During exhalation air flow occurs because of the pressure difference between the alveoli and the surrounding atmosphere. The increase in alveolar pressure acts to compress the air inside the lung and because of this compression the decrease in lung volume during exhalation is always going to be greater than the volume of exhaled air. This effect is known as Thoracic Gas Compression (TGC). The flow rate that occurs for a given alveolar pressure depends primarily on airway resistance. When an individual has airway obstruction their airway resistance is increased and they often attempt to increase their expiratory flow by increasing the amount of force they apply during exhalation. This increased force further compresses the air inside the lung and increases TGC. Numerous researchers have shown that there is usually a substantial differences in TGC between subjects with normal lungs and those with airway obstruction.

A number of physicians and researchers I’ve known and respected have said that in spirometry it always comes down to FEV1 since it is the primary indicator for airway obstruction. Certainly FVC and the FEV1/FVC ratio are important but because patients can stop exhaling early for any number of reasons FVC can be underestimated which in turn can cause the FEV1/FVC ratio to be overestimated so they are not quite as reliable as FEV1.

There have been numerous criticisms of the single-breath DLCO technique, many of them quite valid. In particular, the standard equation for calculating DLCO makes no consideration for the inspiratory and expiratory phases of the maneuver when lung volume and the alveolar capillary surface area are changing. Some investigators have devised ways of correcting for inhalation and exhalation, however other investigators have sidestepped the issue entirely by showing that DLCO can instead be calculated from information acquired only during exhalation.

When DLCO tests are performed my lab’s standard policy to average two or more results that meet the criteria for quality and reproducibility. It is not unusual for us to perform three DLCO tests and have all of them meet quality criteria but to have one test result that is higher than the other two. Unlike spirometry tests, bigger isn’t necessarily better for DLCO, so in a circumstance like this we will average the two closest results rather than choose the highest result. Even though the higher test results can come from a DLCO test with good quality, I think that reproducibility trumps this and that choosing by reproducibility gives us results that are more clinically reliable.

The ATS-ERS standards on lung volume measurements says that measured TLC and RV can be calculated either by RV = FRC – ERV then TLC = RV + SVC or by TLC = FRC + IC then RV = TLC – SVC with the preference going to the first method. Strictly speaking, given the same FRC and SVC measurements either method is going to end up with exactly the same calculated TLC and RV values. Conceptually speaking I believe that TLC = FRC + IC is a more relevant way to think about TLC but this is only because I think that patients find it easier to perform a quality IC maneuver than a quality ERV maneuver.

The Hospital Information Systems (HIS) at different medical centers have grown up mostly in isolation from each other. Even when an HIS is installed by a national vendor, each individual hospital has tended to make its own customizations and to follow past conventions. This is changing and it is changing because there are a number of issues driving rapid improvements in inter-hospital communication. The Meaningful Use (MU) Act is major factor and one that has been helping to set the pace, but because improved communication lowers costs and improves the quality of care insurers and medical institutions have been moving in this direction for their own reasons as well.

The last several decades has seen a complete transition to the use of computers in pulmonary function testing. This has improved Lab efficiency, but it is also the new baseline. Further improvements in technology may improve the reliability and accuracy of test equipment and test results, but it is unlikely to improve PFT Lab efficiency any more than it already has. Report management, which is really information management, has started but hasn’t yet completed the same technological transition and it is here that significant improvements can still be made. These improvement will not only improve the efficiency of the pulmonary function lab, but also its clinical effectiveness for the physicians and patients that are the lab’s customers.

One of the hallmarks of chronic asthma is airway inflammation. This frequently causes an increase in the perfusion of the airways which in turn can appear as an increased DLCO in routine PFTs. A number of investigators have noted that this inflammation can also cause an increase in exhaled air temperature. This increase in exhaled air temperature is not due to an increase in body temperature but to increase in the rate of heat exchange between the airways and respired air due to the increased airway perfusion.

The manager of a nearby PFT Lab made an interesting statement recently, and that was that DLCO measurements made with a sample balloon were superior to those made with a real-time gas analyzer. I think that he is biased to some extent by the fact that the manufacturer of his lab’s testing systems only supports the sample balloon approach to DLCO testing and so that is what he is used to. The same can be said of me however, because all of my lab’s equipment performs DLCO tests using real-time gas analysis and that is what I am used to.

Late in 2012 the Global Lung Function Initiative (GLFI) released their reference equations for spirometry. Although not without some criticism this was an important step towards the development of a single spirometry reference equation that can be used by PFT Labs worldwide. The GLFI recently announced that they were accepting data for their TLCO Task Force. This is project similar to what was done for spirometry, and is intended to create a set of reference equations for TLCO (DLCO) that is applicable to all ages and ethnicities.