Opioids used intra operative increase the amount of opioids needed post operative to control post operative pain. if opioids are given post operative, we assume that more atelectasis and hypoxia takes place even after lung protective ventilation. These postoperative lower oxygen saturations will require giving more frequent oxygen therapy.

This is an observational study where the attending anesthesiologist decides to use opioid free or opioid general anesthesia based on his/her expertise.

Every patient gets a lung protective ventilation and emergence from anesthesia using the principles set forward in the consensus meeting by Young C BJA 2019.

A lung recruitment followed by high PEEP is given when lung compliance decreases below 40 ML/cmH20. The lowest oxygen saturation without adding oxygen therapy is noted post extubation at moment of admission in PACU, after having given an opioid as analgesic and at moment of discharge from PACU..

If post operative satuation drops below 94% oxygen therapy is given and noted.

Inclusion Criteria:

• major surgery where intubation with mechanical ventilation is required
• obese patients with BMI > 35

Exclusion Criteria:

• spontaneous breathing during procedure
• allergic to any of the drugs used in both groups
• severe liver, renal, cardiac or pulmonary disease

Major surgery requiring intubation and mechanical ventilation.