BACKGROUND: The antinociceptive effects of systemically
administered local anesthetics have been shown in various
conditions, such as neuralgia, polyneuropathy, fibromyalgia,
and postoperative pain. The objective of the study was to
identify the peripheral mechanisms of action of low-dose local
anesthetics in a model of experimental pain.
METHODS: In a first experimental trial, participants (n=12) received
lidocaine systemically (a bolus injection of 2 mg/kg in 10 min
followed by an intravenous infusion of 2 mg x kg(-1) x h(-1)
for another 50 min). In a second trial, modified intravenous
regional anesthesia was administered to exclude possible
central analgesic effects. In one arm, patients received an
infusion of 40 ml lidocaine, 0.05%; in their other arm, 40 ml
NaCl, 0.9%, served as a control. In both trials, calibrated
tonic and phasic mechanical and chemical (histamine) stimuli
were applied to determine differentially the impairment of
tactile and nociceptive perception.
RESULTS: Mechanical sensitivity to touch, phasic mechanical
stimuli of noxious intensity, and heat pain thresholds remained
unchanged after systemic and regional application of the anesthetic.
In contrast, histamine-induced itch (intravenous regional
anesthesia), axon reflex flare (systemic treatment), and
development of acute mechanical hyperalgesia during tonic
pressure (12 N; 2 min) of an interdigital web was
significantly suppressed after both treatments.
CONCLUSIONS: Increasing painfulness during sustained pinching
has been attributed to excitation and simultaneous sensitization of
particular Adelta- and C-nociceptors. This hyperalgesic
mechanism seems to be particularly sensitive to low
concentrations of lidocaine. These findings confirm clinical
experience with lidocaine in pain states dominated by