The beach chair position is the most commonly used position during shoulder surgery and offers the surgeon numerous advantages over the lateral decubitus position. However, the beach chair position can also lead to hemodynamic changes, including hypotension and cerebral hypoperfusion. It is therefore the anesthesiologist's job to prevent hypotension during the procedure.

The Acumen Hypotension Predictor Index or HPI for short is a software program that can predict the occurrence of low blood pressure during surgery shortly before it occurs. Studies have already demonstrated the effectiveness of HPI in various operations, where hypotension is defined as a mean arterial blood pressure < 65mmHg for at least 1 minute. To our knowledge, the low blood pressure prediction index has not yet been investigated for the prevention of low blood pressure in shoulder surgery in the beach chair position.

The HPI is built into a monitor that is switched on in addition to the standard monitoring for monitoring blood pressure, pulse, saturation, among other things. The monitor gives a signal when an episode of low blood pressure will occur within 15 minutes. This gives the anesthesiologist the opportunity to anticipate in time so that the occurrence of low blood pressure can be avoided.

Goal The aim of this study is to evaluate the efficacy of the HPI in predicting and reducing the incidence of hypotension during shoulder surgery in the beach chair position.

Procedure A total of 144 patients will participate in this study. Patients will be randomized to the intervention group or the control group according to a 1:1 allocation. In addition to standard monitoring, the HPI monitor will be turned on for all patients. For patients in the control group, the HPI monitor screen will be blinded and the alarms muted.

Patients between 18 and 85 years of age with a BMI ≤ 40 kg/m2 undergoing elective shoulder surgery are eligible for the study.

Inclusion Criteria:

• BMI < 40 kg/m2
• Patient can give informed consent
• ASA I-IV
• Invasive blood monitoring
• General anesthesia combined with interscalene block

Exclusion Criteria:

• - Refusal of participation
• Hypotension before surgery (MAP< 65 mmHg)
• History of stroke
• Dementia
• Contra-indication for interscalene block
• Contra-indication for general anesthesia
• Dialysis