Determination of the minimum alveolar concentration of isoflurane in llamas

Objective-To determine the minimum alveolar concentration (MAC) of isoflura ne (ISO) in Ilamas. Study Design-Prospective study. Animals-Eight adult neutered male Ilamas (9 +/- 1 years [(x) over bar +/- S D], 177 +/- 29 kg). Methods-Anesthesia was induced and maintained in otherwise unmedicated Ilam as with a mixture of ISO in oxygen administered through a standard small-an imal, semi-closed circle system using an out-of-circle, agent-specific vapo rizer. The time from mask placement to intubation was recorded. Inspired an d end-tidal (ET) ISO was sampled continuously. At each anesthetic concentra tion, a constant ET ISO was maintained for at least 20 minutes before appli cation of a noxious electrical stimulus (50 volts, 5 Hz, 10 ms for up to 1 minute). A positive or negative response to the stimulus was recorded, and ET ISO then increased (if positive response) or decreased (if negative resp onse) by 10% to 20%. Individual MAC was the average of multiple determinati ons. Body temperature was maintained at 37 +/- 1 degrees C. Selected cardio pulmonary variables (heart rate [HR], respiratory rate [RR], arterial blood pressure [ABP]) and ET ISO were recorded at hourly intervals from first IS O. Arterial blood was collected for pH, PCO2, PO2 analysis and measurement of packed cell volume (PCV) and total protein (TP) at 2 hour intervals. Fol lowing MAC determination, the anesthetic was discontinued and Ilamas were a llowed to recover. Duration and quality of recovery were noted. Results-The time from start of induction by mask to completion of intubatio n took 19.1 +/- 4.8 minutes. The MAC of ISO corrected to one atmosphere at sea level (barometric pressure 760 mm Hg) in these Ilamas was 1.05 +/- 0.17 %. Mean ABP increased from 70 +/- 26 mm Hg at the end of the first hour of anesthesia to 102 +/- 7 mm Hg measured at the end of the sixth hour of anes thesia. ET ISO decreased from 2.06 +/- 0.10% to 1.27 +/- 0.07% over the sam e time period, but MAC did not change with time. The duration from first IS O to discontinuation of ISO averaged 6.19 +/- 0.9 hours. Animals were able to support their heads in a sternal posture at 23 +/- 10 minutes, and stood 62 +/- 26 minutes following discontinuation of the anesthetic. Conclusion-The MAC for ISO is similar to, but slightly lower than, values r eported for other species. Clinical Relevance-Knowledge of MAC may facilitate appropriate clinical use and provide the basis for future investigation of ISO in Ilamas. (C)Copyri ght 1999 by The American College of Veterinary Surgeons.