This study is an observatory single center study investigating heart rate variability during on and off states of the spinal cord stimulator, in patients with failed back surgery syndrome.

Inclusion Criteria:

• Subject is at least 18 years old.
• Diagnosis of FBSS (Failed conservative treatments for pain including but not limited to pharmacological therapy and physical therapy) and currently being treated with spinal cord stimulation.
• Cognitive and language functioning enabling coherent communication between the examiner and the participant.

Exclusion Criteria:

• The presence of one or more coexisting conditions known to affect HRV analysis (including but not limited to atrial fibrillation, numerous atrial or ventricular extra beats, paced rhythm, left ventricular bundle branch block, cancer, kidney or hepatic failure, and diabetes mellitus (autonomic nervous system dysfunction)).
• The use of medication that directly modifies autonomic control (for example beta-blockers).