Measurement of Ventilatory Function
A great deal can be learned about the lungs' mechanical properties from measurements of forced maximal expiration and inspiration. The spirometer (built-in 1846 by Hutchinson) measures ventilatory function (dynamic lung volumes and maximal flow rates).
Spirometry
Conventionally, a spirometer is a device used to measure timed expired and inspired volumes. From these, we can calculate how effectively and quickly the lungs can be emptied and filled. The measurements that are usually made are as follows:
- VC (vital capacity) is the maximum volume of air which can be exhaled or inspired during either a forced (FVC) or a slow (VC) manoeuvre
- FEV1 (forced expired volume in one second) is the volume expired in the first second of maximal expiration after a maximal inspiration and is a useful measure of how quickly full lungs can be emptied
- FEV1/VC is the FEV1 expressed as a percentage of the VC or FVC (whichever volume is larger) and gives a clinically useful index of airflow limitation
- FEF25-75% is the average expired flow over the middle half of the FVC manoeuvre and is regarded as a more sensitive measure of small airways narrowing than FEV1. Unfortunately, FEF25-75% has a wide range of normality, is less reproducible than FEV1, and is difficult to interpret if the VC (or FVC) is reduced or increased
- PEF (peak expiratory flow) is the maximal expiratory flow rate achieved and this occurs very early in the forced expiratory manoeuvre.
Miller 1996[1]) shows a normal spirogram showing the measurements of forced vital capacity (FVC), forced expired volume in one second (FEV1) and forced expiratory flow over the middle half of the FVC (FEF25-75%).
Static lung volume and capacity
Static Lung volume tests evaluate air movement within the pulmonary tract with no time limitations. McArdle et al. 2000)[3] show the various static lung volume measurements that can be made.
- TV - Tidal Volume - Volume inspired or expired per breath
- Average values Male 600mL, Female 500mL
- IRV - Inspiratory Reserve Volume - Maximum inspiration at the end of tidal inspiration
- Average values Male 3L, Female 1.9L
- ERV - Expiratory Reserve Volume - Maximum expiration at the end of tidal expiration
- Average values Male 1.2L, Female 800mL
- TLC - Total Lung Capacity - Volume in lungs after maximal inspiration
- Average values Male 6L, Female 4.2L
- RLV - Residual Lung Volume - Volume in the lungs after maximum expiration
- Average values Male 1.2L, Female 1L
- FVC - Forced Vital Capacity - Maximum volume expired after a maximum inspiration
- Average values Male 4.8L, Female 3.2L
- IC - Inspiratory Capacity - Maximum volume inspired following tidal expiration
- Average values Male 3.6L, Female 2.4L
- FRC - Functional Residual Capacity - Volume in the lungs after a tidal expiration
- Average values Male 2.4L, Female 1.8L
Breathing volumes
The subject breathes in and out of a sealed chamber through a mouthpiece. As the chamber inflates and deflates, a pen recorder traces the breathing movements onto a chart. The volume of air breathed in and out can be measured using a spirometer. The machine is calibrated so that breathing volumes can be calculated. The air breathed in and out in a regular cycle is called the tidal volume, usually about 500 cm³.
We can breathe in and out to a greater extent, and these extra air supplies are called the inspiratory reserve volume and the expiratory reserve volume, respectively. All three volumes together add up to the vital capacity - the maximum possible tidal volume - usually about 4 to 5 cm³.
When we breathe out as hard as we can, there is still some air in the lungs. It is called the residual volume, add this to the vital capacity, and we have the total lung volume, usually between 5 to 7 cm³.
Some of the air we breathe does not reach the alveoli but remains in the air passages, occupying the so-called dead space. These volumes and the effects of exercise are shown on Wasserman's 1999[2] spirometer trace.
Predicted Normal Values
To interpret ventilatory function tests in any individual, compare the results with reference values obtained from a well-defined population of normal subjects matched for gender, age, height and ethnic origin and using similar test protocols and calibrated instruments.
Typical predicted values for ventilatory function generally vary as follows (Wasserman 1999)[2]:
- Gender: For a given height and age, males have a larger FEV1, FVC, FEF25-75% and PEF but a slightly lower FEV1/FVC%
- Age: FEV1, FVC, FEF25-75% and PEF increase and FEV1/FVC% decrease with age until about 20 years old in females and 25 years in males. After this, all indices gradually fall, although the actual rate of decline is probably masked due to the complex interrelationship between age and height. The fall in FEV1/FVC% with age in adults is due to the more significant decline in FEV1 than FVC
- Height: All indices other than FEV1/FVC% increase with standing height
- Ethnic Origin: Caucasians have the largest FEV1 and FVC; of the various ethnic groups, Polynesians are among the lowest. The values for people of African origin are 10 to 15% lower than for Caucasians of similar age, sex and height because their thorax is shorter for a given standing height. Chinese have been found to have an FVC about 20% lower and Indians about 10% lower than matched Caucasians. There is little difference in PEF between ethnic groups.
References
- MILLER, A. (1996) Pulmonary Function Tests in Clinical and occupational disease. Philadelphia: Grune & Stratton
- WASSERMAN, K. et al. (1999) Principles of exercise testing. Baltimore Lippincott Williams & Wilkins
- McARDLE, W.D. et al. (2000) The Physiology Support System. In: McARDLE, W.D. et al., 2nd ed. Essentials of Exercise Physiology, USA: Lippincott Williams and Wilkins, p. 235
Page Reference
If you quote information from this page in your work, then the reference for this page is:
- MACKENZIE, B. (2004) Measurement of Ventilatory Function [WWW] Available from: https://www.brianmac.co.uk/spirometer.htm [Accessed