I’ve had a number of reports across my desk in the last couple of weeks with both elevated and reduced FRC’s that were associated with a more-or-less normal TLC. I reviewed the raw data from all of these tests (I review the raw data from all lung volume tests) and in only a few instances did I make any corrections to the report. This made me think however, about what, if anything, is an abnormal FRC trying to tell us?
The answers to that question range from “a whole bunch” to “not much” to “darned if I know”. When you measure lung volumes TLC is really the only clinically important result. RV can be useful at times but although the other lung volume subdivisions may play a role in the measurement process they have only a limited diagnostic value. All lung volume measurements start with FRC, however, and if you don’t know you have an accurate FRC how do you know that TLC is accurate?
FRC is a balance point of opposing forces in the lung and thorax. Lung tissue wants to collapse, the rib cage wants to spring open and the diaphragm wants to do whatever muscle tone, gravity and the abdomen allows it to do. All of these forces are to one extent or another dynamic and can change over time. These changes can occur both slowly and rapidly, and are the primary reason why isolated changes in FRC don’t tend to have a lot of clinical significance. For all lung volume measurements however, one primary assumption is that FRC does not change during the test and this isn’t necessarily true.
One of the simplest ways to affect FRC is by changes in body posture or by changes in ventilatory rate and volume. All of the lung volume test procedures attempt to control this first by having the subject sit upright and not change position during the test, and second by having the subject breathe quietly and regularly (at least during the times that are meant to establish FRC). We’ve always taught the importance of these things to our technicians but I’ll never forget the time I walked into one of our testing rooms during a FRC test only to find the patient slouched way back in the chair with one leg crossed over his other knee because the technician was letting the patient be “comfortable”. I’d like to hope this was an isolated incident (and it probably really was) but when I review a report there’s no way for me to know what position the patient was in and I have to wonder if some of the change in FRC between tests (we always try to perform lung volume tests more than once) is due to differences in the patient’s posture.
Trying to control a patient’s ventilation is in many ways a more difficult proposition. FRC tends to rise when a person hyperventilates (although the amount it increases is probably related to the degree of airway obstruction). Many patients are scared or anxious when they come to the PFT lab and this can affect their “normal” breathing pattern. Even the calmest patient can become hyper-conscious of their own breathing and adopt an unusual breathing pattern. In one sense, a non-normal breathing pattern doesn’t matter as long as the patient breathes the same way throughout the test since a change in FRC due to a change in breathing pattern does not by itself affect the TLC or RV. I’ve often seen that once the patient had done their first test that they relaxed significantly during the second test. When this happened the FRC was often quite different between the two tests but TLC and RV didn’t really change.
The helium dilution technique is probably most susceptible to error from short-term changes in FRC because it tends to take the longest amount of time to make a measurement and because the patient needs to breath at FRC during most of the test. For both the N2 washout and plethysmographic techniques, FRC is determined at the beginning of the test during a short period of quiet breathing. This short period would seem to limit the exposure to possible changes in FRC but it also means that the measured FRC comes from only a few breaths and that in itself increases the potential for error.
Realistically though, I consider the effect of posture and ventilation to be part of the background noise of lung volume testing. It’s always going to be there, but unless there is something particularly unusual occurring with either of these the amount of error they introduce into the lung volume measurement is probably small and more likely to affect the FRC than the TLC or RV. When I see a reduced or elevated FRC I am far more inclined to worry about testing errors, but which errors I look for depend on which test was used to measure lung volumes.
Both the helium dilution and N2 washout techniques are sensitive to leaks and leaks always lead to an overestimation of FRC. Large leaks are relatively obvious since they usually prevent the gas analyzer readings from stabilizing. Intermittent leaks (which usually occur when the patient adjusts their lips around the mouthpiece or tries to swallow) can show up as abrupt spikes or drops in analyzer readings. Smaller leaks though, can be very difficult to detect since they don’t necessarily prevent the patient from equilibrating but instead just slow the process down. Test length is related to the degree of airway obstruction so when I see a helium dilution or N2 washout test that seems too long in a patient with a normal-ish FEV1 I become suspicious.
A leak during plethysmography can lead to either over- or under-estimation of FRC. Leaks most often occur during the closed-valve panting phase of the test and tend to shift the baseline of the tidal breathing following it. FRC is determined by the tidal breathing before panting and since the SVC maneuver is performed after the panting an upwards shift will cause TLC to be overestimated and a downwards shift will cause it to be underestimated. I would say that leaks during tidal breathing are relatively easy to detect because they cause the tidal baseline to angle upwards or downwards, but after the panting phase has ended and the valve has re-opened, I’ve seen patients take a deep breath which can shift their tidal baseline upwards and it can take a half dozen breaths or longer for them to return to their original FRC baseline and it can be hard to differentiate between this and a leak.
Since FRC (TGV really) is measured from the angle of the loops made during the closed-valve panting phase of plethysmography, the wrong angle will of course cause it to be inaccurate. Despite the best efforts of technicians and patients loops can have odd contours and determining the angle can be difficult. I acknowledge this, but after each test our lab software displays the loops as a composite (all loops in one graph) and the technician has to right-click and select an option from a context menu in order to show the loops from the individual pants. All too often I’ve seen where the composite loop looked okay but when the individual loops were inspected, one or more of them was either suboptimal and shouldn’t have been used or just had the wrong angle calculated for it. Our technicians know they are supposed to inspect the individual loops so I’ll be kind and say they occasionally overlook this step because the composite loops look fine but this is one reason why I inspect the raw data for each loop from all plethysmograph tests.
About the only way I know for helium dilution or N2 washout measurements to underestimate FRC is for the test to end too soon. This should be a no-brainer but all too often I’ve seen our technicians rely on the testing system to say when the analyzer readings have stabilized and I know that the software algorithm for determining this is simplistic. A company service technician recently chided one of our technicians for not ending a helium dilution test at the 90 second mark because the software said it had reached equilibration. I disagree with this first because it’s been my experience that an average patient with normal lungs usually takes around 2-1/2 minutes to equilibrate but more importantly I’ve also seen that the end-of-test algorithm is often fooled by a process I call overshoot. Overshoot is usually caused by the test system adding too much oxygen to the breathing circuit too quickly. In this instance FRC will be overestimated if the test is ended when the software says it should because there is a period where the analyzer reading stabilizes at a low value before returning to its true equilibration point which is at a higher helium concentration.
When all is said and done however, and you are reasonably certain that FRC has been accurately measured and it’s still higher or lower than it “should” be, is this telling you anything clinically about the patient? This is where the answer of “not much” comes into play. There is an association between obesity and a reduced FRC, but not all patients that are morbidly obese have a reduced FRC. There are also plenty of patients with a normal BMI that have a reduced FRC so you certainly can’t say that a reduced FRC is solely due to obesity. Unless TLC is reduced I think that commenting on a reduced FRC in the presence of obesity is not particularly meaningful, and if it is then I think the comment should be about the TLC instead of the FRC anyway.
There is an association between COPD and an elevated FRC, but again not all patients with moderate to severe airway obstruction have an elevated FRC and there are lots of patients with an elevated FRC that don’t have any airway obstruction. To my mind, in COPD it’s not gas trapping or hyperinflation if the RV is not elevated as well as the FRC and even then RV can be overestimated by a suboptimal SVC maneuver so you need to be sure all your ducks are in a row before making a comment in that direction.
It is possible that an abnormal FRC seen in association with a normal TLC, normal spirometry and normal DLCO is trying to tell us something about the patient. It’s also possible that changes in FRC from one visit to another is also saying something. I’ve not seen this particular question addressed in any research study I’ve read however, so “darned if I know”.
There’s not a lot of clinical significance to FRC. It’s dynamic and can change significantly simply because of changes in posture or breathing. Population studies that have developed reference equations for lung volumes have a higher standard deviation and coefficient of error for FRC than they do for TLC and RV. The primary importance of an elevated or reduced FRC is to alert us to possible errors in the lung volume measurements but the fact is that I’m skeptical about all lung volume measurements until I’ve seen the raw test data and even then I can be hard to convince.
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