A disease disturbs function of all organs of the body. The function of the lung can be measured with a relative ease. Hutchinson described the possibility of measuring the lung vital capacity by spirometry in the year 1846 (1). Since that time spirometry became an important tool in clinical investigations in the field of pneumology. Lung function analysis plays an important role for the diagnosis, the prognosis, the treatment, and the surveying in clinical practice. The quality of pneumology depends highly on such investigations. Even though spirometry has been around for such a long time, it is still believed to have its place in pneumology. But later on, other methods have been adopted for lung function measurements, when difficult problems concerning spirometry were realized. Spirometry uses different variables of lung function, such as forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1), peak expiratory flow (PEF), maximal flows at 75%, 50% and 25% of forced exhalation (MEF). The variables of the flow-volume-curve play an essential role in spirometry. In view of many recommendations, FEV1 is the dominant variable and some believe that the use of FEV1 would be enough to answer most clinical questions put forward. But generally speaking that does not seem true. What are the problems? Two most important issues that are difficult and unavoidable are these:

• Spirometry depends on the cooperation of the investigated subject so much or even more as on the investigator’s skills and approach. Full inspiration or expiration is difficult to achieve. A survey of ten medical centers, where well-trained technicians worked, showed that nearly 50% of the results were unacceptable due to a failure to complete the test (2).
• Results of spirometry depend on the predicted values. These values use body height, age, and sex for the prediction. It is believed that the body height is a chief determinant of the size of lungs. This is not precise enough. At the same height, the intrathoracic gas volume (IGV) can be very different and with a greater IGV lung function variables increase; the increase may not be in proportion to that of IGV, as it is often greater. For instance, if the IGV is 10% bigger, this is followed by an increase of FVC and FEV1% by 40%.

The interindividual FEV1% scatter, based on large population studies, ranges from 75% to 138% or shows standard deviations (SD) of ±20%. The scatter of such a magnitude is unacceptable for drawing conclusions as to the individual values. However, reproducibility of individual results is much better, as it shows about half the foregoing interindividual variation, SD of ±10%. Thus, the results based on the following of the body height-predicted thoracic size or volume are bound to vary and may not properly reflect the innate lung function.

What follows? We need precise individual values, taken as early as possible in the course of life of an individual, for comparability purposes in a follow-up later on. Only would this procedure allow a correct assessment of spirometric results. There are now methods available that need no cooperation of the subject investigated. Therefore, measurement reproducibility is better and SD is smaller. Body plethysmography allows measuring airway resistance (Rt) and the IGV in a short while and without cooperation. But even with this method, larger lungs exert influence on Rt. An increase of IGV of 60% is followed by a 50% decrease of Rt of 0.1 kPa/l/s. In healthy persons the correlation between FVC% and Rt is significant. Persons with smaller Rt values have greater FVC% values. All these correlations show that we should keep in mind that the body height is not an adequate parameter for the assessment of the lung. The use of this parameter is one cause of the varied results of lung function.

Another method used in the assessment of lung function is the measurement of airways reactivity. Here we use different substances that increase the tonus of the bronchial system, e.g. methacholine used most commonly. Persons are required to inhale different amounts of such substances and then we define at what concentration of a substance the FEV1% decreases by at least 15%. Another method is to inhale for 1 min an aerosol of a substance of which we know that healthy persons do not react to. Both methods have their own advantages. A somewhat similar approach is to investigate bronchodilation in patients with airway obstruction as, e.g., in emphysema. Patients who are able to increase their FEV1 by at least 15% have been termed ‘responders’. But again this is not precise. The correlation between FEV1% and Rt is not linear. At low values of FEV1% below 40%, Rt may change much without a corresponding change of FEVl%. At such low baseline levels of FEV1%, FEV1% changes only by a very small amount during bronchodilation. Also, the FEV1% nearly does not change when Rt increases during a normal breath. Rt is mainly determined at the end of a breath where the FEV1% is never influenced so much. There are other new methods under development (3), such as the resistance-volume curve, which do not need person’s cooperation and which provide reliable results for an early diagnosis. Airway resistance increases sharply at the beginning of expiration. During the latter part of expiration, a decrease in resistance follows. In contrast, airway resistance increases over the entire expiration in patients with obstruction. The resistance-volume curve provides much more information about lung function in different diseases and conditions. New predicted values should be worked out for the new methods to improve the reliability of measurements.

Lung function measurements used by far are not so good as they should be. Older approaches to lung function will be sidelined with new methods that provide better diagnosis and improved treatment. But these new methods have developed from spirometry and basically remain in its realm, which shows that the old idea of lung function analysis has been viable. The importance of lung function makes it worthwhile to perform further investigative search for a more precise interpretation of measurements, which is a key goal of pneumology.


REFERENCES

1. Hutchinson J. On capacity of lungs in respiratory functions with view of establishing precise and easy method of detecting disease by spirometer. Trans Med-Chir Soc London 1846; 29: 137
2. Hankinson J L. Office spirometry: does poor quality render it impractical? Chest 1999; 116: 276-277.
3. Schaefer T, Happel A, Islamova S, Schlaefke ME, Ulmer WT. Stability of lung function parameters: spirometry, body plethysmography and mouth occlusion pressure (P0.1). Pneumologie 2002; 56: 679-683.