Evaluating spirometry predicted equations for the elderly

Posted on October 12, 2012 by Richard Johnston

I’ve mentioned previously that the PFT Lab I am associated with recently went through a major hardware and software update. As part of this update we decided to change spirometry predicted equations to NHANESIII. The lab has used the Morris equations for at least the last 25 years and this has caused us to revisit a number of issues associated with interpretation of results, one of which is age.

The software update included the NHANESIII equation set but when we selected it we found that the software would not calculate predicted values for patients over the age of 80. The manufacturer said that this was because that was as far as the age range went in the original NHANESIII study and for this reason they could not extend it. Furthermore, their recommendation was to use the Crapo or Knudsen equations for ages above 80 because they were “more linear”. Continue reading →

Is the MVV clinically relevant?

Posted on October 10, 2012 by Richard Johnston

The respiratory system is in part a mechanical pump or bellows. The Maximum Voluntary Ventilation test (MVV, aka Maximum Breathing Capacity, MBC) is intended to measure the maximum ventilation a patient is capable of. As such the results are dependent on a patient’s lung volume, respiratory muscle strength and endurance, airway resistance and overall inertia of the thoracic cage.

When I started doing PFT’s in the early 1970’s the MVV was a standard part of a complete workup. This has long since changed and I have not performed the MVV test routinely in over 25 years but I’ve always wondered what the MVV test is actually supposed be measuring in a clinical sense.

The ATS/ERS statement on spirometry recommends that the MVV test be 12 seconds long and that for optimum results the patient’s tidal volume should be approximately 50% of their VC at a respiratory rate of 90 breaths per minute. Tidal volume is accumulated during exhalation and at the end of the test the accumulated volume is then multiplied by 5. The ATS/ERS statement also suggests that MVV values that are less than FEV1 (L) x 40 indicate suboptimal patient effort.

Continue reading →