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.