fNIRS allows for ongoing measures of brain functions during surgery. The ability to evaluate cumulative effects of painful/nociceptive events under general anesthesia remains a challenge. Through observing signal differences and setting boundaries for when observed events are known to produce pain/nociception, a program can trigger when the concentration of oxygenated hemoglobin goes beyond ±∆0.3 mM from 25 seconds after standardization. fNIRS signals were retrieved from patients undergoing knee surgery for Anterior Cruciate Ligament (ACL) repair under general anesthesia. Continuous fNIRS measures were measured from the primary somatosensory cortex (S1), which is known to be involved in evaluation of nociception, and the medial polar frontal cortex (mPFC), which are both involved in higher cortical functions (viz., cognition, emotion). A ± ∆0.3 mM threshold for painful/nociceptive events was observed during surgical incisions at least twice, forming a basis for a potential near-real time recording of pain/nociceptive events. Evidence through observed true positives in S1 and true negatives in mPFC are linked through statistically significant correlations and this threshold. Our results show that standardizing and observing concentrations over 25 seconds using the ±∆0.3 mM threshold can be an arbiter of the continuous number of incisions performed on a patient, contributing to a potential intraoperative pain load index.

Significance: Quantitative measurement of perisurgical brain function may provide insights into the processes contributing to acute and chronic postsurgical pain.Aim: We evaluate the hemodynamic changes in the prefrontal cortex (medial frontopolar cortex/ mFPC and lateral prefrontal cortex) and the primary somatosensory cortex/S1 using functional near-infrared spectroscopy (fNIRS) in 18 patients (18.2 AE 3.3 years, 11 females) undergoing knee arthroscopy.Approach: We examined the (a) hemodynamic response to surgery and (b) the relationship between surgery-modulated cortical connectivity (using beta-series correlation) and acute postoperative pain levels using Pearson's r correlation with 10,000 permutations. Results:We show a functional dissociation between mFPC and S1 in response to surgery, where mFPC deactivates, and S1 activates following a procedure. Furthermore, the connectivity between (a) left mFPC and right S1 (original r ¼ −0.683, p permutation ¼ 0.001), (b) right mFPC and right S1 (original r ¼ −0.633, p permutation ¼ 0.002), and (c) left mFPC and right S1 (original r ¼ −0.695, p permutation ¼ 0.0002) during surgery were negatively associated with acute postoperative pain levels.Conclusions: Our findings suggest that greater functional dissociation between mFPC and S1 is likely the result of inadequately controlled nociceptive barrage during surgery leading to more significant postoperative pain. It also supports the utility of fNIRS during the perioperative state for pain monitoring and patient risk assessment for chronic pain.