H-reflex depression by propofol and sevoflurane is dependent on stimulus intensity

Objective The H-reflex has been widely used to investigate effects of drugs on motoneuron excitability in humans. However, up to now no systematic investigation has been done to examine the effects at different stimulus intensities. Here, the M. soleus recruitment curves were compared under influence of propofol and sevoflurane with control conditions to investigate these stimulus intensity dependent effects. Methods The study was performed in 10 volunteers for propofol and sevoflurane each, aged 23–32 years. The M. soleus H-reflex was evoked by stimulation of the tibial nerve. Recruitment curves were gained by increasing the stimulation current stepwise from below the threshold of a minimal H-reflex up to a maximal (m-response. Measurements were performed under the influence of the respective drug (2 mg/l propofol, 0.8 vol% sevoflurane) and compared to control measurements before and after drug administration. Results The relative amount of depression of the H-reflex at high stimulus intensities is for both drugs significantly (p < 0.001, Friedman’s test) lower than at low stimulus intensities. Conclusions Stimulus dependent effects have to be taken into consideration when experimental settings to investigate the effects of drugs on the H-reflex are being designed. According to the size principle of motoneuron excitation, it can also be assumed that under the influence of propofol and sevoflurane larger motoneurons are not depressed in the same amount as smaller motoneurons. Significance Different drug effects on the H-reflex at different stimulus intensities are not only of methodological importance, but also indicate different drug effects on motoneurons of different sizes