Subdissociative‐dose ketamine (SDDK) is used to treat acute pain. We sought to determine if SDDK is effective in relieving acute exacerbations of chronic pain.
This study was a randomized double‐blind placebo‐controlled trial conducted May 2017 to June 2018 at a public teaching hospital (ClinicalTrials.gov #NCT02920528). The primary endpoint was a 20‐mm decrease on a 100‐mm visual analog scale (VAS) at 60 minutes. Power analysis using three groups (0.5 mg/kg ketamine, 0.25 mg/kg ketamine, or placebo infused over 20 minutes) estimated that 96 subjects were needed for 90% power. Inclusion criteria included age > 18 years, chronic pain > 3 months, and acute exacerbation (VAS ≥ 70 mm). Pain, agitation, and sedation were assessed by VAS at baseline and 20, 40, and 60 minutes after initiation of study drug. Telephone follow‐up at 24 to 48 hours used a 10‐point numeric rating scale for pain.
A total of 106 subjects were recruited, with three excluded for baseline pain < 70 mm. After randomization, 35 received 0.5 mg/kg ketamine, 36 received 0.25 mg/kg ketamine, and 35 received placebo. Three subjects receiving 0.5 mg/kg withdrew during the infusion due to adverse effects, and one subject in each group had incomplete data, leaving 97 for analysis. Initial pain scores (91.9 ± 8.9 mm), age (46.5 ± 12.6 years), sex distribution, and types of pain reported were similar. Primary endpoint analysis found that 25 of 30 (83%) improved with 0.5 mg/kg ketamine, 28 of 35 (80%) with 0.25 mg/kg ketamine, and 13 of 32 (41%) with placebo (p = 0.001). More adverse effects occurred in the ketamine groups with one subject in the 0.25 mg/kg group requiring a restraint code for agitation. A total of 89% of subjects were contacted at 24 to 48 hours, and no difference in pain level was detected between groups.
Ketamine infusions at both 0.5 and 0.25 mg/kg over 20 minutes were effective in treating acute exacerbations of chronic pain but resulted in more adverse effects compared to placebo. Ketamine did not demonstrate longer‐term pain control over the next 24 to 48 hours.