Evaluation of the Effect of Ketamine on Remifentanil-induced Hyperalgesia

The aim of this study was to determine if the addition of ketamine reduces remifentanil-induced hyperalgesia, improves its analgesic effect, inhibits IL(interleukin)-6 and IL-8 (inflammatory cytokines), and stimulates IL-10 (an anti-inflammatory cytokine).

Opioids are very effective in pain relief, but they might lower pain threshold, making the patient more sensitive to a pain stimulus, a condition known as hyperalgesia \[Angst; Clarck, 2006\]. Opioid-induced hyperalgesia (OIH) is usually defined as a reduction in nociceptive thresholds in the peripheral field of the sensitized fibers \[Koppert et al., 2003\], and it is associated with increased pain and higher demand for postoperative analgesia \[Guignard et al., 2000\]. This phenomenon adversely impacts pain control, and has been suggested to occur in the peri-operative context, especially associated with the use of remifentanil, a short-acting opioid \[Guignard et al., 2000\].

Several mechanisms have been proposed to explain the hyperalgesia phenomenon, but the most important seems to be the activation of N-methyl-D-aspartate (NMDA) receptors \[Célèrier et al., 2000\]. Ketamine is a NMDA receptor antagonist that has been shown to reduce postoperative pain and the need for postoperative anesthetics and analgesics. Therefore, it is proposed that ketamine could prevent hyperalgesia, resulting in more effective and long-lasting postsurgical analgesia \[Célèrier et al. 2000\].

The results of studies of low dose of ketamine in the prevention of remifentanil-induced hyperalgesia are controversial. Joly et al. \[2005\] demonstrated a reduction in the consumption of opioids and in hyperalgesia assessed with monofilaments. However, Engelhardt et al \[2008\] showed no differences in pain scores or in postoperative opioid consumption.

In addition, some authors observed higher levels of proinflammatory cytokines, associated with increased pain in mice receiving chronic opioid (morphine) infusion \[Johnston et al., 2004; Liang et al., 2008\]. Also, administration of proinflammatory cytokine inhibitors reduced phosphorylation of NMDA receptors \[Zhang et al., 2008\]. However, no study has examined the relationship between the use of remifentanil, the most frequently implicated opioid in OIH \[Guignard et al., 2000\], ketamine (drug capable of inhibiting NMDA-receptors and cytokines) \[Dale et al., 2012\], and the inflammatory response.

The aim of this study was to determine if the addition of ketamine reduces remifentanil-induced hyperalgesia, improves its analgesic effect, inhibits IL-6 and IL-8 (inflammatory cytokines), and stimulates IL-10 (an anti-inflammatory cytokine) in patients submitted to laparoscopic cholecystectomy, a procedure with an usually neglected potential for postoperative pain and that has been poorly investigated in association with OIH.

Eligibility Criteria:
Inclusion Criteria:

* ≥ 18 years old
* both sexes
* ASA physical status I or II
* undergoing laparoscopic cholecystectomy

Exclusion Criteria:

* chronic users of analgesics or had used opioids within 12 h of surgery
* history of drug or alcohol abuse or psychiatric disorder
* contraindications to self-administration of opioids (ie, unable to understand the patient-controlled analgesia \[PCA\] device)
* contraindication for the use of ketamine, such as a psychiatric disorder, acute cardiovascular disorder, or unstable hypertension