Carbon dioxide and oxygen levels in disposable individually ventilated cages after removal from mechanical ventilation

Abstract

Disposable individually ventilated cages have lids that restrict air exchange when the cage is not mechanically ventilated. This design feature may cause intracage CO2 to increase and O2 to decrease (hypercapnic and hypoxic conditions, respectively) when the electrical supply to the ventilated rack fails, the ventilated rack malfunctions, cages are docked in the rack incorrectly, or cages are removed from the ventilated rack for extended periods of time. We investigated how quickly hypercapnic and hypoxic conditions developed within disposable individually ventilated cages after removal from mechanical ventilation and compared the data with nondisposable static cages, disposable static cages, and unventilated nondisposable individually ventilated cages. When disposable individually ventilated cages with 5 adult mice per cage were removed from mechanical ventilation, CO2 concentrations increased from less than 1% at 0 h to approximately 5% at 3 h and O2 levels dropped from more than 20% at 0 h to 11.7% at 6 h. The breathing pattern of the mice showed a prominent abdominal component (hyperventilation). Changes were similar for 4 adult mice per cage, reaching at least 5% CO2 at 4 h and 13.0% O2 at 6 h. For 3 or 2 mice per cage, values were 4.6% CO2 and 14.7% O2 and 3.04% CO2 and 17.1% O2, respectively, at 6 h. These results document that within disposable individually ventilated cages, a hypercapnic and hypoxic microenvironment develops within hours in the absence of mechanical ventilation.

Figure 1.

A) Bottom view of the IVC cage lid for the dIVC, showing air inflow and outflow ports, receptacle for the water bottle, and the filter surrounding the outflow port. B) The plastic piece that holds the 6.2 × 7.3 cm, rectangular, Reemay spun-fiber filter surrounding the outflow port has been removed, showing the filter and the 3.0 × 5.6 cm trapezoid-shaped opening in the lid (large red arrow). C) Two of 14 approximately 3-mm-diameter holes in the plastic holder (small red arrows) that allow air exchange through the filter.

Figure 2.

A) Top view of cage lid for the dSC, showing the 2 raised filters. B) The plastic pieces that hold the Reemay spun filters have been removed. C) Larger plastic filter holder, showing the grid with 286 9-mm² square holes through which air exchange occurs. Note that the holes in the plastic filter holder (small red arrows) of the dSC are more numerous per square area than are those in the plastic filter holder of the outflow port of dIVC lids (Figure 1 C).

Figure 3.

Top view of cage lid for the ndIVC, showing the absence of any filter and the 2 spring-loaded, flap ports through which incoming and outgoing air exchange occurs once the IVC is mounted on the ventilation rack.

Figure 4.

CO₂ levels in dIVC after removal from the ventilation rack (unvent), dSC, ndIVC after removal from the ventilation rack (unvent), and ndSC. Each cage contained 5 adult mice.

Figure 5.

O₂ levels in dIVC after removal from the ventilation rack (unvent), dSC, ndIVC after removal from the ventilation rack (unvent), and ndSC. Each cage contained 5 adult mice.

Figure 6.

CO₂ levels in dIVC after removal from the ventilation rack. Each cage contained 2 to 5 adult mice.

Figure 7.

O₂ levels in dIVC after removal from the ventilation rack. Each cage contained 2 to 5 adult mice.