The goal of this clinical trial is to find the minimum effective volume of IV crystalloid that should be given with phenylephrine to prevent hypotension caused by spinal anesthesia in healthy, term pregnant adults having elective cesarean delivery. The main questions are: What is the minimum effective volume (MEV90, mL/kg) of crystalloid co-loading that prevents spinal-anesthesia-induced hypotension in ≥90% of participants? What maternal side effects and newborn outcomes occur with this strategy (e.g., nausea/vomiting, need for extra vasopressors, total fluids/blood loss, Apgar scores, and umbilical cord blood gases)? There is no separate comparison group; this is a single-arm, adaptive dose-finding study. Participants will: Receive a predefined volume of IV crystalloid over \~10 minutes during spinal anesthesia while phenylephrine is infused. Have blood pressure and symptoms monitored; receive rescue treatment if needed. Allow the next participant's fluid volume to be adjusted based on whether hypotension occurred (biased-coin design). Be followed through postoperative day 2 for maternal and newborn outcomes.
Study Type
INTERVENTIONAL
Allocation
NA
Purpose
PREVENTION
Masking
NONE
Enrollment
45
Balanced crystalloid is infused intravenously immediately after intrathecal injection as a co-load. A predefined volume (mL/kg) is administered over \~10 minutes. The volume for successive participants is adapted by a biased-coin up-and-down algorithm to estimate the minimum effective volume (MEV90) that prevents spinal-anesthesia-induced hypotension. Rescue fluids are allowed per protocol.
Minimum Effective Volume (MEV90) of Intravenous Crystalloid Co-Loading (mL/kg)
The smallest crystalloid co-loading dose (mL/kg) that prevents spinal-anesthesia-induced hypotension in ≥90% of participants when given with prophylactic phenylephrine.
Time frame: From intrathecal injection (start of spinal anesthesia) to end of surgery
Incidence of intraoperative hypotension (mmHg)
Hypotension is defined as a drop in systolic blood pressure (SBP) to \< 80% of the participant's baseline SBP (mmHg). Baseline SBP is the mean of the last three ward SBP measurements obtained before operating room admission.
Time frame: From intrathecal injection to end of surgery
Incidence of intraoperative hypertension (mmHg)
Hypertension is defined as any occurrence of systolic blood pressure (SBP) \> 120% of baseline SBP (mmHg) after spinal anesthesia. Baseline SBP is the mean of the last three ward SBP measurements obtained before operating room admission.
Time frame: From intrathecal injection to end of surgery
Additional bolus phenylephrine requirement
Administration of rescue phenylephrine bolus in addition to the protocolized prophylactic phenylephrine infusion to treat or prevent hypotension after spinal anesthesia. Hypotension is defined as SBP \< 80% of baseline SBP (mmHg) or MAP \< 65 mmHg. Baseline SBP is the mean of the last three ward SBP measurements obtained before operating room admission. When hypotension occurs, phenylephrine 100 µg is administered as an IV bolus to restore blood pressure. The outcome is recorded as number of phenylephrine boluses.
Time frame: From intrathecal injection to end of surgery
Total Phenylephrine Dose (µg)
Total intraoperative phenylephrine dose administered after spinal anesthesia, defined as the sum of the prophylactic infusion dose and all rescue bolus doses. he prophylactic infusion starts at 1,500 µg/h and the infusion rate is reduced if systolic blood pressure (SBP) exceeds 120% of baseline SBP (mmHg). Baseline SBP is the mean of the last three ward SBP measurements obtained before operating room admission. Rescue bolus protocol: phenylephrine 100 µg IV per hypotensive episode (SBP \< 80% of baseline SBP \[mmHg\] or MAP \< 65 mmHg), repeated as needed. Dose will be abstracted from the anesthesia record (infusion rate × time plus boluses) and summarized in micrograms (µg).
Time frame: From intrathecal injection to end of surgery
Estimated blood loss (mL)
Calculated as (suction canister volume - irrigation volume) + gravimetric estimate of soaked sponges/drapes (1 g ≈ 1 mL). For cesarean delivery, amniotic fluid is excluded.
Time frame: From skin incision to end of surgery
Incidence of Maternal Nausea Associated With Hypotension
Occurrence of nausea temporally associated with hypotension after spinal anesthesia. Recorded as yes/no (≥1 episode) and number of episodes.
Time frame: From intrathecal injection to end of surgery
Incidence of Maternal Vomiting Associated With Hypotension
Occurrence of vomiting temporally associated with hypotension after spinal anesthesia. Recorded as yes/no (≥1 episode) and number of emetic events.
Time frame: From intrathecal injection to end of surgery
Incidence of Maternal Dizziness/Lightheadedness Associated With Hypotension
Occurrence of dizziness/lightheadedness temporally associated with hypotension after spinal anesthesia. Recorded as yes/no (≥1 episode) and number of episodes.
Time frame: From intrathecal injection to end of surgery
Neonatal Apgar score at 1Minute
The Apgar score (range 0-10) assessed 1 minute after birth by clinical staff according to standard practice. It is the sum (0-2 each) of heart rate, respiratory effort, muscle tone, reflex irritability, and skin color. Recorded as the total score (0-10) and, if available, the five component scores.
Time frame: At 1 minute after birth
Neonatal Apgar Score at 5 Minutes
The Apgar score (range 0-10) assessed 5 minutes after birth by clinical staff according to standard practice. It is the sum (0-2 each) of heart rate, respiratory effort, muscle tone, reflex irritability, and skin color. Recorded as the total score (0-10) and, if available, the five component scores.
Time frame: At 5 minutes after birth
Umbilical Arterial pH
Umbilical cord arterial blood gas pH obtained immediately after birth from a double-clamped cord segment and analyzed per routine laboratory practice.
Time frame: At delivery (immediately after birth)
Umbilical Arterial Base Excess (mmol/L)
Umbilical cord arterial blood gas base excess measured immediately after birth from a double-clamped cord segment, processed per routine laboratory practice.
Time frame: At delivery (immediately after birth)
Pulmonary edema
New-onset pulmonary edema within 48 hours after surgery, defined by physician diagnosis supported by clinical findings (e.g., dyspnea, crackles) and/or objective evidence (e.g., new supplemental oxygen requirement or SpO₂ ≤ 92% on room air, chest imaging consistent with interstitial/alveolar edema). Record as yes/no (≥1 event).
Time frame: Through postoperative day 2
Clinically Significant Generalized Edema
Generalized pitting edema present within 48 hours after surgery, defined by ≥1 of the following objective criteria (excluding localized, surgical-site-limited edema): (A) Physical exam: pitting grade ≥2+ in ≥2 anatomic regions (e.g., bilateral pretibial/ankle, hands/forearms, sacral, or periorbital), using the standard indentation scale - 2+: \~4 mm pit, brief rebound; 3+: \~6 mm pit, 10-60 s rebound; 4+: \~8 mm pit, \>60 s rebound. (B) Anthropometry: body weight increase ≥2% (or ≥2 kg) from the preoperative baseline or bilateral mid-calf circumference increase ≥2 cm from baseline. (C) Therapeutic criterion: initiation or escalation of diuretic therapy (e.g., IV/PO loop diuretic) for generalized edema as judged by the treating clinician. Recorded as yes/no (≥1 criterion met)
Time frame: Through postoperative day 2
Acute Heart Failure
New diagnosis of acute heart failure within 48 hours after surgery, characterized by symptoms/signs (e.g., dyspnea, orthopnea, pulmonary congestion), objective evidence (e.g., chest radiographic congestion, elevated BNP/NT-proBNP if available, or echocardiographic findings), and requirement for treatment (e.g., IV diuretics, vasodilators, inotropes, or noninvasive ventilation). Record as yes/no (≥1 event).
Time frame: Through postoperative day 2
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