To assess the effect of hemodynamic supportive medication on spinal vasculature, patients scheduled for arterial dilation of the lower limb were chosen as our study population. The spinal cord perfusion is not compromised in these patients, however, most patients suffer from hypotension during this kind of surgery, due to the non-invasive type of surgery and the vasodilating effects of the anesthetics. Therefore, continuous administration of vasoactive medication (phenylephrine or dobutamine) is often required in order to increase blood pressure.

The aim of our study is to evaluate the effect of a continuous administration of phenylephrine or dobutamine on the spinal oxygenation, assessed by 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.

The endovascular surgical procedure can start without delay and the duration of surgery will not be prolonged because of the measurements and blood pressure management

Inclusion Criteria:

• Patients ≥ 18y
• Patient is scheduled for dilation of arterial blood vessels of the lower limb

Exclusion Criteria:

• Age < 18y
• BMI > 30
• severe valvular disease
• previous aortic surgery
• paraplegia/ paraparesis
• kidney replacement therapy
• pacemaker
• pregnancy
• lactating participants
• preoperative use of ACE inhibitors.
• No sinus rhythm on preoperative ECG or at induction of anesthesia (patients with a history of atrial fibrillation can be included if they have a sinus rhythm on their preoperative ECG)