Study of lung function is now widely recognized. It is an essential procedure in cardiorespiratory practice. In many lung diseases symptoms can be misleading and physical signs inadequate, when only the lung function tests provide the vital information. Respiratory function may be severely impaired as a result of disease and yet the lungs appear normal on X-ray chest. On the other hand gross radiological abnormalities may be associated with little impairment of physiological function.
With the availability of simple and sophisticated equipments and development of convenient methods these tests are now routinely employed in clinical practice. Facilities for pulmonary function tests are available at most centres in our country and by necessity have gained the same importance in clinical medicine as they have in the developed countries. The object of this booklet is to present the theory and practice of pulmonary function testing in concise form.
USES OF LUNG FUNCTION TESTS
Study of respiratory function enables clear understanding of the impairment in the specific lung function caused by the disease process. Such assessment has practical significance with regard to diagnosis, therapy and thoracic surgery.
Lung function studies remain the only reliable and objective method of evaluating the course of the disease and effectiveness of the treatment instituted. The tests enable early detection of pulmonary insufficiency in many a patient considered normal on clinical and radiologic examinations. They also provide quantitative measurement of the respiratory impairment which is hardly ever obtainable by radiology or any other method. The tests can differentiate the breathlessness of cardiac origin from that of pulmonary origin and can also differentiate between organic disease of lung and functional symptoms which mimic cardiorespiratory disorder. They assess fitness for high altitude, air travel and diving.
In industrial medicine, the tests are used to assess the degree of disability and the expectation of life due to occupational lung disease. They are now inevitable for the assessment of the immediate risks and ultimate expectation of life before and after thoracic surgery. However, the tests of respiratory function are not sufficiently specific to identify with certainty any individual disease. They are valuable in that they enable the practitioner to identify the contributions of ventilatory failure and diffusion defects as well as to estimate degree of respiratory failure present. Essentially the tests show how disease has altered respiratory function.
It is necessary to be familiar with the general structure of the bronchopulmonary segments, for a better understanding of pulmonary physiology.
Lungs are divided into lobes with fissures in between. Right lung has three lobes, namely, the right upper, the middle and the right lower lobe. Left lung is divided into two lobes, the left upper and the left lower lobes. The left upper lobe consists, however, of an upper division corresponding to the right upper lobe, and a lower or lingular division analogous to the right middle lobe. Each lobe is composed of two or more bronchopulmonary segments. A bronchopulmonary segment consists of a wedge of lung tissue supplied by a single bronchus and the corresponding pulmonary artery. Thus, each bronchopulmonary segment is a unit with respect to circulation and innervation as well as respiration.
The bronchial tree (Fig. 1.1) begins by division of the trachea into right and left main bronchi. Each of the main bronchus in turn, divides to give a branch to each of its lobes. Thus, the right main bronchus divides into the right upper, the middle and the lower lobe bronchi, while left main bronchus divides into the left upper and lower lobe bronchi. These, in turn, divide giving a branch to each of their segments - the segmental bronchi. A bronchopulmonary segment is known after its parent segmental bronchus.
A bronchopulmonary segment is divisible into subsegments of various orders down to the lung lobule (Figs 1.2A and B). The lobule consists of a respiratory bronchiole, its first, second and third division continued as alveolar duct into the atria which communicate with alveolar sacs. The alveolar sac is a collection of alveoli separated from one another partially or completely by connective tissue containing blood vessels, lymphatics and nerves. Thus, an alveolus is the simplest and basic unit of the lung tissue.
Fig. 1.2A: Primary respiratory lobule: General scheme of a primary respiratory lobule in which the sub-divisions of the bronchial tree, pulmonary artery and vein are shown. B-Respiratory bronchiole which divides into two alveolar ducts, ALV-Alveoli along respiratory bronchiole and alveolar ducts, A-atria, SAL-Alveolar sacules, P-Pleura, 1-Pulmonary artery, 2-Branches of pulmonary artery, 3-Pulmonary vein, 4-Bronchial arteries
The trachea and bronchi mainly act as a pathway for air entering and going out of the lungs. They have no respiratory function, i.e. no gaseous exchange takes place, till the terminal respiratory bronchioles are reached. During a normal inspiration, about two-thirds of the inhaled air enters the alveoli, the remaining one-third ventilates the “conducting passage” only. Hence the non-respiratory tracheobronchial space is also known as “anatomical dead space.”
The primary function of the respiratory system is to maintain oxygen and carbon dioxide gas-tension of the arterial blood within narrow physiologic limits. Three processes are required to carry out this respiratory function. They are: (1) Ventilation, (2) Diffusion and (3) Perfusion.
- Ventilation is a process of drawing in of the atmospheric air to reach the alveoli (inspiration) and removal of gases back to atmosphere (expiration). Here the lungs act as a pair of bellows.
- Diffusion is a process of gas exchange between alveoli and blood for which the lungs provide an interface of enormous area through 700 millions of alveoli equal to a total surface area of about 55 square meters of a membrane. Across this alveolo-capillary membrane, oxygen passes from alveolar gas to pulmonary capillary blood and carbon dioxide from capillary blood to lung alveoli.
- Perfusion is a process that constitutes adequate pulmonary circulation across the alveolar surface for such exchange of gas all throughout the lung tissue.
Naturally the three processes, viz. ventilation, diffusion and perfusion are intimately linked in health, but the disease condition may affect any of them alone or together. It is convenient therefore to consider them separately.
Ventilation is the most commonly affected process of lung function in clinical practice. Compared to the inadequacy of ventilation, the disturbance of gas exchange occurs far less frequently. Tests of ventilation are also simpler and extensively used. Hence study of ventilation is of great practical importance.