Objective Monitoring of Pain Using High Frequency Heart Rate Variability—A Narrative Review

Managing pain when a patient cannot communicate, during anesthesia or critical illness, is a challenge many clinicians face. Numerous subjective methods of evaluating pain have been developed to address this, for instance, the visual analog and numerical rating scale. Intraoperatively, objective monitoring of pain in anesthetized patients is assessed through hemodynamic parameters; however, these parameters may not always accurately reflect pain perception. The high-frequency heart rate variability index (HFVI), also known as analgesia nociception index (ANI), is a commercially available device developed by MDoloris that objectively assesses nociception based on patient electrocardiogram, sympathetic tone, and parasympathetic tone. The monitor displays a value from 0–100, where <50 indicates nociception and >50 indicates anti-nociception. Given its potential to objectively monitor pain, numerous studies have utilized this device in clinical and non-clinical settings. As such, we conducted a literature review using various search terms in PubMed and selected HFVI studies based on our inclusion criteria for this review. In this review, we discuss the mechanisms by which numerous available nociception monitors assess pain along with the results of clinical and non-clinical HFVI studies to provide a comprehensive summary for clinicians interested in or considering the use of novel pain monitoring.