When the spirometer was Quack Medicine

The beginning of pulmonary function testing can best be dated to 1846 when John Hutchinson invented the spirometer. Although similar devices had been used by previous scientists he turned it into a precision instrument and approached the study of lung function systematically. His work inspired innumerable other researchers and inventors, and within a few years improved versions of his spirometer were appearing across Europe and America. By my count within twenty years there were at least four other methods for measuring expiratory volume (at least one of which is still in use) had been applied to a new generation of spirometers.

The initial focus of spirometry was entirely on exhaled volume and the use of spirometers for any other purpose did not begin to change until the 1930’s when the MVV test was proposed and formalized. During this time the accuracy of spirometers improved and many serious researchers laid the groundwork for our current understanding of pulmonary medicine and physiology. This was also a period of time in which what we now call quack medicine flourished. A quick glance in almost any of the newspapers or magazines of that time will show a variety of ads extolling the virtues of different medicines, treatments and devices, most of which now appear to be silly, ineffective or just plain dangerous. As much as we might like to ignore or belittle this sordid period in our medical history, the truth is that spirometers were also one of these quack medicine devices.

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SVC and the FEV1/FVC Ratio

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?

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