Hyperbaric whole-body plethysmograph for rodents

Described is a rodent size whole-body plethysmograph constructed of two concentric Plexiglas cylinders. Chamber temperature is controlled by circulating water of constant temperature between the cylinders, the entry gas also being preconditioned to a desired temperature. An aluminum plate partitions the chamber into an animal compartment (1.4 liters without the rat) and a transducer compartment (1.1 liters). Dynamic calibration indicates the system response is uniform between 60 and 270 cycles/min. Static calibration shows a linear correlation between the transducer output (in microV) and the calibration volume. Under hyperbaric conditions such linear correlation also exists, but the transducer outputs are greater for a given calibration volume. For a given volume of injection the transducer output is a linear function of log-ambient pressure. Respiratory variables measured with this plethysmograph are comparable to those reported in the literature both for 1 ATA and under hyperbaric conditions. The plethysmograph is also adaptable for measurement of oxygen consumption and CO2 production, under elevated ambient pressure. Caution for this application is given in regard to 1) the protracted time requirement for reaching a steady state, 2) accumulation of CO2 in the chamber, and 3) diminishing gas concentration of interest between the entering and existing gases, suggesting a trade-off in selecting chamber size, ventilation rate, and level of acceptable CO2 in the chamber.