The Monitoring Paradox: Frontal Electroencephalography Reveals Profound Cortical Depression Despite Cardiovascular Stability in Dogs Undergoing Tibial Plateau Leveling Osteotomy with Isoflurane Anesthesia and Multimodal Analgesic Protocols

Conventional monitoring relies on autonomic reflexes but fails to directly measure cortical hypnosis, creating a critical information gap regarding true brain state. The objective of this article was to evaluate the cardiorespiratory and cortical responses using frontal EEG and parasympathetic tone activity (PTA) in dogs under isoflurane anesthesia and distinct analgesic protocols. Twenty-one adult dogs (n = 7 per group) undergoing tibial plateau leveling osteotomies were assessed at 12 procedural time points using heart rate, blood pressure, Patient State Index (PSI), suppression ratio (SR), and PTA. Treatments included isoflurane maintenance with either intravenous fentanyl, epidural morphine, or regional local blocks. Despite clinically acceptable cardiorespiratory indices, significant treatment-by-time interactions occurred for PSI (p=0.003) and SR (p=0.050). The fentanyl group experienced the deepest cortical suppression and highest Double-Low state incidence compared to epidural morphine (p=0.024). Rapid emergence from these suppressed states yielded dysphoric recoveries in 9 of 21 dogs. In conclusion, evaluating cardiorespiratory and cortical responses demonstrates that standard autonomic monitoring can mask severe intraoperative cortical depression; integrating EEG and PTA offers a valuable approach to optimizing neurophysiological stability.