Facial Sensory Evoked Muscle Potentials During Labour, A Continuous Objective Monitor of Adequacy of Analgesia?

We prospectively studied the relationship between upper facial sensory-evoked muscle potential amplitude, uterine contraction amplitude and heart rate in 20 healthy parturients during the first stage of labour. Monitoring began when minimal pain was reported and continued until severe pain occurred and extradural analgesia was established. Pain intensity was assessed by each patient using a 10 cm visual analogue score. Heart rate was recorded using a pulse oximeter. Sensory evoked muscle potential was derived from frontalis biopotentials that were rectified, integrated and digitised by an Anaesthesia and Brain activity Monitor (Datex, Helsinki, Finland). An additional signal filter was included in the pre-amplifier to improve the signal-to-noise ratio. The rms amplitude of successive 10s samples of sensory evoked muscle potential, as well as heart rate and the output from an external tocotransducer, were then graphically displayed to allow assessment of temporal relationships. The data were assessed using correlation analysis and hypergeometric probability distribution. Before extradural analgesia (mean pain score 7.5 cm, SD 1.5 cm) a significant relationship was demonstrated between sensory evoked muscle potential and uterine contraction, p < 0.001). No constant relationship was seen between heart rate and uterine contraction. Following extradural analgesia (mean pain scores of 2.1 cm, SD 1.3 cm) a variable relationship between sensory evoked muscle potential and uterine contraction amplitude, and heart rate and uterine contraction, was recorded. A continuous display of the relationship between filtered sensory evoked muscle potentials of the frontalis muscle and uterine contraction may, in the future, permit an objective assessment of the adequacy of analgesia following extradural analgesia for the pain of labour.