Determining Residual Volume

 

To maximize accuracy of body composition assessment by hydrostatic (underwater) weighing, residual volume (RV) must be measured rather than estimated.  Residual volume is the amount of air that remains in the lungs following a maximal exhalation, which is performed during an underwater weighing.  RV can be estimated [RV = vital capacity * 0.24 (for males; or RV = vital capacity * 0.28 (for females)], but the potential for error is quite large resulting in decreased accuracy of the body composition assessment and making it no better than that from a good skinfold measurement.  Wilmore (1969) developed a method of indirectly measuring RV using an oxygen dilution technique.  Although the protocol can be performed with gas analyzers in our laboratory, the TrueMax 2400 cart is equipped to perform the measurement more easily.  Follow the instructions below:

 

  1. Turn on the TrueMax 2400 cart and allow it to warm up for at least 15 min, preferably 30 min.
  2. Set the black rocker switch, located on the left side in the back of the analyzer unit, from the 25% setting to 100%.  This causes a statistical change in the scale on which the O2 analyzer is calibrated and "fools" it into thinking that 100% O2, rather than the 16% O2 is being used as the calibration gas. 
  3. Flush and fill at least three of the black Douglas rebreathing bags with 5 L of 100% O2.  With the bag hanging down from the nozzle, continue filling the bag until the creases disappear from the middle half of the bag.  These points are painted on the bags.
  4. Disconnect the sampling line from the mixing chamber (located on the right rear side of the cart), and then connect an extender line between it and the port on the breathing valve on a Douglas rebreathing bag.
  5. Calibrate the gas analyzers.  Note: When prompted to save the calibration settings, you may get a warning message saying that the calibration parameters changed >3% from the previous parameters with a question as to whether you want to update the current calibration parameters.  If this message appears, indicate that you want to update the new parameters.
  6. From the main menu, select the RV / FRC option, then click on RV, and then OK which indicates that you've calibrated the O2 analyzer in the 100% setting.
  7. Input the subject's characteristics, but don't click OK until the subject is on-line and everything is ready to begin a test.
  8. In a seated position, the subject should be fit with a nose clip and the mouthpiece inserted with the valve turned so that the subject is breathing ambient air.
  9. When everything is ready to begin, click OK; have the subject breathe normally for several breaths.
  10. After several normal breaths, press Trial # X, have the subject do a maximal inhalation followed by a maximal exhalation and momentarily hold his/her breath at the end of the exhalation.  At this point, either the subject or lab technician should turn the mouthpiece valve so that the subject begins rebreathing from the bag.
  11. The subject should take deep breaths (about 2/3 maximal) until the N2 concentration stabilizes (5-8 breaths).  At this point, click End Trial # X and remove the subject from the rebreathing bag.
  12. If the bar on the O2 isn't on the steady-state reading, move it with the mouse, move it to the appropriate point.
  13. If performing another trial, allow the subject to breathe normally for >2 min, reconnect him/her to another rebreathing bag, and click OK.  Repeat the procedures beginning with step #10. At least two trials should be performed, and if the trials differ by more than 0.1 L, perform a third trial. 
  14. If finished, click OK; this will return you to the beginning another test, but click on End Test which will take you to the Residual Volume (RV) Summary page.  On this page, a trial can be edited, and a summary result obtained based on all or selected trials.  There is also an option to perform an additional trial.  When finished, click OK, and follow the prompts to save and print the results.
  15. If finished with RV testing, exit the program, and properly close down Windows.  Also, re-connect the sampling line to the mixing chamber and return the O2 analyzer rocker switch to the 25% setting.