Hypoventilation is defined as ventilation below that which is needed to maintain adequate gas exchange. It can be a feature in lung diseases as diverse as chronic bronchitis and pulmonary fibrosis but determining whether it is present of not is often complicated by defects in gas exchange. When desaturation occurs during a CPET (i.e. a significant decrease in SaO2 below 95%) this is a strong indication that the primary exercise limitation is pulmonary in nature and from that point the maximum minute ventilation and the Ve-VCO2 slope can show whether the limitation is ventilatory or instead due to a gas exchange defect. But in this circumstance what what does it mean when both the maximum minute ventilation and Ve-VCO2 slope are normal?

During this last week I was contacted by two different individuals who were asking for help in understanding their PFT results. In both cases they had a markedly elevated TLC and the interpretation included the notation that they had gas trapping and hyperinflation. Even though the amount of information they provided was minimal I am extremely skeptical that the TLC measurements were correct. Gas trapping usually only occurs with severe airway obstruction. Hyperinflation, which at minimum consists of a chronically elevated FRC and RV, usually only occurs after prolonged gas trapping. An elevated TLC usually occurs only with prolonged hyperinflation and given the improvements in the care and treatment of COPD I’ve seen over the last several decades, has become relatively uncommon.

A low FEV1/VC ratio is the primary indication for airway obstruction. From ATS/ERS Interpretive Strategies for Lung Function tests, page 956. The ATS/ERS statement on interpretation says “The VC, FEV1, FEV1/VC ratio and TLC are the basic parameters used to properly interpret lung function (fig. 2). Although FVC is often used in place of VC, it is preferable to use the largest available VC, whether obtained on inspiration (IVC), slow expiration (SVC) or forced expiration (i.e. FVC).”

One of the more recognizable flow-volume loop contours is the one associated with severe airway obstruction. Specifically, this type of loop shows an abrupt decrease in flow rate following the peak flow with a more gradual decrease in flow rates during the remainder of the exhalation. This abrupt decrease in flow rates was first described on a volume-time curve and the inflection point was called a “kink” but this point also corresponds with the inflection point on the flow-volume loop. This feature has also been called a “notch” or a “spike” but a number of researchers have called this the Airway Collapse pattern (AC) and it is more formally defined as a sharp decrease in flow rate from peak flow to a discontinuity point at less than 50% of the peak flow and occurring within the first 25% of the exhaled vital capacity.

Dozens of articles have been written about the correlation between different abnormal flow-volume loop contours and pulmonary disorders. In contrast very little has ever been written about what constitutes a normal flow-volume loop and what this looks like has been primarily anecdotal. Interestingly, the ATS/ERS standard for spirometry includes an example of a “normal” flow-volume loop but its source and what makes it normal is not explained. From the ATS/ERS standard on spirometry, page 327.

Recently my lab has had some turnover with a couple of older staff leaving and new staff coming on board. While reviewing reports I’ve found a number of instances where the incorrect FVC and FEV1 were reported. Taking these as “teachable moments” I’ve been annoying the staff with emails whenever I find something notably wrong. I had thought that our rules for selecting the best FVC and FEV1 were fairly straightforward but given the number of corrections I’ve made lately it seemed like it would be a good idea to revisit our policy on this subject.

Recently a report came across my desk from a patient being seen in the Tracheomalacia Clinic. The clinic is jointly operated by Cardio-Thoracic Surgery and Interventional Pulmonology and among other things they stent airways. The patient had been stented several months ago and this was a follow-up visit. Given this I expected to see an improvement in spirometry, which had happened (not a given, BTW, some people’s airways do not tolerate stenting), but what I didn’t expect to see was a significant improvement in lung volumes and DLCO.

Everyone uses the FEV1/FVC ratio as the primary factor in determining the presence or absence of airway obstruction but there are differences of opinion about what value of FEV1/FVC should be used for this purpose. Currently there are two main schools of thought; those that advocate the use the GOLD fixed 70% ratio and those that instead advocate the use the lower limit of normal (LLN) for the FEV1/FVC ratio.

This relatively odd DLCO testing error came across my desk today. Although it’s fairly unusual it brings up some interesting points about how the Breath-Holding Time (BHT) is determined and what effect it has on DLCO. Specifically, at the beginning of the DLCO test the patient took a partial breath in, then exhaled, then took a complete breath in. The patient performed the DLCO test three times and did exactly the same thing each time despite being coached by the technician to only take a single breath in. I’m sure this says something about human nature but I’m not exactly sure what.

This idea originates, as far as I know, from Michael Sims, president and CEO of NspireHealth. I got it second hand and suspect that it is a small part of a larger presentation but this one point is worth discussing by itself. Specifically, we call the places we work “Pulmonary Function Laboratories” and this is at best an outdated and somewhat obscure term that doesn’t do much to make it clear what we do.