Minimum Effective Dose of Ropivacaine for Spinal Anesthesia for Cesarean Delivery

Some unexpected adverse reactions such as respiratory depression, vomiting, especially supine hypotensive syndrome could caused by a high dose of anesthetic in cesarean section. The investigators assumed that there be a minimum effective dose that will be associated with the best possible performance of ropivacaine for different intervertebral spaces(L2-3 and L3-4 in usual). That maybe enhance the comfort of parturient and safety of fetus.

This study is conducted as a prospective, randomized, up-down sequential dose of isobaric ropivacaine in 3 mL that will provide effective analgesia for 50% of parturients in cesarean section. The investigators use the combined spinal-epidural anesthesia(CSEA) technique in the study. The initial dose of ropivacaine is chosen as 12 mg in L2-3 group and 15 mg in L3-4 group, and the testing interval is 0.5 mg with subsequent doses being determined by the outcome of the previous injection in the same group. If the previous response is ineffective, the next patient will receive 0.5 mg more than the last patient. If the response of the previous patient is effective, the next patient decrease 0.5mg.

The visual analogue scale (VAS) is used to rate the pain, where 0 is no pain and 10 is the worst pain imaginable. Criteria for evaluation:(1) effective: after spinal anesthesia finish, a dose that provide adequate sensory dermatomal anesthesia to pinprick to T6 or higher within 10 minutes, and the VAS is lower than or equal to 3 within 60minutes after skin incision; (2) ineffective: if the initial plane of sensory dermatomal anesthesia is lower than T6 or VAS is greater than 3 within 60 minutes after skin incision, the dose of ropivacaine is considered inadequate and additional lidocaine is given through epidural catheter.

Motor assessments are performed at the start of operation. Motor block in the lower limb is assessed by a modified Bromage scale (0=no paralysis, 1=unable to raise extended leg, 2=unable to flex knee,3=unable to flex ankle). All of the assessments are made by an anesthetist who is blinded to the group assignment as well as to the dose injected.

On the other hand, the investigators will compare the incidence of adverse reaction between the L2-3 and L3-4 group.

Using the Dixon and Massey up-and-down method study design, 60 patients scheduled for elective cesarean section will be included in the study, 30 for each group.

Inclusion criteria

• Full term parturient(> 37 and <42 weeks gestation)
• Above 20 years and below 35 years
• American Society of Anesthetists(ASA) I to II
• Singleton pregnancy
• Elective cesarean section
• Body weight during 60-85kg, height during 150-170cm

Exclusion criteria

• Patient refusal
• Hypertension
• Diabetes mellitus
• Heart diseases
• Asthma
• Abnormal fetus or placenta
• Contraindications to combined spinal-epidural anesthesia(CSEA)
• Allergy to ropivacaine