The goal of this observational study is to learn about the analgesia nociceptive index (ANI) is associated with pain sensitivity in patients between the ages of 9-17 who are experiencing chronic pain. The main question it aims to answer is:

- Does the ANI score correlate with the change in the pain sensitivity in patients aged 9-17 undergoing an interventional block.

Patients already receiving an interventional block as part of their regular medical care for chronic pain will have their level of pain recorded during clinic appointments

Inclusion Criteria:

• Experiencing chronic pain defined by: persistent or recurrent pain at least once a week for at least three months in their electronic medical charts or by reference of the patient's physician

Exclusion Criteria:

• Taking medications known to affect the sympathetic or parasympathetic nervous systems such as cholinomimetics (e.g., pilocarpine), anticholinergics (e.g., ipratropium), sympathomimetics (e.g., salbutamol), and adrenergic antagonists (e.g., propranolol).
• Diagnosed cardiac or neurological conditions, including patients with arrhythmias, heart block, postural orthostatic tachycardia syndrome, Guillain-Barre syndrome, or spinal cord injury.
• Active cancer diagnosis
• Conditions that may interfere with the ability to understand instructions or complete assessments including: cognitive, or developmental delay.
• Patients who do not speak English or French.

All patients who are offered an interventional block at the Edwards Family Interdisciplinary Center for Complex Pain of the Montreal Children's Hospital will be invited to participate. Initial contact will be through the medical team in contact with the patient. They will ask them if they agree to be contacted for a research study. Once the patient consents to be contacted, the research coordinators will reach out the patient and explain the study.