Spinal anesthesia is widely accepted as the anesthetic method of choice for Cesarean section. However, the incidence of spinal hypotension is 50-70%, and the decrease in blood pressure is often rapid and severe. Heart rate variability is influenced by various factors such as the sympathetic nervous system, parasympathetic nervous system, temperature regulation, baroreflex, and endocrine regulation, but high frequency heart rate variability above 0.15 Hz very specifically reflects the parasympathetic nervous system. The ANI monitor calculates heart rate variability mediated by changes in the parasympathetic nervous system. This study aims to determine whether ANI monitor can predict hypotension in patients undergoing a caesarean section under spinal anaesthesia while applying the preemptive vasopressor phenylephrine infusion protocol.

Inclusion Criteria:

• full-term parturients undergoing cesarean delivery

Exclusion Criteria:

• emergent surgery, placenta previa, Morbid obesity with a BMI ≥ 40 kg/m2, Gestational age < 36 or ≥ 41 weeks, Contraindications to spinal anesthesia, cardiac arrhythmia, implantable pacemaker, diseases affecting the autoimmune system (such as immune disease or diabetic neuropathy), use of medications affecting ANI monitoring (antimuscarinics, alpha-agonists, beta blockers), illiteracy, or foreigners

pregnant women

ANI monitor will be applied to all patients, but will be covered with black papar so that it could not be observed during anesthesia. ANI data recorded at 1 second intervals will be downloaded at the end of anesthesia.