Prevention of Hypotension During Elective Cesarean Section Under Spinal Anaesthesia With Fixed-rate Metaraminol Infusion Versus Fixed-rate Norepinephrine Infusion: A Double-blind Randomised Controlled Clinical Trial.

Spinal anaesthesia is the anaesthetic technique of choice for elective cesarean section.

Spinal anaesthesia can be complicated by hypotension, with incidence exceeding 80% occasionally. Hypotension can lead to nausea, emesis and a subjective feeling of discomfort due to cerebral hypoperfusion. If left untreated, severe or sustained hypotension can lead to decreased uteroplacental flow and fetal distress of premature or compromised fetuses while severe complications to the parturient might ensue, such as loss of consciousness, aspiration, apnea or cardiac arrest.

Prophylactic vasoconstrictor infusion is an effective strategy for preventing maternal hypotension during regional anaesthesia. Phenylephrine is widely used because it acts quickly and causes less neonatal acidosis than ephedrine, but high doses may reduce maternal cardiac output. Norepinephrine and metaraminol, a sympathomimetic amine that acts both directly and indirectly, mainly by stimulating alpha-1-adrenergic receptors, with a weak effect on beta-receptors, have emerged as potential alternatives. While the optimal norepinephrine infusion dose has been defined in several studies, research on metaraminol remains limited, with insufficient data on its ideal dosing and efficacy in obstetrics. Most comparative studies evaluate metaraminol against phenylephrine-some suggesting better fetal acid-base outcomes-but no studies directly compare metaraminol with norepinephrine. Further research is therefore needed before metaraminol can be routinely recommended in obstetric anaesthesia.

This study will include pregnant adults who are normotensive, without complications, with a single pregnancy, who are not in active labor, and who are scheduled to undergo an elective cesarean section at the Gynecology-Obstetrics Clinic of Aretaieion Hospital.The study will use randomized, concealed allocation to assign participants to receive either metaraminol or norepinephrine infusions. Study group allocation will take place according to a computer-generated sequence of random numbers. A non-involved healthcare professional will reveal each participant's assignment, and a nurse will prepare identical 50 mL syringes labeled "study infusion," containing the drug at concentrations that deliver either 100 µg/min (200 µg/mL at 30 mL/h) of metaraminol or 4 µg/min (8 µg/mL at 30 mL/h) of norepinephrine. The anaesthesiologist, patient, and data analyst will remain blinded to group allocation.

All parturients will receive standard pre-anesthetic evaluation and standard hemodynamic monitoring will be applied. Next, on the hand opposite the one with the cuff for measuring NIBP, a special finger cuff connected to the Edwards Lifesciences ClearSight system will be placed on the index, middle, or ring finger. The ClearSight system from Edwards Lifesciences is a non-invasive medical system that allows continuous monitoring of blood pressure (cBP), as well as a range of other hemodynamic parameters such as cardiac output (CO), stroke volume (SV), and systemic vascular resistance (SVR).