Determination of the minimum alveolar concentration of sevoflurane in Holstein steers

Abstract

Objective: To determine the minimum alveolar concentration (MAC) of sevoflurane in Holstein steers using electric stimulation. Study design: Prospective experimental study. Animals: A total of 15 Holstein steers aged 7.3 ± 1.2 months and weighing 121 ± 25 kg. Methods: Animals were anesthetized with sevoflurane at 8% in oxygen at 5 L minute–1 via facemask and were intubated with an orotracheal tube of a compatible size. After 15 minutes of stabilization of the initial expired concentration of sevoflurane (FE′Sevo) at 2.6%, electrical stimulation on the thoracic limb was initiated with a sequence of 2 × 10 ms followed by 2 × 3 second electrical currents of 50 V and 50 Hz, 5 seconds apart. Following each stimulus with a negative response, the FE′Sevo was decreased by 0.2% and a 15 minute interval was awaited before the next stimulus. The procedure was repeated until the first FE′Sevo value with a positive motor response was obtained. The FE′Sevo was then increased by 0.1%, followed by a new stimulus, until a negative response was obtained. The value of MAC was calculated as the arithmetic mean between the lowest FE′Sevo associated with a negative motor response and the highest FE′Sevo associated with a positive response. Results: The mean MAC for the 15 steers was 2.0 ± 0.3%, which corresponds to 2.1 ± 0.3% at sea level. Conclusions: Based on the proposed methodology, the MAC of sevoflurane for healthy Holstein steers is 2.1 ± 0.3% at sea level. Clinical relevance: This FE′Sevo value can be used to guide depth of anesthesia in steers weighing approximately 120 kg in clinical practice.