Low-Dose Ketamine Infusions for Postoperative Analgesia in Patients on Chronic Opioids Therapy in General Inpatient Units: safety Signals and Monitoring Needs in a Retrospective Cohort

Patients with chronic pain often have difficult postoperative pain and high opioid needs. Ketamine is a useful adjuvant, but uncertainty about monitoring requirements limits utility. To evaluate incidence and severity of ketamine-related adverse effects under a low-resource monitoring protocol on general inpatient units, we conducted a retrospective chart review at a tertiary center including adults with chronic pain on long-term opioids who received postoperative ketamine infusions (0.1-0.3 mg/kg/h). Monitoring included vital signs, sedation, and mental status at predefined intervals. Data included demographics, surgery type, ketamine doses, preoperative opioid use, and adverse effects (hallucinations, nightmares, nystagmus/diplopia, hypersalivation, seizures), classified as mild (no pharmacologic/escalation) or severe (intervention or higher-acuity transfer). Eighty-nine patients were included (abdominoperineal 53.9%, orthopedic 42.7%). Mean 0-24 h and 24-48 h administered ketamine doses were 249.2 ± 167.5 mg and 235.6 ± 293.2 mg. No severe adverse effects occurred. Mild effects were reported in 9.0% (hallucinations 7.9%; nystagmus/diplopia 1.1%); all resolved with dose reduction or discontinuation; no patient required pharmacologic treatment or higher-level monitoring. Ketamine dosing, surgery type, age, sex, and preoperative opioid use were not associated with hallucinations. These findings support resource-conscious general inpatient units' protocols, while prospective studies should refine patient selection, dosing, and monitoring frequency.