Influence of Continuous Administration of Phenylephrine Versus Dobutamine on Spinal Oxygen Saturation, Measured With Near-infrared Spectroscopy (NIRS).

All patients receive standard anesthesia care during the surgical procedure. Before induction of anesthesia, baseline MAP will be defined and 6 additional sensors (stickers) will be applied to the back of the patient at three levels: 1 at the upper thoracic level(T3-T4), 2 at the lower thoracic level (T9-T10) and 2 at the lumbar region (L1-L2) and 1 on the deltoid muscle of the upper arm. Two sensors are routinely applied to the forehead to measure cerebral oxygenation and a BIS sensor is applied to measure depth of anesthesia. In all patients a non-invasive cardiac output monitor (Clearsight; EdwardsTM LifeScience, Irvine, CA, USA) is routinely used. This monitor provides a continuous arterial pressure waveform in a non-invasive way and facilitates continuous evaluation of blood pressure. Hereby, efficient adaptation of the administration of vasopressors is feasible. Through an intravenous line, anesthetics will be administered. Vasopressor agents will be administered through a dedicated second intravenous line. After induction of anesthesia, an endotracheal tube is placed.

A vasopressor agent will be administered continuously after intubation in order to maintain blood pressure in a range from MAPbaseline -20% to normal (preoperative) values (MAPbaseline). If MAP decreases to a value lower than MAPbaseline -20%, a higher dose of vasopressor will be administered. If MAP increases to a value above MAPbaseline, the dosing rate will be decreased. According to the group to which the patient has been randomized, phenylephrine or dobutamine will be administered and the dose will be adjusted to MAP during a 30-minute study period.

The total drug amount for phenylephrine and dobutamine will not exceed 5 mg, resp. 30 mg.

The study will be completed after 30 minutes of continuous medicamentous hemodynamic support administration (i.e. 30 minutes after intubation) or if administration of phenylephrine or dobutamine exceeds 1 µg/kg/min or 10 µg/kg/min, respectively.

If the administration of the study medication does not achieve the desired result, management of the hemodynamics will be left to the discretion of the attending anesthesiologist and the patient will be excluded from further data analysis.