Closed-circuit liquid injection isoflurane anesthesia in the horse

Six horses were administered isoflurane anesthesia by liquid injection into a closed breathing circuit according to the square root of time model. The unit dose (UD) was calculated using Lowe's formula to provide an end-tidal concentration of 1.3%, or the minimum alveolar concentration of isoflurane. The mean UD was 4.2 +/- 0.2 mL. The mean end-tidal isoflurane concentration (ETiso) for each interval after injection, and the peak and minimum concentrations for each injection interval, did not change beginning with the second injection, indicating that the square root of time model accurately predicted isoflurane uptake in the horse. Mean ETiso measured for the interval after the first injection was 0.68 +/- 0.06%, which was significantly (p < .05) lower than the mean concentration after all subsequent injections (1.1 +/- 0.1%). Mean peak end-tidal concentration was 1.1 +/- 0.25% after the first injection and 1.7 +/- 0.26% for all other injections. Mean minimum end-tidal concentration was 0.77 +/- 0.13% for all injection periods. This model proved to be an acceptable technique for administration of isoflurane anesthesia to horses.