What is the purpose of the Non-Linear Current Adjustment Mode?
The idea is to offer the anaesthetist preset Current and Pulse Width settings which will provide a more predictable, linear progression of the distance the needle moves from setting to setting.
Starting at the highest setting, the charge will decrease exponentially at the following ratio 1/r2 with every anti-clockwise adjustment of the wheel. The goal is that the corresponding needle movement will be linear, in other words, one will have to move the needle 1mm (for argument sake) closer to the nerve for every decreasing position (Current and Pulse Width setting).
How can the user use this mode to customize his preferred current settings?
This mode offers the anesthetist a simple way to adjust the Current and Pulse Width settings by entering his preferred settings in 20 different preset positions.
How do I know the Stimpod is in Non-Linear Mode?
The '
' indicator on the top right of the display of the device tells the user that the device is in Non-Linear Mode.
Can I adjust Pulse Width while operating in Non-Linear mode?
No, the Pulse Width button is disabled in this mode. This is because the preset adjustment values contain both Current and Pulse Width preset values.
The principle of using the required threshold current to estimate the distance from the stimulated nerve forms the basis of nerve locating, when using neuromuscular stimulation. The distance from the nerve vs. the stimulation current required is governed by Coulombs law: E=K (Q/r2) where E is the current required, K a constant, Q the minimal current, and r the distance. The presence of the inverse square means that a very high stimulus is needed when the tip is some distance from the nerve. Thus, the threshold current required to facilitate excitation of a nerve is proportional to the square of the distance from the electrode to the nerve fibre. [1,2,3] This relationship holds true as long as the needle is subjected to a homogenous environment regardless of the distance from the nerve. In reality such a relationship is more complex, due to the fact that every tissue through which the needle moves, presents a different impedance spectrum.
The STIMPOD has the default values presented in the table programmed for Non-Linear Current Mode. It is up to the User to define different values if he prefers. For instance if you would normally start your Nerve Locating procedure at 1.6 mA at 100µs, then enter those values in position 20. If the lowest value you might us is 0.1mA at 100µs, enter it in position 1. Fill in positions 2 -19 with a progression of other preferred values. When the unit starts up it will be set at 0.0mA and 100µs. When the user turns the wheel clockwise it will start to progress through settings from position 1 – 20. If the user turns the wheel anti-clockwise the setting will progress the other way ie. From position 20 – 1.
| Position | Current (mA) | Pulse Width (ms) | Charge (µC) |
| 1 | 0.3 | 0.1 | 0.03 |
| 2 | 0.43 | 0.1 | 0.043 |
| 3 | 0.58 | 0.1 | 0.058 |
| 4 | 0.76 | 0.1 | 0.076 |
| 5 | 0.97 | 0.1 | 0.097 |
| 6 | 1.2 | 0.1 | 0.12 |
| 7 | 1.4 | 0.1 | 0.14 |
| 8 | 1.7 | 0.1 | 0.17 |
| 9 | 2 | 0.1 | 0.2 |
| 10 | 2.3 | 0.1 | 0.23 |
| 11 | 2.7 | 0.1 | 0.27 |
| 12 | 3 | 0.1 | 0.3 |
| 13 | 3.4 | 0.1 | 0.34 |
| 14 | 3.8 | 0.1 | 0.38 |
| 15 | 4.3 | 0.1 | 0.43 |
| 16 | 4.8 | 0.1 | 0.48 |
| 17 | 1.8 | 0.3 | 0.54 |
| 18 | 2.1 | 0.3 | 0.63 |
| 19 | 2.4 | 0.3 | 0.72 |
| 20 | 2.7 | 0.3 | 0.81 |
If you would like to change it back to Linear Mode. Hold the Menu (Pulse Width) Key down until the Stimpod display the Menu Options. Choose Current Adjustment Mode and follow the prompts.
