Port of Choice: Induction Chamber Inlet Position Affects Speed of Isoflurane Induction in Mice

Various factors affect inhalant anesthetic induction speed in mice. We investigated the effects of different inhalant anesthetic inlet port positions on isoflurane induction speed in 3 mouse strains. We considered 3 anesthetic inlet port positions: conventional (port located in a traditional location approximately halfway between the induction box lid and base), top (port incorporated into the induction box lid), and bottom (port located adjacent to the base, but on the side wall). We hypothesized that positioning the anesthetic inlet port at the bottom of the induction chamber would result in a shorter induction time compared with the conventional inlet placement across all 3 mouse strains. This crossover study design involved randomly assigning mice undergoing isoflurane anesthetic induction with one of the custom-designed inlet positions: conventional, top, or bottom inlet port. C57BL/6, NSG, and Alb-Cre Foxm1b-/- male and female mice were used (n = 60, 20 mice per strain). The anesthetic induction speed was evaluated as the (1) time to immobility and (2) time to loss of righting reflex (LORR) - the time to loss of righting reflex by slightly tilting induction chamber. In C57BL/6 mice, the time to immobility was shorter using the bottom inlet port than with the conventional inlet. In NSG mice, the time to immobility was longer using the top inlet port than with the conventional inlet. In Alb-Cre Foxm1b-/- mice, inlet port position did not affect the time to immobility. For the time to LORR, in C57BL/6 mice, the bottom inlet resulted in shorter times to LORR compared with the top inlet, while no difference was detected between the bottom and conventional inlets. For Alb-Cre Foxm1b-/- mice, the bottom inlet produced shorter LORR times compared with both the top and conventional inlets. In NSG mice, inlet position did not significantly affect LORR times. This study suggests that a bottom gas anesthetic inlet port position provides the shortest (fastest) anesthetic induction for C57BL/6 and Alb-Cre Foxm1b-/- mice, but not for NSG mice.