The investigators aim to investigate whether administration of a short-acting opioid (remifentanil) guided by a pain monitor (nociceptive level monitor) during anesthesia, can reduce pain in children after surgery.

The investigators hypothesize that pain monitor-guided administration of remifentanil can reduce pain postoperatively compared with standard care.

NOL-guided analgesia has not previously been assessed in children, and it is unknown whether this additional monitoring has any clinical benefits in a pediatric population.

In the present study the investigators aim to investigate whether administration of perioperative remifentanil guided by the CE-certified Pain Monitoring Device monitor-PMD200™, also called NOL-monitor, can reduce pain (primary endpoint), opioid consumption and agitation in children aged 3-16 years undergoing surgery with intravenous anesthesia.

The investigators hypothesize that NOL-guided perioperative remifentanil administration can reduce postoperative pain compared with standard clinical care (remifentanil dosing based on hemodynamic variables).

All patients will receive standard analgesia and antiemetic administered perioperatively:

• IV paracetamol 15 mg/kg
• IV Ibuprofen 10 mg/kg, unless contraindicated
• IV Morphine 25-100 μg/kg according to the extent of surgery and the departments' standard, administered 30 minutes before end of surgery (will not be included in the total perioperative or postoperative opioid consumption)
• In case of minor surgery, 1-2 μg/kg fentanyl, according to the department's standard, may be administered instead of morphine at the discretion of the anesthetist (will not be included in the total perioperative or postoperative opioid consumption)
• Regional anesthesia may be provided at the discretion of the anesthetist
• IV ondansetron 100 μg/kg

Inclusion Criteria:

• American Society of Anesthesiology (ASA) class of 1-3
• Scheduled surgery requiring intraoperative opioid administration
• Planned maintenance anesthesia with propofol and remifentanil
• The trial subject's custody holders' must be able to understand the trial protocol, risks, and benefits, and provide signed informed consent

Exclusion Criteria:

• Inability of the trial subject's custody holder to read or write Danish
• Children who cannot cooperate to the study assessments based on the investigators' evaluation. This may be children with mental disorders, visual disturbances, hearing or speech impairment etc. interfering with assessments.
• Arrythmias, either known arrythmias preoperatively or arrythmias detected within the first minutes perioperatively (in which case the patient will be excluded post- randomization).
• Allergy to the medicines used in the study
• Daily intake of opioids (morphine, oxycodone, ketobemidone, methadone, fentanyl) during the last 4 weeks
• Weight < 10 kg

The patients and their relatives, outcome assessors, data analysts and PACU nurses will be blinded. However, the anesthesiologists and anesthesia nurses anesthetizing the child according to the NOL monitor/standard care cannot be blinded. The anesthesiologist and anesthesia nurses anesthetizing the child will not be involved in patient care after the patient leaves the theatre, nor in any clinical assessments, data collection or interpretation of adverse events.