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The first demonstration of electrical nerve stimulation was performed as early as 1780 by Luigi Galvani on a frog. Galvani touched the nerves of the frog's spinal cord with metal electrodes which caused contractions of the leg muscles. However, the electrophysiologic mechanisms behind this experiment were not understood until much later.
Although these ancient principles of nerve stimulation were well known and understood, its practical use for location of nerves for plexus anesthesia has become popular only within the past two decades.
The reliability of peripheral nerve stimulation was greatly improved by the introduction of constant current sources for the electronic stimulators making it the procedure of choice for safe and reliable nerve location in plexus anesthesia. 2. Neurophysiology and Electrical Nerve Stimulation 2.1 Basics The basic principles underlying peripheral nerve stimulation are the electrophysiological properties of nerves and muscles: Ohm's Law and Coulomb's Law. All living cells exhibit a voltage of approximately 90 mV (measured inside the cell relative to the outside) across their membrane. This voltage is called the membrane potential or resting potential (see Figures 1a, 1b, and 2). Nerve and muscle cells have the unique capability of creating short electrical impulses, so-called action potentials, in response to appropriate stimuli. The stimuli cause a depolarization of the membrane (decrease of the membrane potential). Action potentials (nerve impulses) are of uniform amplitude (approximately 120 mV), shape, and duration (approximately 1ms in vertebrates) [see Figure 2].
 Figure 1a Once the stimulus exceeds a certain strength (threshold level), an action potential is generated and propagated along the membrane of the nerve cell — especially along its long axon or nerve fiber which connects to other nerve cells or muscles.
 Figure 1b Depending on the strength and duration of the stimulus, a series of action potentials is generated whereas the amplitude of the stimulus is mainly represented by the frequency of the nerve impulses. In this way, information is encoded and transmitted through our nervous system from the periphery (coming from receptor cells) to the central nervous system (CNS); where the information is processed; and from the CNS to the periphery (via so-called motoneurons) where the muscles are activated to move our body. 2.2 Sensory and Motor Nerve Fibers There are various types of nerve fibers in our body which differ in their diameter and kind of insulation; and thus have different velocities of impulse propagation. Low speed (»1 m/s) pain fibers (transmitting numb pain) have a small diameter and no specialized insulation (see Figure 2). The very fast (up to 120 m/s) motor neurons supplying (for example) the muscles in our limbs have a large diameter and a special insulation (myelin sheath).
 Figure 2 In addition to the differences in impulse propagation velocities, they also differ in their excitability to external electrical stimuli because more charges need to be moved in non-insulated fibers to reach the same level of depolarization of the membrane; which means that it takes more time for a given stimulus to reach the threshold level (see Figures 1a and 1b). 2.3 Rheobase and Chronaxy
 Figure 3 Figure 3 shows the overall threshold curves for different stimulus amplitudes and durations for low speed and high speed fibers. The rheobase is defined as the lowest stimulus amplitude eliciting an action potential using long stimulus durations (stimulus duration is also called impulse width). Chronaxy is defined as the stimulus duration at the point where the threshold amplitude is two times the Rheobase. Electrical stimuli with the duration of the Chronaxy are very efficient (at relatively low amplitudes) to elicit action potentials. If the stimulus duration is too short, the stimulus may not elicit an action potential at any amplitude (e.g. pain fibers are not excited with stimulus durations of 0.1 ms, see also Figure 2). 2.4 Ohm's Law, Current Sources In order to move the appropriate amount of charges at the nerve membrane to depolarize it up to the threshold level and to elicit an action potential, it is necessary to apply a stimulus with a given duration (e.g. 0.1 ms) at a sufficient amplitude level (current). In order to achieve such a well defined stimulus, it is essential to use a stimulator device providing a constant current source which compensates for the variation in tissue resistance while the stimulation needle is approaching the nerve and passing through different layers of tissue. Ordinary current sources only provide the selection of the output voltage, which means that the current would increase/decrease when the resistance in the circuit decreases/increases (Ohm's Law). A constant current source provides selection of the current and automatically adjusts the output voltage properly if the resistance changes. 2.5 The Stimulation Needle Approaching the Nerve
3. Standards for Good Nerve Stimulation Equipment
Electrical Features (according to Kaiser 1990)5
Stimulation Needles
Needle Insulation
Non-insulated needles exhibit a relatively flat threshold curve (see Figure 5a, top), i.e. the nerve localization is not very accurate — especially after the needle tip has already passed the nerve. Use of this type of needle may lead to higher than anticipated block failure rates.
 Figure 5a Insulated needles with conductive bevels concentrate the stimulation current to the bevel area and thus provide good nerve location capabilities (steep threshold curve, see Figure 5a, bottom).
 Figure 5b Completely insulated needles with a pin-point electrode provide optimal conditions for accurate nerve location (see Figure 5b). The complete coating of the bevel results in non-cutting atraumatic puncture characteristics. The 15° bevel (Figure 5b, top) requires less puncture force as compared to the 30° bevel type (Figure 5b, bottom).
Connectors
4. Distinction Between PNS and TENS Units
4.1 Use of PNS (Peripheral Nerve Stimulation)
PNS is the localization of nerves for the purpose of peripheral nerve blockade. The blocks are typically applied in a dedicated block area, the pre-op holding area, or in the operating room.
TENS is the intra-operative assessment and control of muscle relaxation. The stimulating currents are typically applied only in the operating room.
The strong current pulses may cause nerve damage, and, if current flows transthoracic, it may cause severe arrythmia or even cardiac arrest.
4.3 Resumé
5. Summary
The B. Braun peripheral nerve stimulators - Stimuplex® Dig RC and the new Stimuplex HNS11 - have been designed according to the most modern technical aspects and requirements which originate from the theory and practice of peripheral electrical nerve stimulation. Click below to learn more about either device.
Stimuplex Dig RC | Stimuplex HNS11 |
In 1912, Perthes developed the first electrical nerve stimulator for selective stimulation of nerves and the corresponding muscles.
The amount of current or charge, respectively, received by the nerve membrane primarily depends on the distance between the electrode (needle) tip and the nerve. This means that the muscle reaction increases when the needle is approaching the corresponding nerve and the current can be decreased again to the threshold level.
This situation is illustrated in figures 5a and 5b. Step-by-step the current is decreased while approaching the nerve until the desired threshold current (usually between 0.2 and 0.5 mA at 0.1 ms stimulus duration) is reached. The needle tip is now close enough to the nerve for injecting the local anesthetic, but is still at a safe distance regarding the risk of touching the nerve.
Again, the procedure described above is more valid if the nerve stimulator device provides an accurate constant current source.
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