Vital signs are measured in the preparation area before entrance in the operation theater. The following measures are taken in the supine position every three minutes for three measures: blood pressure and cardiac frequency. The mean blood pressure and cardiac frequency become the reference measure for each patient.
Upon entrance in the operation theater, anesthesia care is given in a standardized manner:
* Supine positioning of the parturient on the operating table
* Standard monitoring of vitals signs every minute for the time of the study, including systolic, diastolic an mean blood pressures, cardiac frequency and SpO2
* 18G intravenous catheter insertion on the forearm
* Randomization of the patient in one of the two groups followed by blind administration of ondansetron or saline
* 5 minute wait period before sitting the patient for spinal anesthesia
* Standardized spinal anesthesia: Whitacre 25G spinal needle, L4-L5 intervertebral space, hyperbaric bupivacaine 12mg + fentanyl 10 mcg + morphine 100 mcg
* Upon injection of the spinal medication: 1000 ml Ringer lactate coloading and start of the iv norepinephrine infusion upon study protocol.
* Evaluation of the sensitive block level at 5 and 10 minutes, a level of T6 is needed to start surgery
* Nausea and vomiting prophylaxis in both groups with metoclopramide 10 mg and dexamethasone 4 mg if no contraindication. Nausea and vomiting per cesarean section will be treated with dimenhydrinate 25 mg
* Bradycardia (HR below 60) will be treated with atropine 0,4 mg
* Multimodal post-operative analgesia with intra rectal acetaminophen 1000 mg and indomethacin 100 mg
Norepinephrine infusion protocol:
* The rate of the infusion pump is blindly set in mcg/kg/min by one of the investigators before the entrance of the patient and the anesthesia team in the operating room. The setting is hidden from the main physician at all times and the perfusion is started when the spinal medication is injected.
* For the first patient of each group (saline and ondansetron) the norepinephrine infusion is started at 0,05 mcg/kg/min until the end of the study
* Hypotension (as defined as a value inferior or equal to 80% of the systolic blood pressure reference value) is treated by the anesthesiologist with a 4 mcg bolus of norepinephrine from a prepared syringe of norepinephrine 4 mcg/ml
* Hypertension (as defined as a value superior or equal to 120% of the systolic blood pressure reference value) is treated by stopping the norephineprine infusion and restarting it when the value is inferior to 120% of the reference value.
* For the following patients, the norepinephrine infusion rate will be adjusted by up and down allocation following the results of the previous patient: if no hypotension occurred (no bolus needed): the infusion rate is decreased by 0,005 mcg/kg/min (for example from 0,05 mcg/kg/min to 0,045 mcg/kg/min) / if at least one hypotension occurred (at least one bolus needed): the infusion rate is increased by 0,005 mcg/kg/min (for example from 0,05 mcg/kg/min to 0,055 mcg/kg/min)
The study is stopped at the fetal delivery.
