Opioid analgesic drugs are the main treatment of patients during anesthesia. Although highly effective, their use is not without problems. One is the increasing requirement of these address the same nociceptive stimulus.

Opioid induced hyperalgesia could be an explanation studies in animal models. Through mechanisms where N-methyl-D-aspartate receptors, glutamatergic system disturbances and changes in intracellular calcium regulation involved.

The hyperalgesia induced by intrathecal opioids is controversial. The investigators propose a model study in patients undergoing cesarean section to study the secondary hyperalgesia induced based on the study of nociceptive thresholds with two methods opioids: Von Frey filaments and digital algometer.

If intrathecal fentanyl is used in spinal anesthesia for elective cesarean section, then, an increase in sensitivity will occur. This increase can be measured by von Frey filaments, expressed in increased requirement clinically opioids.

Inclusion Criteria:

• Pregnant 37 to 41 weeks.
• American Society of Anesthesiologists classificationI or II.
• Between 18 and 40 years.
• Scheduled for elective cesarean section under spinal anesthesia in the Maternity Service of the Clinical Hospital of the Catholic University.

Exclusion Criteria:

• Background Of high risk pregnancy. Twin-pregnancy.
• Obesity> 30 kg / m 2 before pregnancy.
• Background Of psychiatric illness.
• Using Chronic analgesic.
• AllergyDrug used in protocol.
• require General anesthesia during surgery