Intra-operative Ketamine Infusions in Patients With Chronic Lower Back Discomfort Undergoing Laminectomies.

Noxious stimuli occurring intra-operatively and post-operatively generate central sensitization, decreasing pain thresholds and ultimately increasing analgesic requirements. The pathophysiology of central sensitization is thought to involve excitatory amino acid receptors that have been implicated in the prolongation of painful states in animal models. The N-methyl-d-aspartate (NMDA) receptor is one such excitatory amino acid receptor (1, 2).

The underlying mechanism of central sensitization is thought to involve c-fiber associated injury occurring with incision. Crile and Wall brought about the concept that attenuation of central sensitization could be accomplished via the provision of analgesic interventions (opioids, local anesthesia) prior to the noxious insult. They termed the central sensitization attenuation preemptive analgesia. The concept of preemptive analgesia was later expanded to implicate both pre and post-incisional noxious stimuli as part of this process, resulting in studies designed to provide interventions throughout the surgical intervention (peri-procedural) (3). Reduction in analgesic requirements or pain scores for more than five half-lives (1st order kinetics) following the provision of the intervening analgesic agent peri-procedurally is now known as preventative analgesia. The term preemptive analgesia is now reserved for interventions that occur only before the noxious stimuli.

Multiple studies have investigated the concepts of preemptive analgesia and preventative analgesia by providing a variety of analgesic interventions at various times throughout the surgical insult in addition to more conventional means of anesthesia provision, including opioids, Non-Steroidal Anti-Inflammatory Drugs (NSAID)s, Cyclooxygenase-II (COX-2) inhibitors, alpha-2 agonists, and ketamine (4, 5, 6). Preemptive and preventative analgesia using a variety of pharmacological agents with at least partially known mechanisms of actions has provided some insight into potential mechanisms of central sensitization.

Ketamine is a N-methyl-d-aspartate (NMDA) receptor antagonist that has been shown to be useful in the reduction of acute postoperative pain and analgesic consumption in a variety of surgical interventions with variable routes of administration. It has also been shown to be effective in the presence and absence of opioids, suggesting that it has more than one mechanism of action in preemptive and preventative analgesia, including but not limited to decreasing central excitability, decreasing acute post-operative opioid tolerance, and a possible modulation of opioid receptors (7). Ketamine is a common anesthetic agent and has been in use since the Vietnam War. Clinically, ketamine provides pain relief with minimal respiratory depression, and at higher doses (1-2mg/kg) can induce general anesthesia while maintaining blood pressure and cardiac output.

Recently, a qualitative systematic review of the role of NMDA receptor antagonists was completed. Twenty-four studies investigating the role of ketamine met the inclusion criteria of the study, 58% of which demonstrated a preemptive or preventative analgesic effect. Patients underwent a variety of surgical procedures, both ambulatory and inpatient, and there was no obvious effect of either surgical type or dose of ketamine (range 0.15 to 1mg/kg) on the success of preventative intervention. However, the authors were unable to quantify the degree of reduction in primary end-points (opioid consumption, pain scores, both) due to variability in recording of such data. In addition, most inpatient studies were limited to abdominal procedures while the outpatient studies investigated mainly knee arthroscopies, providing no insight into the degree of impact of NMDA receptor antagonism in the setting of high pre-operative opioid tolerance combined with surgical procedures known to be associated with an invariably high degree of post-operative pain. Of note, only 1/24 studies documented a significant difference in side effects related to ketamine provision in patients who had received 20mg of epidural ketamine (7).