Rhythmic Change of Cortical Hemodynamic Signals Associated with Ongoing Nociception in Awake and Anesthetized Individuals: An Exploratory Functional Near Infrared Spectroscopy Study

Abstract

Background Patients undergoing surgical procedures are vulnerable to repetitive evoked or ongoing nociceptive barrage. Using functional near infrared spectroscopy, the authors aimed to evaluate the cortical hemodynamic signal power changes during ongoing nociception in healthy awake volunteers and in surgical patients under general anesthesia. The authors hypothesized that ongoing nociception to heat or surgical trauma would induce reductions in the power of cortical low-frequency hemodynamic oscillations in a similar manner as previously reported using functional magnetic resonance imaging for ongoing pain. Methods Cortical hemodynamic signals during noxious stimuli from the fontopolar cortex were evaluated in two groups: group 1, a healthy/conscious group (n = 15, all males) where ongoing noxious and innocuous heat stimulus was induced by a contact thermode to the dorsum of left hand; and group 2, a patient/unconscious group (n = 13, 3 males) receiving general anesthesia undergoing knee surgery. The fractional power of low-frequency hemodynamic signals was compared across stimulation conditions in the healthy awake group, and between patients who received standard anesthesia and those who received standard anesthesia with additional regional nerve block. Results A reduction of the total fractional power in both groups—specifically, a decrease in the slow-5 frequency band (0.01 to 0.027 Hz) of oxygenated hemoglobin concentration changes over the frontopolar cortex—was observed during ongoing noxious stimuli in the healthy awake group (paired t test, P = 0.017; effect size, 0.70), and during invasive procedures in the surgery group (paired t test, P = 0.003; effect size, 2.16). The reduction was partially reversed in patients who received a regional nerve block that likely diminished afferent nociceptive activity (two-sample t test, P = 0.002; effect size, 2.34). Conclusions These results suggest common power changes in slow-wave cortical hemodynamic oscillations during ongoing nociceptive processing in conscious and unconscious states. The observed signal may potentially promote future development of a surrogate signal to assess ongoing nociception under general anesthesia. Editor’s Perspective What We Already Know about This Topic What This Article Tells Us That Is New

Near‐infrared spectroscopy: More than just monitoring brain oxygenation

“…They found that injection of local anesthetics (a painful stimulus itself) prior to skin incision induced an instantaneous increase in both oxyhemoglobin and total hemoglobin content the prefrontal cortex are comparable between the awake and anesthetized states. 5 Together, these data should stimulate innovative efforts to improve perioperative nociception. Indeed, as frustrating as it is, our current clinical practice to monitor pain during the intraoperative period is mostly based on indirect proxies such as blood pressure, heart rate, and movements-an approximate and rudimentary practice.…”

Preventing pediatric chronic postsurgical pain: Time for increased rigor

“… 40 In humans, ongoing nociceptive processing has been shown to occur in adolescent patients under balanced general anesthesia. 41 …”

“… 55 , 56 Our group has been evaluating a technology (functional near-infrared spectroscopy, fNIRS) for measures of nociception and pain in adult and pediatric populations during both awake and unconscious/anesthetized states. 41 , 57–59 Low susceptibility to motion artifacts, flexibility in setup, and low overhead cost make fNIRS a useful neuro-investigative tool for a wide range of research and clinical applications involving pediatric patients. It is a popular technique to investigate typical and atypical development in infants, 60–62 young children, 63 , 64 and adolescents.…”

“… 74 Though the frequency of evoked pain may contribute to or exacerbate central sensitization, the ongoing pain may be as important and more difficult to evaluate under general anesthesia. In order to accomplish this, we have established algorithms such as power spectral analysis 75 and functional connectivity analysis that could gauge and measure ongoing pain in a manner similar to that previously reported by fMRI studies of chronic persistent pain. 76 Improvements in these measures will allow us, we believe, to provide a measure of surgical pain load (SPL).…”

Brain-based measures of nociception during general anesthesia with remifentanil: A randomized controlled trial

“…over time-there is no gold standard for the quantitative monitoring of nociception intraoperatively, and anesthesiologists titrate analgesics based on autonomic responses via the nociceptive autonomic responses as surrogates; available nociception monitors generate an analgesic index derived from physiologic (mainly autonomic) signals rather than quantifying nociception directly from the central nervous system [65,66]. We have reported on each of these separately [18,67] but not in terms of overall pain load that may contribute to enhanced central sensitization; further research is necessary to define thresholds for intervention; (2) Definition of sensitivity and specificity of fNIRS measures-although the activation's are context specific, other activations while under anesthesia may need to be evaluated in both awake and anesthetized patients as we have recently reported for painful stimuli [67]; (3) Measures across anesthetic type (e.g., inhalational versus nerve block versus combination) is also necessary; (4) The importance of maintaining optimal analgesia through surgery to inhibit subsequent acute and chronic pain/analgesic use-further work is required to quantitate the relationship of perioperative pain to sensitization and the development of chronic pain; as such, determination of presurgical factors (brain state and physiological/autonomic activity) influence the measurement of pain during surgery; and (5) Technical development-if shown to be useful, the technology will need to be developed into a format that can be used at the bedside/in the operating room.…”