Differential sensitivities of fetal muscle groups to d-tubocurarine

Nine fetal lambs were instrumented for long-term intrauterine recording of electrocorticogram electrooculogram, electyromyogram of the dorsal neck muscle, the quadriceps femoris, and the diaphragm, monitoring of blood pressure, and measurement of heart rate. d-Tubocurarine (0.1 to 0.8 mg) was administered intravenously to the fetus over a 1-minute interval. Increasing doses of d-tubocurarine selectively blocked various muscle groups, with the extraocular muscles being the most sensitive. Comparison of the median effective dose for neuromuscular blockade of the extraocular muscles/nuchal muscle/diaphragm was 1:2.5:6. The duration of action of d-tubocurarine was directly related to the relative sensitivities of the different muscle groups, which are ranked from most sensitive to least sensitive as extraocular muscles, nuchal muscle, and diaphragm. Fetal electrocorticogram continued to cycle despite total neuromuscular blockade. However, there was an increase in synchronized and mixed activity, which was predominantly due to an increase in cycle numbers, and a decrease in desynchronized activity, which was predominantly due to shorter cycle lengths. During total neuromuscular blockade, there was also a significant decrease in heart rate during synchronized and mixed electrocorticogram states but not during a desynchronized electrocorticogram state, thus resulting in a loss of the heart rate difference normally seen between synchronized and desynchronized states. Fetal blood pressure did not change with total neuromuscular blockade.