STABILIZATION OF THE NEUROMUSCULAR RESPONSE WHEN SWITCHING BETWEEN DIFFERENT MODES OF NERVE-STIMULATION AT SURGICAL DEGREES OF NEUROMUSCULAR BLOCKADE

During general anesthesia, different modes of nerve stimulation are us ed for estimation of the degree of neuromuscular blockade. When switch ing between the different modes, it is important to know whether the p receding mode influences the responses to the succeeding mode, and, if so, for how long. The object of our study was to determine the number of stimulations required for stabilization of the muscular response w hen switching between double-burst stimulation (DBS) applied every 20 sec, train-of-four (TOF) applied every 12 sec, and posttetanic count ( PTC) at surgical degrees of neuromuscular blockade.A total of 33 women were anesthetized with fentanyl, thiopental, halothane, and nitrous o xide. A constant degree of neuromuscular blockade was maintained at a twitch height of 4 to 11% of the control twitch height using a continu ous infusion of atracurium. The ulnar nerve was stimulated supramaxima lly at the wrist, and the contraction in the adductor pollicis was mea sured mechanomyographically. At surgical degrees of neuromuscular bloc kade, only the first twitch response to TOF stimulation (T1) and the f irst twitch response to DBS stimulation (D1) are consistently present. When switching from DBS to TOF, 4 to 7 stimulations (56 to 92 sec) we re required for stabilization of the T1 response. When switching from TOF to DBS, 3 stimulations (36 to 52 sec) were required for stabilizat ion of the D1 response, and, finally, when switching from PTC to DBS, 5 to 11 stimulations (81 to 201 sec) were required for stabilization o f the D1 response. Stabilization of D1 was faster following TOF than f ollowing PTC (p < 0.01). We conclude that, when switching between diff erent modes of nerve stimulation, the number of stimulations required for stabilization of the responses to the succeeding mode of stimulati on depends on the preceding mode.