Seated Time for 90% Incidence of Hypotension

The study will determine the 90% effective seated time after intrathecal injection of hyperbaric bupivacaine to avoid hypotension in patients undergoing cesarean delivery.

The study will use a Biased Coin Design (BCD), up-and-down sequential method, to determine the ET90 for absence of hypotension during Cesarean delivery. In this methodology, the first patient will be assigned an a priori initial seated time. Any other patient will be assigned a seated time based on the presence or absence of hypotension in the previous patient. If the previous patient developed hypotension, then the new patient will be seated for a longer time, which will be equal to the seated time of the previous patient plus a pre-determined time increment (up to an also pre-determined maximum allowed time). If no hypotension occurred to the previous patient, the new patient will be assigned one seated time from two choices: The same seated time that has been assigned to the previous patient, or a seated time shorter than that of the previous patient by a pre-determined time decrement (the pre-determined time increment and decrement are the same). The choice will be determined from a biased coin result, with a probability of 0.9 for assigning 'the same as previous' seated time and a probability of 0.1 for assigning a 'shorter seated time'.

Pt will be seated for a predetermined time period. All Patients will receive intrathecal bupivacaine and morphine.

Inclusion Criteria: Patients with singleton pregnancies scheduled for elective cesarean delivery Exclusion Criteria: BMI>35 Unable to communicate in study language ASA greater than 2 Hypertensive disease of pregnancy