I’ve been reading Miller et al’s Laboratory evaluation of Pulmonary Function which was published in 1987. That was an interesting time since PFT equipment manufacturers had mostly transitioned to computerized systems but there were still a lot of manual systems in the field. For this reason the book’s instructions are still oriented mostly around manual pulmonary function testing and there are numerous warnings about double-checking the results from automated systems.
The book includes extensive discussion on the calculations and formulas used for testing which makes it useful as a teaching resource. The authors were also very concerned about the correct way to run a PFT lab so there is a fair amount of discussion about staff requirements for education and training (including the medical director) and staff behavior and conduct. To this end each chapter includes extensive instructions on the proper way to perform tests and treat patients. Although the tone of this is somewhat dated and I’d like to say these kind of reminders shouldn’t be necessary, it doesn’t hurt to set a standard on the level of professionalism we should aspire to.
What caught my eye though, was a section in the chapter on Normal Values titled Interdependence of Normal Values which discussed of the value of deriving predicted TLC from predicted FVC. The authors were concerned that reference equations for different tests (and not just lung volumes) were being selected without concern for how well they fit together. I’ve previously written about the problems that results when inserting the reference equation for FVC into the reference equations for lung volumes. In one instance, the TLC was adjusted so that the final predicted TLC was equal to RV + VC, but this meant that TLC (and IC) were changed from the original reference equations. In another, the FVC was just substituted for SVC without adjustment which meant that RV + VC was not equal to TLC and IC + ERV was not equal to VC and this makes interpreting results problematic. What this means however, is that almost 30 years after this was published, this problem is still around.
As a solution, the authors point out that ratios, such as the FEV1/FVC ratio and the RV/TLC ratio tend to be relatively independent of height.
Since:
TLC = FVC + RV
This can be mathematically re-written as:
Which means that TLC can be derived from predicted FVC if the RV/TLC ratio is known.