Electrical Characteristics Of Peripheral Nerve Stimulators Currently Available In Clinical Practice

To determine which electrical characteristics of the PNS contributed to the localization of a peripheral nerve, the electrical properties of eight commercially available peripheral nerve stimulators and a Grass S-88 stimulator were measured. The PNS was then used in the laboratory to locate a peripheral nerve in an anesthetized cat. The controlled environment in the laboratory allowed each PNS to be used under nearly identical conditions.

Experiment I: Determination of Output Characteristics of Peripheral Nerve Stimulators

To determine the shape and duration of the stimulating pulse, the linearity of the output, and the change in the output in response to a change in load resistance, the peripheral nerve stimulators were connected to a variable resistance and an oscilloscope (Tektronix Type 564B) as shown in Figure 17-6 . The shape, duration, and amplitude of the output were measured on the oscilloscope screen. The linearity of the output was determined by correlating the current output of the PNS with the dial (or slide) setting of the same PNS. The change in the output of the PNS as the load resistance was changed from 1000 Q to 2000 Q was monitored on the oscilloscope. The current output of the peripheral nerve stimulator was calculated from the relation, V=I*R. V is the voltage output as measured on the oscilloscope and R is the known load resistance. The known resistance in this experiment is approximately that encountered when the PNS is connected to a patient.

Result. The effect of increasing the load resistance on the current and voltage output was determined first. For a twofold increase in resistance, the current delivered by the PNS dropped an average of 10% (range: 1%-24%). Next, the linearity of the peripheral nerve stimulators was determined. A completely linear PNS delivered X5 of its maximal output when the dial was set at x%. As can be seen from Figure 17-7 , some PNSs deviated greatly from being linear, whereas others were quite linear. Finally, all but one of the PNSs studied delivered a square wave of duration between 200 ^sec and 1000 ^sec. The exception delivered a triangular pulse.

Figure 17-5 A, Effect of 2 mL of air on amplitude of the compound potential. B, Effect of 2 mL of saline on the median nerve in infraclavicular block. C, Effect of 1 mL of lidocaine on the same nerve. (From Raj PP, Rosenblatt R, Montgomery SJ: Use of the nerve stimulator for peripheral blocks. Reg Anesth 5:14-21, 1980.)

Figure 17-5 A, Effect of 2 mL of air on amplitude of the compound potential. B, Effect of 2 mL of saline on the median nerve in infraclavicular block. C, Effect of 1 mL of lidocaine on the same nerve. (From Raj PP, Rosenblatt R, Montgomery SJ: Use of the nerve stimulator for peripheral blocks. Reg Anesth 5:14-21, 1980.)

Experiment II

To determine how the different electrical properties of the PNS affected the localization of a peripheral nerve, the peripheral nerve stimulators were used in the laboratory to locate a peripheral nerve in an anesthetized cat. Six cats of either sex weighing between 2.5 kg and 3.5 kg were used for this study. The cats were obtained from Kaiser Lake, St. Paris, Ohio, and housed in the Department of Laboratory Animal Medicine until used. On the day of the experiment, the cat was given 100 mg of ketamine and 0.1 mg of atropine intramuscularly. After the appropriate areas for surgery were shared, the cat was intubated and placed on a respirator. Anesthesia was maintained with 0.2% to 0.5% methoxyflurane, 67% N2 O, and the balance of O2 . Venous and arterial lines were established in the external jugular vein and carotid artery, respectively. Blood pressure and heart rate were monitored continuously. Blood gases were taken every 60 minutes to ensure that normal acid-base status was maintained.

To monitor evoked muscle twitches produced by the PNS, the sciatic nerve-tibialis muscle preparation of the cat was chosen as a model. The superficial tendons of the tibialis muscle on the dorsal surface of the right hind paw were exposed. After clamping the quadriceps muscle and the paw to immobilize the leg, the most lateral tendon

Electrical Nerve Stimulator Anesthesia

Figure 17-6 Schematic diagram of the apparatus used in experiment I. The oscilloscope (left), variable resistance (center), and the peripheral nerve stimulator (right) are connected in parallel. (From Ford D, Pither C, Raj P: The use ofperipheral nerve stimulators for regional anesthesia. A review of experimental characteristics, technique, and clinical applications. Reg Anesth 9:73—77, 1984.)

Figure 17-6 Schematic diagram of the apparatus used in experiment I. The oscilloscope (left), variable resistance (center), and the peripheral nerve stimulator (right) are connected in parallel. (From Ford D, Pither C, Raj P: The use ofperipheral nerve stimulators for regional anesthesia. A review of experimental characteristics, technique, and clinical applications. Reg Anesth 9:73—77, 1984.)

was attached to an F-1000 Microdisplacement Muograph, transducer (Norco Biosystems, Inc.). Then the remaining tendons and the Achilles tendon were severed. Finally, the force transducer was connected to a strip recorder (Physiography, Model DMP-4B) to record the evoked muscle twitches ( Fig. 17-8 ).

The needle (6.3 cm*22 gauge [G] spinal needle, uninsulated) was mounted in a one-dimensional manipulator and aligned to approach the sciatic nerve through the hamstring muscle. The needle was advanced through the

Figure 17-7 Linearity of the peripheral nerve stimulators. The percentage of output delivered at a given setting was plotted along the Y axis, and the percentage of dial setting was plotted along the X axis. A linear response is shown by a straight line starting at the origin and going to the (100%) point. (From Ford D, Pither C, Raj P: Electrical characteristics of peripheral nerve stimulators: Implications for nerve localization. Reg Anesth 9:73—75, 1984.)

Peripheral Nerve Stimulator Diagram

Figure 17-8 Schematic diagram of the apparatus used in experiment II. The cathode of the peripheral nerve stimulator was attached to the needle (arrow) and the anode to the cat through a 1000 Q resistance. To measure the current delivered by the peripheral nerve stimulator, the voltage drop across the 1000 Q resistance was measured on the oscilloscope and related to current by: Current = V/1000. To measure the voltage drop delivered by the peripheral nerve stimulator, the line labeled "Current" was disconnected and connected as shown by the dashed line labeled "Voltage." The tibialis muscle was attached to a force transducer and the muscle twitches recorded on a strip recorder. (From FordD, Pither C, Raj P: Electrical characteristics ofperipheral nerve stimulators: Implications for nerve localization. Reg Anesth 9: 73—75, 1984.)

Figure 17-8 Schematic diagram of the apparatus used in experiment II. The cathode of the peripheral nerve stimulator was attached to the needle (arrow) and the anode to the cat through a 1000 Q resistance. To measure the current delivered by the peripheral nerve stimulator, the voltage drop across the 1000 Q resistance was measured on the oscilloscope and related to current by: Current = V/1000. To measure the voltage drop delivered by the peripheral nerve stimulator, the line labeled "Current" was disconnected and connected as shown by the dashed line labeled "Voltage." The tibialis muscle was attached to a force transducer and the muscle twitches recorded on a strip recorder. (From FordD, Pither C, Raj P: Electrical characteristics ofperipheral nerve stimulators: Implications for nerve localization. Reg Anesth 9: 73—75, 1984.)

skin and each PNS was connected in sequence to the cat and the needle. The voltage output, current output, and dial setting, along with the muscle twitch strength, were recorded. The needle was advanced in 0.5-cm increments and the recording repeated until the femur was reached or the needle was clearly past the nerve.

Next, the needle was moved in and out until the position was found where the minimum current caused a muscle twitch. This maneuver simulated the clinical situation of manipulating the needle to find the closest approach to the nerve.

At the conclusion of this experiment, with the needle 1 to 2 cm beyond the sciatic nerve, a cutdown to the nerve revealed how closely the needle approached the nerve ( Table 17-4 ). By withdrawing the needle until the tip was again even with the nerve, the depth of the nerve was determined. A repeat series of measurements was made 2 to 3 cm distally on the nerve.

Result. In rotating the PNS through this series of experiments, five favorable electrical characteristics were discerned. They were (1) a linear output, (2) high and low output ranges, (3) a short stimulation pulse, (4) clearly

TABLE 17-4 -- POLARITY OF STIMULATION Anodal vs cathodal current required to stimulate peripheral nerve X 4.57 x 4.3

Data from Ford DJ, Pither CE, Raj PP: Electrical characteristics of periph eral nerve stimulators: Implications for nerve localization. Reg Anesth 9:73—77, 1984; and from BeMent SL, Ranck JB Jr: A quantitative study of electrical stimulation of central myelinated fibers. Exp Neurol 24:147—170, 1969.

marked polarity, and (5) constant current output. In addition, three design features that also contributed to finding a peripheral nerve were noted. These were a large, easily turned dial; a digital output meter; and a battery check.

Experiment III: Strength Duration Curve

Because all of the commercial PNSs examined used a stimulus pulse width between 2000 microseconds and 1000 microseconds, it was not possible to determine the effect of a shorter pulse (<200 ^sec) on the peripheral nerve localization procedure used in Experiment II. Therefore, to determine the effect of pulse width, the Grass stimulator, which has a variable pulse width, was used to construct a strength-duration curve.

Result. The results of this experiment are shown in Figure 17-9 . The Y-intercept is the rheobase and the pulse duration corresponding to twice the rheobase is the chronaxy. The dotted lines in Figure 17-9 show the change in stimulating current as the needle is advanced toward the nerve from 1 cm away. The shorter the pulse duration, the greater the change. For a short pulse (40 ^sec), there was a change of approximately 29 mA. For a long pulse (1000 ^sec), the change was 2.6 mA. Uninsulated needles gave the same qualitative results, but more current was required (data not shown).

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