5Practice guidelines for obstetrical anesthesia a report by American Society of Anesthesiologists Task Force on obstetrical anesthesia.Anesthesiology, 1999, 90: 600.
6Conell-price J, Evans JB, Hong D, et al.The development and validation of a dynamic model to accoumt for the progress of labor in the assessment of pain[J].Anesth Analg, 2008, 106(5): 1509-1515.