Intraoperative direct cortical stimulation motor evoked potentials: Stimulus parameter recommendations based on rheobase and chronaxie

Objective

To determine optimal interstimulus interval (ISI) and pulse duration (D) for direct cortical stimulation (DCS) motor evoked potentials (MEPs) based on rheobase and chronaxie derived with two techniques.

Methods

In 20 patients under propofol/remifentanil anesthesia, 5-pulse DCS thenar MEP rheobase and chronaxie with 2, 3, 4 and 5 ms ISI were measured by linear regression of five charge thresholds at 0.05, 0.1, 0.2, 0.5 and 1 ms D, and estimated from two charge thresholds at 0.1 and 1 ms D using simple arithmetic. Optimal parameters were defined by minimum threshold energy: the ISI with lowest ${\mathit{rheobase}}^2\times \mathit{chronaxie}$, and D at its chronaxie. Near-optimal was defined as threshold energy <25% above minimum.

Results

The optimal ISI was 3 or 4 (n = 7 each), 2 (n = 4), or 5 ms (n = 2), but only 4 ms was always either optimal or near-optimal. The optimal D was ～0.2 (n = 12), ～0.1 (n = 7) or ～0.3 ms (n = 1). Two-point estimates closely approximated five-point measurements.

Conclusions

Optimal ISI/D varies, with 4 ms/0.2 ms being most consistently optimal or near-optimal. Two-point estimation is sufficiently accurate.

Significance

The results endorse 4 ms ISI and 0.2 ms D for general use. Two-point estimation could enable quick individual optimization.