A self-correcting indirect calorimeter system for the measurement of energy balance in small animals

Abstract

Indirect calorimetry involves measurement of CO2 produced and O2 consumed by an organism. These measurements are then used to calculate energy output, metabolic rate (MR), and respiratory quotient (RQ), a relative assessment of carbohydrate and lipid oxidation. By far the most difficult aspect of indirect calorimetry is measurement of O2. Moreover, the abundance of O2 (20.95%) relative to CO2 (0.03%) in ambient conditions dictates that measurement errors of O2 have greater implications on calculations of MR and RQ. Because compressed air is not feasible for use with animals in long-term experiments, changes in ambient conditions are nearly unavoidable. A self-correcting indirect calorimetry system was designed and constructed utilizing differential O2 and CO2 analyzers and a blank cage to monitor ambient conditions periodically. The system was validated by changing ambient O2 and CO2 concentrations by infusing N2 into the system during a test butane burn. MR and RQ were largely unaffected by these changes in ambient conditions, and inclusion of a blank cage in the system accounted for slight calibration offsets. MR and RQ were measured in mice ( n= 95) with and without correction for any small changes in ambient conditions measured in the blank cage. Coefficients of variation for MR and RQ were significantly decreased by taking into account ambient conditions measured in the blank cage ( P < 0.001), which resulted in a 2.3% increase in precision for measurement of MR. This system will be used to more accurately assess long-term measurements of energy balance in the many murine models of leanness and obesity to gain better insights into pathophysiology and treatment of human obesity.