The role of the sympathetic component of the autonomic nervous system on pain before and after third molar extraction- an observational cohort study.

Background: Assessing heart rate variability (HRV) before a standardized surgery would help to explore further the relationship between the autonomic nervous system and pain.

Methods: A single-center prospective cohort of 117 patients (55% female) scheduled for third molar extraction underwent a preoperative resting measurement of arterial pressure followed by an HRV recording, then potentiated by a Valsalva maneuver and a deep breathing challenge. Finally, pain sensitivity was assessed by hand immersion in hot water. All surgeries were conducted under local anesthesia, with or without sedation. The primary outcome was a composite pain/analgesia score (CPAS) incorporating both pain intensity and analgesic drug intake; it was adjusted to the type of anesthesia by within-subgroup ranking.

Results: The increase in heart rate in the Valsalva maneuver, and the low- to high-frequency ratio (LF/HF) in the deep breathing, were inversely correlated to preoperative heat pain, which was correlated itself to the CPAS (ρ = 0.195; p = 0.035). The only other parameter influencing CPAS was the increase in heart rate in the Valsalva maneuver, with an inverse correlation (ρ = - 0.191; p = 0.046). While age tended to impair HRV, particularly in its parasympathetic component, and while men displayed a stronger parasympathetic response than women, neither age nor sex interacted with these effects. Neither preoperative arterial pressure nor the occurrence of parental hypertension influenced the pain outcomes.

Discussion: Although the identified relationships were not particularly strong, they are consistent with an influence of the sympathetic component of the autonomic nervous system. However, they do not support the interest of HRV assessment to predict postoperative pain in current practice.

Trial registration: Not applicable.