## Lung capacities

These are determined by the size of the thorax and lungs, and consist of two or more volumes:

r * Functional residual capacity (FRC): volume of gas remaining in the lungs at

i the end of a normal expiration. This is around 3000-4000 ml and depends on r

£ the age, sex, height and weight of the subject. It is the sum of the RV and the

ERV. Functional residual capacity represents the volume of gas remaining in a the lungs that continues to take part in gas exchange, and is thus a measure of

oxygen reserve.

* Inspiratory capacity (IC): maximum volume of gas that can be inhaled after normal expiration. 3000-4000 ml.

= volume of gas in the lungs after a maximal inspiration.

The TLC is the largest possible volume of gas in the lungs.

* Forced vital capacity (FVC) is a vital capacity measurement taken as rapidly as possible. It evaluates the resistance properties of the airways and the strength of the expiratory muscles. The normal time required to exhale from a FVC to RV is less than 3 seconds. During a forced expiratory manoeuvre the volume of gas expired in the first second of expiration (FEV1) is usually 75% of the FVC (FEV1/FVC = 0.75). The peak expiratory flow rate occurs early in expiration, usually within the first 0.2 second, and is an index of both large airways resistance and of patient effort. It also depends on age, height and gender. The forced expiratory flow, defined as the slope of the expiratory flow between 25% and 75% of the FVC (FEV25-75), is effort independent and an index of small airways resistance. Forced vital capacity is equal to 50-60 ml/kg. Variability in the peak expiratory flow rate > 15%-20% daily or from day-to-day suggests obstruction. A fall in peak expiratory flow rate >15% following exercise is diagnostic of exercise-induced asthma. An increase in FEV1 > 20% with beta-adrenergic bronchodilator favours asthma over chronic obstructive airways disease.

The values for lung volumes and capacities are generally expressed as percentages of the predicted value. For normally distributed values, 95% of the population values lie within two standard deviations of the mean.

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