Spirometers

A spirometer is an apparatus for measuring the volume of air inspired and expired by the lungs. A spirometer measures ventilation, the movement of air into and out of the lungs. The spirogram will identify two different types of abnormal ventilation patterns, obstructive and restrictive. There are various types of spirometers which use a number of different methods for measurement (pressure transducers, ultrasonic, water gauge).

A spirometer is the main piece of equipment used for basic Pulmonary Function Tests (PFTs). Lung diseases such as asthma, bronchitis, and emphysema can be ruled out from the tests. In addition, a spirometer is often used for finding the cause of shortness of breath, assessing the effect of contaminants on lung function, the effect of medication, and evaluating progress for disease treatment.

The earliest attempt to measure lung volume can be dated back to the period A.D. 129-200. Claudius Galen, a Roman doctor and philosopher, did a volumetric experiment on human ventilation. He had a boy breathe in and out of a bladder and found that the volume did not change. The experiment proved inconclusive.

Even with the numerical precision that a spirometer can provide, determining pulmonary function relies on differentiating the abnormal from the normal. Measurements of lung function can vary both within and among groups of people, individuals, and spirometer devices. Lung capacity, for instance, may vary temporally, increasing and then decreasing in one person’s lifetime. As a result, ideas about what constitutes “normal” are based on one’s understanding about the sources of variabilities and can be left to interpretation.

Traditionally, sources of variation have been understood in discrete categories, such as age, height, weight, gender, geographical region (altitude), and race or ethnicity. Global efforts have been made in the early 20th century to standardize these sources to enable proper diagnosis and accurate evaluation of pulmonary function. However, rather than further aiming to understand the causes of such variations, the primary approach for dealing with observed differences in lung capacity has been to “correct for” them. Utilizing results from comparative population studies, attributes are empirically factored together into a “correction factor.” This number is then utilized to form a personalized ‘reference value’ that defines what is considered normal for one individual. Practitioners may thereby find the percent deviation from this predicted value, known as ‘percent of predicted,’ and determine whether someone’s lung function is abnormally poor or excellent.

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Wikipedia