The speed of mask induction using an in‐circuit vaporizer may be influenced by vaporizer setting. To investigate this in clinical patients, 18 dogs were randomly assigned to one of three groups. Each dog was premedicated and then mask induced with isoflurane using a Stephen's in‐circuit vaporizer set at 1/2, 3/4, or full ON. We determined inspired isoflurane and oxygen concentrations at the level of the mask, respiratory rate, resistance to mask induction, and time to intubation. No significant differences were found between groups in resistance to induction or in time to intubation. At settings of 3/4 and full ON, inspired isoflurane concentrations at time of intubation ranged from 3.3 to 8.25, and were significantly higher than those resulting from the 1/2 setting (range 2.1 to 4.6). We conclude that it may be preferable to avoid settings greater than 1/2 when using the Stephen's vaporizer for mask induction because of the potential adverse effects of high inspired inhalant anesthetic concentrations. In addition, use of higher vaporizer settings may not significantly speed induct
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