I was reading an article recently that made an off-hand reference to the 100% oxygen shunt fraction test. Results from the test were included in the data analysis but the equations the researchers used were not presented nor were they referenced, nor was the procedure described. This is probably because the shunt fraction test and its equations are very much old-school pulmonary physiology but even if the subject is probably covered at one time or another in physiology classes I suspect that some of the issues involved in the calculation are not as well understood as they should be.
There are some similarities between the deadspace-tidal volume ratio (Vd/Vt) and the shunt fraction but even though they are both are involved in gas exchange (and to some extent they also correlate with each other) they are measuring different things. When blood flows through the lung some blood passes through well ventilated alveoli and becomes fully saturated; some blood passes through poorly ventilated alveoli and is only partially saturated; and some bypasses the alveoli entirely. The resulting arterial oxygen content is the summed average of all of these compartments.
There are two different ways that shunt fraction can be measured and calculated; physiological and anatomical. The physiological shunt equation can be performed at any FiO2 (but usually around the FiO2 of room air) and requires that arterial and mixed venous blood samples be taken more or less simultaneously and then analyzed for PO2 and SaO2. The basic formula is:
I often find topics for this blog in a sideways fashion. Recently while searching for something else I ran across an article about the minimum clinically important difference (MCID) of the Residual Volume (RV) in patients with emphysema. I’ve come across the MCID concept before but I had never really followed up on it. This time I started researching MCID and immediately ran across a number of articles about the MCID of the 6-minute walk test (6MWT). This got me to review the articles I have on hand and I found that since I last wrote about the 6MWT I’ve accumulated quite a few new (or at least new to me) reference equations as well as a number of articles about performance issues. Given all this how could I not re-visit the 6MWT?
In addition to the 6 reference equations I had previously I’ve found another 13 female and 14 male reference equations for the 6MWT (total 19 female, 20 male) which is an opportunity to re-visit the selection process. This immediately raises the question about what factors should be used to calculate the predicted 6-minute walk distance (6MWD). Because the 6MWT is essentially an exercise test age has an obvious effect on exercise capacity so it is no surprise that with the exception of one set all of the reference equations consider age to be a factor. It should be noted however, that many of the reference equations are intended to be only applied over a limited range of ages and this may limit their utility.
Given the fact that stride length and therefore walking speed are directly related to height it is somewhat surprising to find that only twelve of the male and eleven of the female reference equations consider height to be a factor. When height is a factor, the predicted 6MWD is usually affected something like this:
Weight also affects exercise capacity but an interesting question is whether the observed 6MWD should be compared to a predicted 6MWD based on a “normal” weight or whether the 6MWD should be adjusted to the individual’s actual weight and assessed accordingly. To some extent this is already an issue in current PFT predicted equations. For example, weight is not a factor in any of the FVC or TLC reference equations and when lung volumes are decreased in the presence of obesity they are considered to be abnormal. On the other hand, the reference equations I use for maximum oxygen consumption during a CPET include weight as a factor and for a number of reasons this is likely the correct approach. For this last reason I would think that weight should be a factor and ten of the reference equation sets consider weight (or BMI) to be a factor. When weight is a factor, the predicted 6MWD is usually affected like this:
For the first dozen or so year that I worked in a pulmonary function lab it was with counter-weighted, volume-displacement water-seal spirometers more or less like this:
Patients would do a series of tests and I’d end up with a bunch of pen traces on kymograph paper that I’d have to measure with a ruler and use a desktop calculator (it was about a foot square, weighed a couple of pounds and had a nixie tube digital display) to create a hand-written report. I’m not going to suggest that these spirometers were in any way better than what we’re using now but I have to say that I would have seen the following problems more or less immediately.
Recently I was reviewing a report from a patient with very severe obstruction and noticed something a bit off about the flow-volume loop. Specifically, the end-exhalation of the tidal loop looked like it was at a significantly higher volume than the end of the FVC effort.
Because the high-frequency sawtooth pattern (from the patient, not the equipment) makes it a little hard to see if this is what was really happening, I downloaded the raw data and re-graphed the volume-time curve with a spreadsheet.