Recent advances in pain management

Adjunctive analgesic therapy

Adjuvant analgesics are drugs that have weak or nonexistent analgesic action when administered alone but can enhance analgesic actions when coadministered with known analgesic agents. Such agents are often administered in cases of refractory pain. For some chronic pain syndromes, however, they may constitute a first-line approach. Because pain is such an individual experience, analgesic regimens may require several drugs at varying dosages to confer a comfortable state. Adjunctive therapies such as the tricyclic antidepressants, anticonvulsants, N-methyl-D-aspartic acid receptor antagonists and low-dose intravenous local anesthetics, to name a few, have proved to be efficacious in relieving certain types of pain, especially neuropathic and cancer pain. Their use in animals is increasing, with anecdotal reports of some success.

Physiology of pain

The substantial increase in our collective knowledge of pain physiology and pharmacology over the past decade has had a significant effect on the practice of clinical veterinary medicine. An overview of the basic anatomical and physiologic components of nociceptive processing is presented, as well as a discussion of the sensitizing events that occur within the nervous system in acute and chronic pathologic pain states. The unique features of visceral and neuropathic pain are also outlined. With the goal of improving the success of our therapeutic interventions, the final section is devoted to the various classes of analgesic drugs and techniques, and how they are best incorporated into pain management strategies.

Patient-controlled epidural versus intravenous pethidine to supplement epidural bupivacaine after abdominal aortic surgery

In a double-blind, randomized, crossover study of 25 patients after abdominal aortic surgery, we compared patient-controlled analgesia (PCA) with epidural versus intravenous pethidine. All patients received continuous epidural infusions of 0.125% bupivacaine adjusted to maintain appropriate sensory levels. The 48 hour study period commenced 36 to 48 hours after surgery and covered postoperative days 2 and 3. There was a crossover in PCA mode (epidural or intravenous) after 24 hours. Plasma pethidine concentration at the end of each 24 hour period and the total 24 hour pethidine dose did not change significantly between postoperative days 2 and 3. Pethidine plasma concentration was lower after 24 hours epidural than after intravenous PCA [125 (SD 108) ng/ml versus 171 (SD 107) ng/ml, P = 0.03], although pethidine dose did not differ significantly [mean 147 (SD 124) mg/24 h]. Visual analog pain scores (VAS) did not differ significantly between postoperative days 2 and 3, or at rest between epidural and i.v. groups. However, VAS with coughing and with abdominal palpation were lower in the epidural PCA group (P = 0.05, 0.008). With a background epidural infusion of 0.125% bupivacaine, PCA with epidural pethidine provided better pain control than PCA intravenous pethidine and this was achieved at lower plasma pethidine concentrations.

Lidocaine concentrations in plasma and cerebrospinal fluid after systemic bolus administration in humans

Preclinical studies suggest that systemic lidocaine acts at the level of the spinal dorsal horn to inhibit hyperalgesia resulting from nerve injury, yet no clinical data are available to support this view. Therefore, we sought to characterize the time course of lidocaine in the plasma and cerebrospinal fluid (CSF) after an IV bolus injection of lidocaine 2 mg/kg in patients scheduled for surgery involving spinal anesthesia. Sixty-five patients were randomly allocated to one of five study groups (n = 13 per group) receiving IV lidocaine before CSF/ plasma sampling at 5, 10, 15, 30, or 60 min. Gas chromatographic analysis of these samples revealed a fast but transient peak (5-15 min) in lidocaine plasma levels (1.7+/-0.16 microg/mL), which declined rapidly thereafter. Only small concentrations of IV lidocaine were found in the CSF (6%- 8% of plasma concentration), but this fraction remained stable from 15 min until termination of the experiment. No statistical correlation was observed between plasma and CSF lidocaine levels. These data suggest that because of the prolonged availability of lidocaine at the spinal dorsal horn level, systemic administration of lidocaine suppresses central sensitization within the spinal cord after nerve injury in humans.

IMPLICATIONS

Cerebrospinal fluid concentrations of lidocaine after its systemic bolus delivery in humans indicate that the spinal cord may be the major site of antinociceptive action by this route of drug administration.