[Nociceptive flexion reflex. Principles and applications of the threshold determination].

The threshold for the pain flexion reflex in the lower limb can be used to determine the potency in man of analgesic drugs acting centrally. A new apparatus is, the "algometre", described. Bipolar electrical stimulation (5 stimuli of 1 ms over a period of 10 ms) is applied to the saphenous nerve just behind the external malleolus. An amplifier detects the muscle response (electromyogramme; EMG) at the muscle-tendon junction in the popliteal space; this occurs about 70-130 ms after the painful stimulus. The whole system is linked to a microcomputer. The EMG and EMG/intensity curves are displayed on a colour screen, and can be copied by a colour tracing table. The whole machine is mobile and can be moved from bed to bed. An increase in the threshold during inhalation of 50% nitrous oxide in oxygen was shown. However, the usefulness of the "algometre" in clinical practice remains to be assessed by further studies.