Effects of low-dose IV ketamine on peripheral and central pain from major limb injuries sustained in combat.

OBJECTIVE Examine response patterns to low-dose intravenous (IV) ketamine continuous infusions on multiple pain outcomes, and demonstrate effectiveness, safety, and tolerability of ketamine administration on general wards. DESIGN Retrospective case series of consecutive patients given low-dose IV ketamine continuous infusions. SETTING Walter Reed Army Medical Center, Washington, DC. PATIENTS Nineteen eligible inpatients with neuropathic pain from major limb injuries sustained in combat with inadequate pain control from multimodal analgesia. INTERVENTIONS A 3-day IV infusion of ketamine at doses ≤ 120 μg/kg/h. OUTCOME MEASURES Daily present (PPI), average (API), and worst (WPI) pain intensity (0-10), global pain relief (GPR) (1 "no relief" to 5 "complete relief"), daily assessments of adverse events, and daily opioid requirements measured during therapy. RESULTS A significant reduction in PPI (P < 0.001) and improvement in GPR (P = 0.031) was noted over time. Higher baseline WPI (≥ 7; N = 4) was associated with a significant decrease in WPI (P = 0.0388), but lower baseline WPI (N = 5) was not. Significant mean percent decreases in PPI with higher baseline PPI (N = 8; P = 0.0078) and WPI with no phantom limb pain (PLP) (N = 10; P = 0.0436) were observed. Mean percent increase in overall GPR was better for those reporting GPR scores ≤ 3 (N = 13) in the first 24 hours of therapy (P = 0.0153). While not significant, mean opioid requirement (IV morphine equivalents) decreased from 129.9 mgs ± 137.3 on day 1 to 112.14 ± 86.3 24 hours after therapy. CONCLUSIONS Low-dose ketamine infusions for complex combat injury pain were safe and effective, and demonstrated response patterns over time and by baseline pain score stratification and presence or absence of PLP.