The Slow Vital Capacity (SVC) maneuver is usually performed as part of lung volume measurements. It is not unusual for the SVC to be larger than the FVC, particularly in patients with airway obstruction. This can have a bearing on the FEV1/FVC ratio and in fact the ATS-ERS recommendations for PFT interpretation say that the largest vital capacity value regardless of which test it comes from should be used to calculate the FEV1/VC ratio. When I review a full panel of tests (FVC, lung volumes, DLCO) I always check to see if the SVC or IVC (from the DLCO test) are larger than the FVC and then re-calculate the FEV1/FVC ratio and its percent predicted if they are. Test results that at first glance look normal will instead show airway obstruction often enough when this has been done that the time spent going through this process is worthwhile.

This only works however, when I have a full panel of tests to extract other vital capacities from. Patients that show airway obstruction when their FEV1/VC ratio is re-calculated have often had only spirometry performed on prior visits and their spirometry results were considered to be within normal limits at those times. Our lab software lets us select and report the “best” FVC and FEV1 from a series of spirometry efforts so this raises an interesting question and that is when and how often should a SVC maneuver be performed instead of a FVC maneuver during a spirometry session in order to get and report the largest VC?

The FEV1/FVC ratio is an important component in determining whether or not airway obstruction is present and to some extent in characterizing the degree of obstruction when it is present. Although it is dependent on the quality of both the FEV1 and FVC determinations my experience has been that the FVC tends to be a lot more variable and more dependent on effort than the FEV1. This is one reason why the FEV1/FEV6 ratio has been proposed as a substitute for the FEV1/FVC ratio. Although the FEV1/FEV6 ratio may be easier to perform and more reliable to obtain one reason this approach has not caught on is that the indication for airway obstruction is subtler and not as clearly evident as the FEV1/FVC ratio, particularly for older patients.

The ATS-ERS statement on spirometry says that a forced vital capacity maneuver should be at least 6 seconds long but at the same time acknowledges that this is a minimum standard and technicians and patients should be encouraged to exceed this whenever possible. Even so, I suspect that a lot of technicians are in the habit of terminating a spirometry effort when 6 seconds has been reached often simply because that is what the testing software says is adequate.

The reality is that older patients and patients with airway obstruction often need more than 6 seconds to exhale completely. Of course, the need for a complete exhalation needs to be counterbalanced with the patient’s safety and I will be the first to admit that knowing when to stop pushing a patient can be difficult to determine. The SVC maneuver is usually significantly less stressful than the FVC maneuver so patients can often exhale longer and produce a larger vital capacity for this reason. Additionally, a forced maneuver often causes a degree of airway collapse and the steady effort of an SVC maneuver can often produce a larger vital capacity in the same time period as a corresponding FVC effort. For these reasons I’d suggest that a SVC maneuver should be performed whenever the FVC is below normal and at least moderate airway obstruction is also present or the patient is elderly (and given my current age I am not willing to indicate where the dividing line between middle-aged and elderly may be except to say it’s always going to be somewhere older than me).

Performing spirometry is part ability and part psychology. Many patients stop exhaling long before they reach 6 seconds not because they are not able to exhale longer but because they don’t “get it” or because they don’t like or even fear the sensation of a complete exhalation. (I can’t remember the number of times I’ve had patients stop exhaling after the initial blast oer and over despite my encouragement and even demonstrating the maneuver multiple times myself. Sometimes after enough tries you could see a light bulb go on over their head and they’d say “oh, you mean like this” and then proceed to perform the maneuver flawlessly.) To help patients learn to exhale longer I’ve frequently had them try a SVC maneuver instead. Showing a patient how much they could exhale when they weren’t worried about doing it quickly helped some of them perform the FVC better and even if it didn’t I often got a larger VC anyway.

Some patients, most often those with asthma or bronchiectasis, show a symmetrically reduced FVC and FEV1 during exacerbations of their disease. It has been estimated that up to 10% of all asthmatics show this pattern at one time or another and it is attributed to non-homogeneous distribution of airway bronchoconstriction or collapse. The flow-volume loops of these patients often show a restrictive pattern (normal peak flow with an elliptical contour). The fact is that they are obstructed not restricted and that a steady effort may sometimes enable them to exhale more air than a forced effort. I’d suggest that a patient who has previously shown evidence of airway obstruction (or comes with a diagnosis of asthma or bronchiectasis) and has a low vital capacity with a normal FEV1/FVC ratio should attempt a SVC maneuver.

There are two different ways to perform a SVC maneuver and the way in which a given testing system’s software analyzes spirometry breathing maneuvers may limit which SVC maneuver can be performed. The SVC can be performed by starting with a maximal inhalation to TLC and then followed by a maximal exhalation to RV or instead by starting with a maximal exhalation to RV and ending with a maximal inhalation to TLC. For physiological reasons patients are often able to produce their largest vital capacity when inhaling from RV to TLC but for many patients this is a conceptually more difficult maneuver to perform than exhaling from TLC to RV.

The order in which inspiration or expiration is performed does not matter in the SVC module in our lab testing software but in the spirometry module the software does not recognize or measure an inspiratory vital capacity when it is performed in the “wrong” order. This has limited us to performing SVC maneuvers only by inhalation to TLC and exhalation to RV. This is not necessarily problem except for those labs that have mandated that the SVC maneuver be performed by an exhalation to RV and inhalation to TLC. It may also only be a problem with our lab’s software. The ATS-ERS statement on spirometry says that when an IVC is performed as part of a FVC maneuver, it can be performed either before the forced exhalation or after it. Despite this our lab’s software will only measure an IVC when it is performed after the forced exhalation. A work-around to this problem would be to switch to the SVC testing module when performing the SVC test but I think this adds a layer of complexity on the patient spirometry testing session and the could make reporting and reviewing results confusing.

Since performing an SVC during spirometry needs to be done in only a minority of patients one problem is training staff to know when and when not to perform the SVC. My suggestion would be to perform an SVC as part of spirometry when:

• the FVC and FEV1/FVC ratio are below normal.
• the FVC is less than 6 seconds.
• the FVC and FEV1 are symmetrically reduced in a patient with a prior history of airway obstruction or a diagnosis of Asthma or Bronchiectasis.

There are a number of reasons why performing an SVC maneuver as part of spirometry in order to obtain a larger vital capacity is a good idea. It is something that should be done for quality patient testing and it can lead to a more accurate patient diagnosis and less overall testing for the patient. This is an observation on my part however, and not part of the ATS-ERS standards so determining whether or not this should be done is something each PFT lab will have to decide for itself.

References:

Brusasco V, Crapo R, Viegi G. ATS/ERS Task Force: Standardisation of Lung Function Testing: Standarisation of spirometry. Eur Respir J 2005; 26: 319-338.

Brusasco V, Crapo R, Viegi G. ATS/ERS Task Force: Standardisation of Lung Function Testing: Interpretive strategies for lung funciton tests. Eur Respir J 2005; 26:v948-968.

Hyatt RE, Cowl CT, Bjoraker JA, Scanlon PD. Conditions associated with an abnormal nonspecific pattern of pulmonary function tests. Chest 2009; 135: 419-424.

Swanney MP, Jensen RL, Crichton DA, Beckert LE, Cardino LA, Crapo RO. FEV6 is an acceptable surrogate for FVC in the spirometric diagnosis of airway obstruction. Am J Respir Crit Care Med 2000; 162: 917-919.

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