should be noted that most of the neuromuscular stimulating response was most likely facilitated in the first 100μs of the stimulus at 5mA. It could then be misleading to simply look at the average stimulating amplitude. In other words, it could be argued that the stimulation as indicated in the first figure and the one in the second figure could elicit a similar neuromuscular stimulating response (contraction) with the electrodes positioned at exactly the same distance from the target nerve. If one then relied on the information presented of an actual average current transferred, one would have the erroneous impression that the cathode in eliciting a response in the case of the second figure would be closer to the nerve than the cathode that elicited the same response in the case of the first figure.