A normal subject can expire 80% or more of his vital capacity in the first second. 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. The determination of maximum breathing capacity (MBA) is an exacting procedure and it is the often not possible to perform it on debilitated patients. It is the volume of air, forcibly breathed out after a deep inspiration in a given time. The abbreviations those are commonly used in the spirometric measurement are given in the Table 8.1. Remarkable reduction in the MBC occurs in diseases like emphysema where both the rate as well as the depth of respiration decrease. 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. It is a test simple to perform and a reliable respiratory function test. It measures also the ‘bellows action’ of the lungs and depends on its elasticity. The value depends upon the degree of motivation of the subject and upon the efficiency of the muscles of respiration. 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. 8.14) through a valve and is instructed to breathe as quickly and as deeply as possible for 12 seconds. The subject is allowed to breathe into a Douglas bag (Fig. Now-a-days called Maximum Ventilation Volume (MVV): However, day-to-day assessment of vital capacity say in a patient with paralysis of respiratory muscles (respiratory poliomyelitis) is of prognostic significance. Significance of vital capacity as a respiratory function test is indeed limited because it is a static test for lung function. Increased rigidity of the lungs (diminished compliance) as may occur during pulmonary congestion, emphysema, chronic asthma or bronchitis will also cause diminished vital capacity. 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. 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 is higher by 30 to 40% in the athletes compared to the subjects with sedentary disposition. Vital capacity diminishes with age and is always lower by more than 10% in old people. Observed vital capacity may show a variation of 10% from the predicted vital capacity in normal subjects. The above figures were obtained from observations based on 100 medical students of Calcutta.Įuropeans 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. This predicted vital capacity in adult male = height in cm × 20 ml and in females height in cm × 16 ml. Best correlation is obtained between height in cm and vital capacity. The exact amount of vital capacity depends on age, sex and size of the individual.
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