In this article we will discuss about the subdivisions of lungs and capacities obtained by a spirometer with the help of suitable diagrams.
1. Lung Volumes:
(i) Tidal Volume (TV = 500 ml):
(ii) Respiratory Minute Volume (RMV):
By definition RMV can be obtained by multiplying tidal volume by respiratory rate per minute and is approximately equal to 500 ml × 12 = 6 litres/minute. The value of course may vary widely and is increased considerably during exercise when both the tidal volumes as well as respiratory rate are increased. At rapid rates of breathing a person usually cannot sustain a tidal volume more than about one-half of his vital capacity.
(iii) Inspiratory Reserve Volume (IRV):
The IRV of air that can be breathed in by maximum inspiratory effort after an ordinary inspiration. It amounts to 2.0 to 3.3 litres.
(iv) Expiratory Reserve Volume (ERV):
The ERV of air that can be breathed out by maximum expiratory effort after an ordinary expiration (about 1 litre).
(v) Residual Volume (RV):
The RV is the amount of air which remains in the lungs after maximal expiration. It can only be expelled out from the lungs by opening the chest and allowing the lungs to collapse (average 1.2 litres). The volume of air which enters the alveoli with each breath is equal to the TV minus the dead space volume.
2. Lung Capacities:
(i) Inspiratory Capacity (IC):
Maximum volume of air that can be inspired from the end-expiratory position, i.e., TV + IRV = about 3.5 litres.
(ii) Functional Residual Capacity (FRC):
Volume of air remaining in the lungs after a quiet expiration i.e., RV + ERV (about 2.2 litres).
(iii) Total Lung Capacity (TLC):
Volume of air that the lung can hold after a maximum possible inspiration, i.e., IC + FRC (about 5.7 litres).
(iv) Vital Capacity (VC):
It is the volume of air that can be breathed out by maximal expiratory effort after a maximum inspiration. By definition it amounts to IC + ERV = (3.5 +1) litres = 4.5 litres.
The exact amount of vital capacity depends on age, sex and size of the individual. It also shows a racial variation. Best correlation is obtained between height in cm and vital capacity. This predicted vital capacity in adult male = height in cm × 20 ml and in females height in cm × 16 ml. The above figures were obtained from observations based on 100 medical students of Calcutta.
Europeans have got a higher vital capacity and the empirical formula – height in cm × 25 ml in case of men and height in cm × 20 ml in case of women may be used for calculation of their predicted vital capacity. Observed vital capacity may show a variation of 10% from the predicted vital capacity in normal subjects. Vital capacity diminishes with age and is always lower by more than 10% in old people.
Vital capacity is higher by 30 to 40% in the athletes compared to the subjects with sedentary disposition. Vital capacity is less in supine position than in standing position because the intra-thoracic blood volume diminishes in standing posture, and the diaphragm can move downwards more easily than in supine position.
Vital capacity diminishes in conditions associated with weakness of the muscles of respiration, or when the movement of the thoracic cage is restricted or in space occupying defects of the chest. Increased rigidity of the lungs (diminished compliance) as may occur during pulmonary congestion, emphysema, chronic asthma or bronchitis will also cause diminished vital capacity.
Significance of vital capacity as a respiratory function test is indeed limited because it is a static test for lung function. However, day-to-day assessment of vital capacity say in a patient with paralysis of respiratory muscles (respiratory poliomyelitis) is of prognostic significance.
(v) Maximum Breathing Capacity (MBC):
Now-a-days called Maximum Ventilation Volume (MVV):
The subject is allowed to breathe into a Douglas bag (Fig. 8.14) through a valve and is instructed to breathe as quickly and as deeply as possible for 12 seconds. The volume of expired air collected into the Douglas bag multi plied by 5 will give his/her value of MBC per minute (ambient temperature and pressure saturated with water vapour, ATPS) which is equal to about 140 liters per minute. The value depends upon the degree of motivation of the subject and upon the efficiency of the muscles of respiration. It measures also the ‘bellows action’ of the lungs and depends on its elasticity. It is a test simple to perform and a reliable respiratory function test.
Instead of Douglas bag – the test can also be performed in a slow moving Spirometer calibrated for the rate of respiration horizontally and the volume of respiration vertically. Remarkable reduction in the MBC occurs in diseases like emphysema where both the rate as well as the depth of respiration decrease. The abbreviations those are commonly used in the spirometric measurement are given in the Table 8.1.
(vi) Forced Expiratory Volume (FEV):
It is the volume of air, forcibly breathed out after a deep inspiration in a given time. The determination of maximum breathing capacity (MBA) is an exacting procedure and it is the often not possible to perform it on debilitated patients.
Useful information regarding respiratory status of a patient can be obtained by analysis of a single forced expiration after a maximum inspiration on a rapidly moving homograph. A normal subject can expire 80% or more of his vital capacity in the first second. Patients with obstructive disease of lung will expire proportionately much less within the first second of expiration (Fig. 8.15).
(vii) Breathing Reserve (BR):
Breathing Reserve = MBV (Maximum breathing capacity) – RMV (respiratory minute volume) = about 136 litres/minute. When the breathing reserve falls to 60 to 70% of MBC dysponea will result.
(viii) Functional Residual Capacity (FRC):
During quiet breathing the pulmonary ventilation is achieved almost entirely by muscles of inspiration. The end-expiratory position of the lungs represents the ‘resting position’ during which all the muscles relaxed. As we know that, the volume of air remaining in the lungs at this level represents ‘functional residual capacity’ and measures about 2300 ml in an adult male.
It provides the air in the alveoli to aerate the blood between the two consecutive respiratory acts, thus preventing marked rise all in the concentration O2 and CO2 in the intervals of respiration. Functional residual capacity (FRC) and functional residual volume (FRV) increase in all conditions associated with overdistension of the lungs, e.g. in Emphysema. Quantitative measurement of residual volume and functional residual capacity is not possible by Spirometry.
Some of the special techniques employed for this purpose is described below:
(a) N2 -Wash-Out Technique:
The man inspires pure O2 (N2 free) and expires into a Douglas bag for a 7-minute period. During this period all the N2 present in his lungs is washed away by inspired O2. The total volume of expired air in the Douglas bag can be determined by a gas-meter. The volume of expired air multiplied by N2% gives the quantity of N2 washed off from the depth of the lungs. Since the lung air contains about 80% of N2– the volume of functional residual air can be easily calculated.
Suppose the volume of expired air over the 7-minute period = 50,000 ml and the concentration of nitrogen in the collected gas is 4%
Total quantity of N2washed off from lungs 50,000 × 0.04 = 2000 ml. Therefore the functional residual capacity = 2000 × 100 = 2500 ml
(b) Body Plethysmograph (Fig. 8.16):
It is simply as air-tight tank in which the subject is placed. He inspires the air within the tank and then expires forcefully against a manometer. The expiratory pressure is recorded. The rise of pressure within the chest decreases the volume of gas in the lungs (Boyle’s law) and the volume of the chest also decreases.
Thus the subject’s body occupies less space within the plethysmograph. A volume recorder records the amount of the reduction which is equal to the degree of compression of gases in the lungs which together with pressure recorded will give the volume of gas in the lungs. Inspiratory capacity is then recorded and deducted from the former value to give functional residual volume (FRV).