According to Merriam-Webster's medical dictionary, Spirometry is the measurement by means of a spirometer of the air entering and leaving the lungs. The same dictionary defines a spirometer as an instrument for measuring the air entering and leaving the lungs. There is circularity in these definitions, unfortunately. A better definition would be simpler: spirometry is the measurement of the air entering and leaving the lungs. A spirometer, then, is any of a number of devices used today or historically to accomplish spirometry.
The Mayo Clinic offers a somewhat more informative definition of spirometry here, and the Lung Association of Canada has a nice 3 minute video explaining the modern spirometry test and its purposes, embeded here:
For a more substantial tutorial (roughly 40 minutes including question and answer time), including a number of case studies and related multiple choice questions to quiz your understanding, the American Lung Association has a 14 October 2010 (the first World Spirometry Day) webinar archived here.
Briefly, spirometry is, as Merriam-Webster's suggests, the measurement of air flow into and out of the lungs. Various aspects of this air flow are measured. In modern spirometry tests, subjects are instructed to take as deep a breath as they can, then to exhale quickly, but also to sustain the exhalation for as long as they can. Generally six seconds of exhalation is required to obtain useable results.
The history of spirometry is an interesting subject in its own right. Some have suggested that the surgeon John Hutchinson invented spirometry. In 1844 Hutchinson presented findings to the Statistical Society of London relating to two devices he had developed to measure the capacity and power of the lungs.1 By capacity, Hutchinson meant, "that quantity of air which an individual can force out of his chest by the greatest voluntary expiration, after the greatest voluntary inspiration." He referred to this as vital capacity, as he believed it would be strongly predictive of health and longevity. Today we call this quantity forced vital capacity (FVC), and it is one of the main measurements taken during a modern spirometry test. Hutchinson had hypothesized that he might be able to predict the vital capacity of a healthy person based on some simple measurements of stature:
My first object was, to discover whether any relation existed between this "capacity" of the lungs, inspiratory and expiratory power and any other external and physical sign; therefore I submitted the whole number of subjects to all the observations enumerated in Table B (page 3), and to my gratification I did discover a relation intimately existing between this capacity and power, and the height of the individual, as I have indicated by the bracket on that table. I shall demonstrate most clearly to this Society, that so uniform is this relation, that if I be allowed to take a man's height, I can tell what the capacity of his lungs and his inspiratory and expiratory powers should be, to constitute him a healthy individual.
In his original paper Hutchinson presented spirometry results on more than 2,000 persons, demonstrating this uniform relationship between lung capacity and height, and also showing that persons with various illnesses, including tuberculosis (consumption), scoliosis, and others, had markedly reduced lung capacities and powers. He remarked further:
Not only does disease in the chest limit the natural capacity; but also an enlargement of any of the visceral organs, acting so as to prevent the arch of the diaphragm freely alternating in its curve. A moderate meal reduces the 'capacity' from 4 to 6 cubic inches, and a plentiful dinner from 9 to 14 inches, according to the powers of the individual at table. The capacity of those who suffer from curvature of the spine is most remarkably small. One person was so low as 27 cubic inches, being the utmost quantity he could throw out of his chest by one full expiration.
And he observed:
The greatest capacity I have ever observed was that of Freeman and Randall, both measuring upwards of 6 feet 11 - 1/2 inches. Freeman's capacity was 432 cubic inches, and Randal's 464 cubic inches. The lowest [healthy] capacity I have examined is that of Robertson, height 3 feet 9 inches, being 80 cubic inches."
Hutchinson reported varying levels of lung capacity and power for different professions or social classes, including such categories as "sailors", "fire brigade", "pugilists", "paupers", and "gentlemen". He noted that expiratory power, which he measured according to the number of inches of mercury a subject could move by exhaling into a tube, was greater for persons in professions that required the regular use of their lungs. Jewelers, for example, "who use the blow-pipe much" could be expected to have a higher than average power of expiration, all else being equal. Another very interesting observation concerned very heavy individuals. His general rule regarding height being predictive of lung capacity held very true for heights from 5 feet up to 6 feet, but he noted that exceptions had to be made, "for very stout and corpulent individuals, whose capacity I find to stand the lowest." John Hutchinson also hypothesized that vital capacity and other measures of lung function would be a strong predictors of mortality, and this has certainly proven to be true, particularly for persons with chronic obstructive pulmonary disease.2-5
A 1978 historical review6 noted that several devices for measuring vital capacity (FVC, and various similar quantities) and other measures of lung capacity or power had been developed prior to Hutchinson's 1844 treatise. One such instrument was developed by the scientist and inventor James Watt, who, according to the article, "took his consumptive son to see Thomas Beddoes and to breathe the new medicinal gases; they all became high on nitrous oxide with the poets Coleridge and Southey, and Roget of Thesaurus fame." This article noted, however, that the spirometers in common use at that time (1978) were essentially the same as that developed by Hutchinson, but for improvements in size and the addition of various bells and whistles, including built-in graphic and timing devices. This perhaps adds some weight to John Hutchinson's being considered the inventor of what would become modern-day spirometry.
In addition to FVC, a number of other measurements or calculations are recorded in a modern spirometry test. Of particular interest are the volume exhaled during the first second of what generally should be six seconds or more of exhalation, known as the forced expiratory volume in one second (FEV1); and the ratio of FEV1:FVC, which gives the proportion of total exhaled lung volume that occurs in the first second of exhalation. When one or another of FVC, FEV1 or the ratio FEV1:FVC is lower than normal for a person of comparable height and age this may be indicative of impaired lung function, and treatment may be indicated. Furthermore, lower values of FVC, FEV1, and especially the ratio of FEV1 to FVC are associated with greater all-cause mortality risk and are used to define the level of severity of COPD.4,5 There is much more to spirometry than this, and I again refer the reader to the very informative webinar at the American Lung Association for further information on how the tests are done and how the results are interpreted. For more on the history of spirometry, the articles cited above, and others, make for very interesting reading.1,6,7,8
- Hutchinson J. Contributions to vital statistics, obtained by means of a pneumatic apparatus for valuing the respiratory powers with relation to health. Journal of the Statistical Society of London, Vol. 7, No. 3 (Sep., 1844), pp. 193-212.
- Beaty TH, Cohen BH, Newill CA, Menkes HA, Diamond EL, Chen CJ. Impaired pulmonary function as a risk factor for mortality. Am J Epidemiol. 1982 Jul;116(1):102-13.
- Hole DJ, Watt GC, Davey-Smith G, Hart CL, Gillis CR, Hawthorne VM. Impaired lung function and mortality risk in men and women: findings from the Renfrew and Paisley prospective population study. BMJ. 1996 Sep 21;313(7059):711-5; discussion 715-6.
- Mannino DM, Diaz-Guzman E. Interpreting lung function data using 80% predicted and fixed thresholds identifies patients at increased risk of mortality. Chest. 2012 Jan;141(1):73-80.
- Garcia-Aymerich J, Serra Pons I, Mannino DM, Maas AK, Miller DP, Davis KJ. Lung function impairment, COPD hospitalisations and subsequent mortality. Thorax. 2011 Jul;66(7):585-90.
- Spriggs EA. The history of spirometry. Br J Dis Chest. 1978 Jul;72(3):165-80.
- Kiraly A. History of spirometry. Journal for Pre-Health Affiliated Students, University of Illinois at Chicago. 2005;4(1). Accessed on 29 April 2013 here.
- Petty TL. John Hutchinson's mysterious machine revisited. Chest. 2002 May;121(5 Suppl):219S-223S.