Meeting Abstract
One inadvertent consequence of many commonly-employed respirometry equations is that they produce biologically unrealistic estimates of animal metabolism in situations where the composition of respiratory gases (i.e., O2 and CO2) differs strongly from the atmosphere (i.e., hyperoxia, hypoxia, or hypercapnia). This suggests that an alternative set of respirometry equations might prove useful under these specific experimental conditions. We measured changes in fractional concentrations of respiratory gases in laboratory mice and other animals across a range of ambient O2 and CO2 concentrations to illustrate the nature and magnitude of these potential errors. We show that the fractional changes in animal O2 and CO2 caused by metabolism are relatively independent of the ambient O2and CO2 concentrations in acute experiments – in agreement with conventional wisdom – but in clear contrast to the results that would be obtained from many respirometry equations. The magnitude of these errors increases exponentially as experimental conditions increasingly deviate from atmospheric conditions. In fact, some of the most frequently used equations were found to overestimate metabolic rates by several-fold. We conclude that respirometry equations comprising both a numerator and denominator term, which may work perfectly well for normoxia gas conditions, are particularly error-prone, and that the denominator terms [e.g. (1-FinspiredO2) or (1-FexpiredO2)] drive the error in the metabolic estimates. We also demonstrate these errors in an R software computer simulation. We attribute these errors to the propagation of assumptions in a paper published over 60 years ago, and expect that countless reports of metabolic rates have may have propagated these them. We conclude by proposing a simple set of respirometry equations that researchers can use to calculate metabolic rate independent of ambient gas concentrations, thus overcoming this particular limitation of the main equations advocated for use in respirometry.
