Minimum Effective Volume of Crystalloid Co-Loading to Prevent Spinal Anesthesia-Induced Hypotension in Cesarean Section

The smallest crystalloid co-loading dose (mL/kg) that prevents spinal-anesthesia-induced hypotension in ≥90% of participants when given with prophylactic phenylephrine.

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.

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.

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.

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).

Calculated as (suction canister volume - irrigation volume) + gravimetric estimate of soaked sponges/drapes (1 g ≈ 1 mL). For cesarean delivery, amniotic fluid is excluded.

Occurrence of nausea temporally associated with hypotension after spinal anesthesia. Recorded as yes/no (≥1 episode) and number of episodes.

Occurrence of vomiting temporally associated with hypotension after spinal anesthesia. Recorded as yes/no (≥1 episode) and number of emetic events.

Occurrence of dizziness/lightheadedness temporally associated with hypotension after spinal anesthesia. Recorded as yes/no (≥1 episode) and number of episodes.

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.

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.

Umbilical cord arterial blood gas pH obtained immediately after birth from a double-clamped cord segment and analyzed per routine laboratory practice.

Umbilical cord arterial blood gas base excess measured immediately after birth from a double-clamped cord segment, processed per routine laboratory practice.

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).

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)

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).