Ketamine, an NMDA receptor antagonist, reduces postoperative pain and opioid use but carries a risk of hallucinations. In a systematic review of 53 trials (2839 patients), prophylactic intravenous ketamine (median 0.4 mg/kg) lowered pain intensity on a 0–10 scale by 0.89 cm at 6 hours, 0.42 at 12 hours, 0.35 at 24 hours, and 0.27 at 48 hours, and reduced 24-hour morphine consumption by 15.7 mg, without affecting morphine-related adverse effects. Awake or sedated patients receiving ketamine without benzodiazepine had a 2.32 times higher odds of hallucinations (number needed to harm 21). Under general anesthesia, hallucination risk was low regardless of benzodiazepine use. The overall role of ketamine in perioperative analgesia remains unclear.
The stream of consciousness is thought to be composed of discrete brain states, reflected in EEG microstates—transient, stable patterns of global neuronal activity lasting fractions of seconds. In 23 surgical patients, high-density EEG was recorded continuously from wakefulness to unconsciousness induced by step-wise increasing propofol concentrations. Under surgical anesthesia, microstate sequences became sparser, longer-lasting, and less complex. However, moderate sedation initially increased the temporal dynamics and complexity of microstates, producing a distinctive U-shaped pattern that may correspond to paradoxical excitation. These findings suggest that normal consciousness relies on a metastable balance between order and chaos, enabling flexible state transitions, and that altered consciousness reflects changes in this balance.