This study is designed to determine the ED90 for a single dose of phenylephrine for the treatment of spinal induced hypotension in parturients presenting for an elective CD. The ED90 is the effective dose at which 90% of subjects will have a "positive" response to phenylephrine. The primary outcome measure is the ED90 for bolus phenylephrine. Secondary outcomes include the need for additional vasopressors, glycopyrolate to treat bradycardia, and the presence of hypertension following administration of phenylephrine.

Inclusion Criteria:

1. Non-emergent cesarean delivery with planned regional anesthesia (i.e. elective planned cesarean delivery for malposition, patient choice, cervical pelvic disproportion, previous cesarean delivery and other diagnosis that require a predetermined cesarean delivery)
2. American Society of Anesthesia physical status class I & II (ASA I - Healthy, ASA II - mild and controlled systemic disease, eg. controlled essential hypertension)
3. Age ≥ 18 years (Standard within the obstetrical literature)
4. Term gestational age
5. English-speaking

Exclusion Criteria:

1. Morbid Obesity (Body Mass Index ≥ 45 kg/m2) (Morbidly obese parturients require a dose of local anesthetic less than the standardized dose in this study, typically have an exaggerated response to vasopressors, and the blood pressure cuff occasionally needs to be replaced by an intraarterial catheter due to limitations in size)
2. Height < 5'0" (Women < 5'0" are likely to require a dose of local anesthetic less than the standardized dose in this study)
3. Laboring women
4. Urgent or emergency cesarean delivery
5. Severe hypertensive disease of pregnancy defined as systolic blood pressure (SBP) > 160mmHg, diastolic blood pressure (DBP) > 110mmHg and/or requiring antihypertensive treatment or associated with significant proteinuria
6. Severe maternal cardiac disease
7. Diabetes type I
8. Subjects on monoamine oxidase inhibitors (MAOI's) or tricyclic antidepressants
9. Fetal anomalies
10. Failed spinal anesthesia
11. Subject enrollment in another study involving a study medication within 30 days of CD
12. Any other physical or psychiatric condition which may impair their ability to cooperate with study data collection