Intravenous Bolus of Phenylephrine vs. Norepinephrine in Preventing Hypotension After Spinal Anesthesia

This study will be a prospective, randomized, active treatment controlled trial.

After written and informed consent are obtained, the study participants are randomly assigned using a computer generated table to 1 of 2 treatment groups prior to cesarean delivery.

Group A will consist of: An intravenous bolus of 0,2 mcg/Kg of norepinephrine to maintain systolic blood pressure (SBP) within 80-120% of baseline before the spinal.

Group B will consist of: An intravenous bolus of 100mcg of phenylephrine to maintain systolic blood pressure (SBP) within 80-120% of baseline before the spinal.

Patients will be admitted to holding area. Baseline arterial blood pressure and heart rate will be measured in supine position, with left uterine displacement. Baseline blood pressure will be calculated by taking the mean of three consecutive SBP measurements taken 5 minutes apart. 500 mL of Lactated Ringers solution will be administered immediately after induction of spinal anesthesia at the outflow rate of 100ml per hour.

The primary endpoint is the number of provider interventions needed to maintain the SBP within 100-120% of baseline for both groups. The secondary endpoint is nausea measured with each provider intervention after induction of spinal anesthesia, and immediately following delivery with 11 point verbal rating scale (0 = no nausea, 1 = nausea). Vomiting will be recorded whenever present during the surgical procedure.

Bradycardia (HR less than 50 BPM) will be treated with Atropine 0.4mg IV or ephedrine 3 mg - 12 mg IV bolus.

Study participants will receive a standard spinal anesthetic consisting of 0.5% hyperbaric bupivacaine (2 mL) plus sufentanil (5 mcg) at L3-4 or L4-5. Prior to surgical incision, the spinal sensory level will be tested to the bilateral T6-T4 dermatomal level. The patients will be positioned supine with a wedge placed under the right hip to avoid aortocaval compression. Both the patient and the researcher's assistant (who will collect data) will be blinded as to the administered Phenylephrine or norepinephrine bolus. The study will end when cesarean section is completed and the patient transferred to the post-operative care unit.

Measured variables will include systolic, diastolic and mean non-invasive blood pressure, the number and type of interventions for control of blood pressure, heart rate, incidence of nausea and vomiting (NV), incidence of arrhythmia and fetal cord blood analysis (pH) at delivery.