| Respirometry fundamentals
These issues -- and how LabAnalyst deals with them -- are addressed in more detail below.
Note: if you are a respirometry geek -- and who else would waste time reading this? -- you may have noticed that I'm not using the correct abbreviations for rates of oxygen consumption and carbon dioxide production. Proper usage includes a dot over the V in VO2 and VCO2 to indicate rates instead of volumes.
Alas, I haven't found a convenient way to make the 'Vee-dot' symbol in html.
In constant volume respirometry, the organism is placed in a sealed chamber, and over time its respiration changes the gas concentrations in the chamber. You measure rates of gas exchange by determining gas concentrations (O2 and/or CO2) at the start and end of a period of measurement, and then using the cumulative difference in concentrations and the elapsed time to compute the average rate of change. The most straightforward way to handle constant volume calculations with LabHelper and LabAnalyst is as follows:
- First, collect samples of 'initial' and 'final' gas from the animal chamber(s) and inject them through a gas analyzer while continually recording the concentration (in %) with LabHelper. Between injections, flush the analyzer with reference gas (or fluid). You should get a data file with a series of 'peaks', one for each injection of 'final' gas (or fluid).
- Next, in LabAnalyst, use the baseline function to set the 'initial' values at zero. The 'final' values now show the % change during the measurement period.
- Finally, for each peak, find the maximum deflection from baseline with the 'maximum value ' option in the ANALYZE menu. Click the 'C.V.R. options...' button to set up the constant volume calculations.
A more versatile method for computing C.V.R. is in the SPECIAL menu. This offers more options than the C.V.R. window in the maximum value calculatorIf you have an oxygen or CO2 sensor within the sealed chamber (or if the respiratory fluid is circulated past a sensor), you can measure the decline in pO2 or increase in pCO2 continuously, and then take the derivative of the change to get time-specific rates.
- With any gas analyzer, 'extra' gases dilute the concentrations of O2 and CO2. Unless you quantify this, you might not calculate VO2 or CO2 accurately. Most animals don't emit enough of these gases to cause much of a dilution problem, but you need to be aware of the potential.
- With some oxygen analyzers that have high-temperature measurement cells (like the zirconia cells used by Applied Electrochemistry/Ametek S-3As), another error can result when organic gases oxidize -- combust -- in the cell. The S-3A's cells operate at more than 500 °C, so there is plenty of potential for this to occur. Oxidization removes oxygen from the gas stream. It also produces CO2, water, and other reaction products that lower the concentration of the remaining O2 and further increase the error (generating an artifactually high VO2).
If your animal does produce substantial quantities of non-standard respiratory gases, the solution is to remove the problematic gas species from analysis air by using an appropriate scrubbing filter upstream of the gas analyzer(s). This, too, incurs operational penalties, such as reduced response time and more scrubbing tubes to keep fresh.