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Peak Pain Relief Is Delayed and Duration of Relief Is Extended Following Intravenous Phentolamine Infusion:: Preliminary Report

Background and Objectives. Intravenous phentolamine infusion (ivP) has been shown to produce pain relief in some patients with sympathetically maintained pain. Animal models of neuropathic pain have reported decreases in pain behavior lasting 5 days following phentolamine injection. However, no clinical study has prospectively assessed pain relief for longer than 24 hours. The aim of this preliminary report was to assess prospectively daily pain relief scores for 1 week following ivP. Methods. Thirty-seven consecutive patients with neuropathic pain were asked to complete a pain relief scale for 7 days following ivP. All patients were first administered 35 mg ivP. At least 1 week later, 16 patients were administered ivP at a higher dose, 50 mg or 75 mg. Data from a total of 45 infusions were collected. Results. Sixteen patients experienced pain relief from ivP. Twenty-seven infusions resulted in pain relief. Peak pain relief was delayed in 25 of 27 (93%) positive ivP: 7 patients experienced the onset of peak response the night immediately following an infusion, 13 the next day, 3 2 days later, and 1 each 4 and 5 days after infusion. All 16 patients who reported pain relief following ivP experienced at least 2 days of relief with each infusion. Eight patients experienced at least 1 week of pain relief. No differences in pain relief scores were seen with higher-dosage ivP. Conclusions. A delay in peak pain relief was reported following 25/27 ivPs. Peak pain relief occurred most often at least 24 hours after infusion completion. In these patients, duration of pain relief was at least 2 days. Placebo controlled studies are needed to verify these observations.