Dualstim/Deluxe Model Ns-2ca/Dx Peripheral Nerve Stimulator

When using the DualStim/DeluxeTM, model NS-2CA/DX nerve stimulators, the auditory volume control knob should never be set at above 7. Set at any level below 7, this otherwise acceptable nerve stimulator will perform well and the LCD will consistently and accurately indicate the intensity of the delivered stimulus.

Peripheral nerve stimulator (PNS) assisted neuronal blockade is a commonly used technique for nerve localization during peripheral neuronal blockade. Because the intensity of the stimulating current gives an indirect indication of the needle-nerve distance, the ability of PNS to deliver a constant and precise current is crucially important.

For anesthesiologists who use DualStim/Deluxe^TM, model NS-2CA/DX nerve stimulators (Life-Tech, Inc., Houston, TX), it is important to know that the delivered current output of the nerve stimulator may be substantially less than that indicated by the LCD display. This happens when a control knob regulating the volume of the emitted beep signal that accompanies each stimulus delivered by the stimulator is set at its highest values (8 or 9). For instance, the three-digit LCD (Liquid Crystal Display) indicates that a 0.2 to 0.35 mA stimulus is being delivered even when the electrodes are not connected to the patient, or when the stimulus amplitude control dial is set to zero.

Because it is important to know the intensity of the delivered stimulus during the nerve localization, we tested 2 units of the Model NS-2CA/DX DualStim/Deluxe^TM nerve stimulators in our laboratory using an oscilloscope and a dummy load of 2 kohm. In both units the LCD indicated that a stimulus of 0.25 to 0.35 mA was delivered when the volume knob for the "beep" was set at 8 or 9 (the highest volume values), even when the stimulus intensity control knob was set at zero. At this setting, however, no measurable stimulus was delivered, regardless of whether the electric circuit was closed or open. Additionally, when the beep volume control knob is set at a value of 8 or 9, the LCD consistently displayed a stimulus intensity of 0.25 to 0.35 mA higher than actually delivered over the full range of current outputs. When the "beep" volume is decreased to 7 or below, however, the LCD will correctly indicate the intensity of the delivered stimulus.

The clinical implications of this findings are as follows:

1. Anesthesiologists who prefer to begin nerve localization using a lower stimulating current (33% of anesthesiologists begin nerve localization using stimuli of lesser intensity) may be misled by the display and attempt nerve localization while the nerve stimulator is not delivering a stimulus.
2. Since the stimulus being delivered is 0.25 to 0.35 mA weaker than the one displayed, this may mislead anesthesiologists to keep trying to obtain a nerve response with stimuli of lesser intensity (preferably 0.5 mA or less), while the observed response may already be adequate.
3. The LCD may indicate that the stimulus of 0.25 to 0.3 mA is being delivered even if the lead attachments become loose or disconnected. This in return may result in unnecessary attempts at nerve localization.

Being aware of this limitation, the manufacturer points out that the DualStim stimulators have been on the market for 15 years and have had an excellent acceptance and safety record. Since the electronic design in DualStim stimulators uses a constant voltage generator circuit, this unit is currently being replaced by a constant current generator unit. The new model, MaxiStim, Model ST4 overcomes the deficiencies of the older model and additionally, enables the user to vary the frequency of the stimulation, pulse width and pattern duration.

Bottom line

When using the DualStim/Deluxe^TM, model NS-2CA/DX nerve stimulators, the auditory volume control knob should never be set at above 7. Set at any level below 7, this otherwise acceptable nerve stimulator will perform well and the LCD will consistently and accurately indicate the intensity of the delivered stimulus.

Reference

• Hadzic A, Vloka JD, Koorn R. Effects of the auditory volume control knob on the stimulus amplitude display of the DualStim/Deluxe^TM model NS-2CA/DX peripheral nerve stimulator. Anesthesiology, 1997; 87:714-715.