Effects of ventilation and isoflurane end-tidal concentration on intracranial and cerebral perfusion pressures in horses

Objective - To measure the effects of isoflurane end-tidal concentration and mode of ventilation (spontaneous vs controlled) on intracranial pressure (ICP) and cerebral perfusion pressure (CPP) in horses. Animals - 6 adult horses of various breeds. Procedures - Anesthesia was induced and maintained with isoflurane in O₂ in 6 healthy, unmedicated, adult horses. Using a subarachnoid strain gauge transducer, ICP was measured. Blood gas tensions and carotid artery pressures also were measured. Four isoflurane doses were studied, corresponding to the following multiples of the minimum alveolar concentration (MAC): 1.0 MAC, 1.2 MAC, 1.4 MAC, and 1.6 MAC. Data were collected during controlled ventilation and spontaneous ventilation at each dose. Results - Increasing isoflurane end-tidal concentration induced significant dose-dependent decreases in mean arterial pressure (MAP) and CPP but no change in ICP. Hypercapnic spontaneous ventilation caused significant increases in MAP and ICP compared with normocapnic controlled ventilation; no change in CPP was observed. Conclusions and Clinical Relevance - Hypercapnia likely increases cerebral blood flow (CBF) by maintaining CPP in the face of presumed cerebral vasodilation in healthy anesthetized horses. The effect of isoflurane dose on CBF, however, remains unresolved because it depends on the opposing influences of a decrease in CCP and cerebral vasodilation.