Neonatal Acid-Base Status After C Section With Maternal Tilt vs. Supine

This study aims to compare the neonatal acid-base status of women who remain supine horizontal, with women who are tilted to the left side by 15° during elective cesarean delivery at term under spinal anesthesia, with systolic BP maintained at baseline with a phenylephrine (PE) infusion. The null hypothesis is that the position of the patient (supine horizontal or tilted) during cesarean section will make no difference to neonatal wellbeing. The primary outcome will be the neonatal umbilical arterial base deficit (BD) at birth. The investigators will also examine the effects of maternal position during cesarean section under spinal anesthesia on maternal cardiac output (CO) and whether these changes have a correlation with the neonatal umbilical cord blood acid-base status. The secondary outcome will be the total phenylephrine dose requirement at 15 minutes after spinal injection.

Methods: This will be a randomized trial structured as an equivalence trial. Subjects will be American Society of Anesthesiologists (ASA) I and II women aged ≥18 years, non-laboring, at term (\>37 weeks gestation) with singleton pregnancies in cephalic presentation, scheduled for elective cesarean delivery under spinal anesthesia. Patients will receive spinal anesthesia for cesarean section in the sitting position, after which they will lay either supine horizontal or the surgical table will be tilted to the left by 15°, depending on the group to which they were randomized (SUPINE or TILT). An IV phenylephrine infusion will be administered to maintain systolic BP at baseline until delivery. A noninvasive hemodynamic monitoring system (NICOM, Cheetah Medical Inc., Vancouver, WA, USA) will continuously monitor CO and stroke volume. Neonatal umbilical vein and arterial blood gas results will be recorded.The null hypothesis is that the position of the patient (supine horizontal or tilted) during cesarean section will make no difference to neonatal wellbeing.

Statistical Design: The estimated sample size is the minimum number of study subjects required for rejecting the alternative hypothesis (i.e., accepting the null hypothesis). A significant difference between groups in the primary outcome, umbilical artery BD (which is a measure of fetal well being at the time of delivery and an indirect indicator of the adequacy of uteroplacental perfusion) would be of the order of magnitude of 2 mmol/L. Standard deviation of BD tends to be of this magnitude also. It would therefore require 22 subjects per group for 90% power to detect this magnitude of difference with alpha 0.05. Because BD is often not normally distributed, it is appropriate to increase sample size by 10-20%, and because the investigators have important secondary outcomes (PE usage, UA and UV pH, UV BD) the investigators intend to recruit 50 subjects per group to allow sufficient power for analysis of these secondary outcomes with appropriate correction for multiple comparisons, and allowing for 10% dropout. PE usage in the investigators previous study tends to have a standard deviation of about 30% of the mean, also roughly the magnitude of a clinically significant difference.

Eligibility Criteria:
Inclusion Criteria:

* ASA I and II women aged ≥18 years
* Non-laboring
* At term (\>37 weeks gestation) with singleton pregnancies in cephalic presentation
* Scheduled for elective cesarean delivery under spinal anesthesia.
* Maternal height will be between 150 - 180 cm and the body mass index (BMI)≤ 40 kg/m2.

Exclusion Criteria:

* Ruptured membranes
* Severe polyhydramnios or oligohydramnios
* Nonreassuring fetal heart rate
* Intrauterine growth restriction
* Abnormal lie - e.g. breech, transverse lie
* Multiple gestation
* Maternal comorbidities: hypertension, preeclampsia, other cardiovascular disease, renal failure, diabetes mellitus\> 10 years, severe scoliosis or kyphosis, uterine abnormalities (e.g. large fibroids, bicornuate uterus)
* Medications - anti-hypertensive agents
* Current smoking or illicit drug use
* Failed spinal (sensory level \< T6 after 15 minutes), need to convert to general anesthesia before delivery (exclusion from data analysis