EMG basics part 7: Stimulus intensity (minimum and maximum upper stimulus with waveform)

This content includes a lot of grammatical and vocabulary errors.Please cut me some slack from Japan.


No waveform at 30mv x

No max maximal upper stimulus at 50mv.

This time, I would like to talk about the correct method of electrical stimulation. In evoked electromyography, CMAP and SNAP are derived by applying electricity from the stimulating electrode.

But improper stimulation position and method will increase the voltage to mischief => patient will be in pain.

Keep in mind that it is against the medical principle of ‘Do no harm’ to bring disadvantage to the patient by your inexperience.

Minimal Stimulus

The minimum stimulus for CMAP/SNAP waves is about 10mv in the upper extremities and face, and 15-20mv in the lower extremities. Therefore, if the waveform does not appear at 20mv in the upper extremities and face and 30mv or more in the lower extremities, it is the fault of the person performing the test.

Note, however, that in the distal part of the lower extremity (foot) or in the knee socket, it is sometimes necessary to apply close to 40 mv due to edema or obesity.

Now, the main reasons why waveforms do not appear even at 30 mv are almost always due to the following two points: (1) the stimulation site is off in the first place, and (2) the stimulating electrode is not firmly pressed against it.

Please study and get used to (1).

As for (2), many people are too gentle.

Electrical stimulation is applied to the skin, but what you really want to stimulate is the nerve located 0.5cm~3cm below the skin. If you do not press down strongly, it is difficult for the stimulation to reach the nerve.

Since the electrode is pressed so hard that it leaves a mark immediately after the test, there is no need to mark it with a pen.

Also, electrical stimulation is painful, so the patient unconsciously shows an escape reaction. If this happens, the stimulating electrode will be separated from the skin and inaccurate results will be obtained. Therefore, it is important to press the extremities firmly against the bed and to apply contraction from the opposite side on the face.

Supramaximal Stimulation

On the other hand, the stimulation intensity at which the waveform amplitude (potential) does not increase any further is called “maximal supramaximal stimulation”.

However, this does not mean that you should increase the stimulation intensity blindly. If the stimulation intensity is too strong, nerves other than the target nerve may respond, or the waveform itself may become too small or invisible due to CMAP in SNAP.

In general, maximal supramaximal stimulation is achieved when the stimulation intensity is about twice the minimum stimulation intensity. However, in the case of SNAP, if it exceeds 15mv, CMAP will have appeared, so around 10mv is appropriate.

Note that if the electrode is not firmly applied here, the patient’s escape reaction and muscle contraction may cause the electrode to shift easily, resulting in a smaller waveform or a change in shape.

Minimum stimulation: 20 mv on the face of the upper extremities, 30 mv or less on the lower extremities

Maximum upper stimulation: 40mv or less for upper face and 60mv or less for lower limbs.

Stimulation electrodes should be pressed firmly.

That is all. Thank you again.