FACIAL SENSORY EVOKED MUSCLE POTENTIALS DURING LABOR - 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 s evere pain occurred and extradural analgesia was established. Pain int ensity was assessed by each patient using a 10 cm visual analogue scor e. Heart rate was recorded using a pulse oximeter. Sensory evoked musc le potential was derived from frontalis biopotentials that were rectif ied, integrated and digitised by an Anaesthesia and Brain activity Mon itor (Datex, Helsinki, Finland). An additional signal filter was inclu ded in the pre-amplifier to improve the signal-to -noise ratio. The rm s amplitude of successive 10s samples of sensory evoked muscle potenti al, as well as heart rate and the output from an external tocotransduc er, were then graphically displayed to allow assessment of temporal re lationships. The data were assessed using correlation analysis and hyp ergeometric probability distribution. Before extradural analgesia (mea n pain score 7.5 cm, SD 1.5 cm) a significant relationship was demonst rated 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 m uscle potential and uterine contraction amplitude, and heart rate and uterine contraction, was recorded. A continuous display of the relatio nship between filtered sensory evoked muscle potentials of the frontal is muscle and uterine contraction may, in the future, permit an object ive assessment of the adequacy of analgesia following extradural analg esia for the pain of labour.