| Gas exchange: units, etc.
Units: As a last step, you need to select the units for gas exchange. In the final window of the sequence, you are presented with 4 or 5 edit fields containing mass, B.P., Ta, etc. Adjust these as necessary. You can select from a variety of conversion units in the pop-up menu (shown at right with ml/min selected).
If you are calculating VO2 or VCO2 in watts or joules/time, you also need to set the correct value for heat equivalency (an example for a VO2 calculation is shown at right). The defaults (20.1 J/ml for VO2 and 25.0 J/ml for VCO2 ) are reasonable for mixed diets. Theoretical heat values are shown for pure carbohydrate, fat, and protein diets; see Gessaman and Nagy (1988, Physiol. Zool. 61:507-513) for a discussion of gas exchange conversion factors for metabolic calculations.
You can click buttons to select appropriate values for carbohydrate, lipid, or protein substrates. It is possible to directly enter your own value into the edit field, however.
You can show the conversion equation by clicking the 'show equation' button:
The 'instantaneous' correction helps compensate for the volumetric washout characteristics of respirometry systems, using the 'Z' correction approach modified for exponential washout kinetics (Bartholomew, Vleck, and Vleck 1981, Journal of Experimental Biology 90, 17-32). It works best in systems with large flow rates relative to effective volume. In general, it is prudent to avoid this somewhat rude manipulation of data if the system volume is large and the flow rate is low, unless the chamber is very well stirred (as in a recirculating wind tunnel or a chamber with mixing fans). In particular, be aware of the following:
NOTE: Movements of animals within chambers, relative to the position of input and output ports, can introduce very ugly errors in instantaneous calculations because of transient artifactual enrichments or rarifactions of local gas concentrations that have nothing to do with real metabolic rates.
Basically, the instantaneous conversion algorithm works from estimations of how rapidly a gas concentration that has been transiently deflected from a constant value will return to a state of zero deflection if no other changes occur. Another way of putting it is as follows: if an animal instantly changes its metabolism to a new steady-state value, how long does it take for the excurrent gas concentrations to achieve their new steady state values? An instantaneous change in metabolism will be detected at the gas analyzer as a gradual approach to new steady-states, as shown schematically here:
What the 'instantaneous' conversion does is back-calculate from the measured concentration changes to approximate the real event:
For VO2 calculations the algorithm assumes incurrent oxygen concentration is 20.95% and offset so as to read zero at 20.95% oxygen. For carbon dioxide and water vapor it assumes that incurrent concentrations are zero % (not offset). Calculations will yield inaccurate results if these assumptions are violated. The so-called effective volume is necessary for these calculations. It can be derived from a washout curve for the system being used.
The response correction option in the TRANSFORMATIONS routines (EDIT menu) can also adjust for mixing problems and washout characteristics. It is based on an unmodified Z-transform, and in some cases may be easier to use than the instantaneous correction.