Labor analgesia is an essential health caring procedure for women. However, epidural
analgesia cannot be performed on all subjects for different contraindications, such as lower
platelet counter, back infection at the puncture site, and fear of epidural injection etc.
Therefore, intravenous analgesia is an alternative for such conditions. Given the influence
of intravenous administration of drugs on fetus, the drug selection is very important.
Remifentanil, a super-short efficacious opioid, can last for 3-4 minutes after injection,
which is similar in both maternal and fetal environment. Thus the fetus-associated side
effects would be less than other drugs. The investigators hypothesized that remifentanil
would be a superior intravenous drug used with patient-controlled technique for labor
analgesia.