Lung volumes are divided into two categories (Figure 135-1). Static lung volumes are measured with slow breathing, whereas dynamic lung volumes are measured with fast or forced breaths. The lung volumes and capacities measured during spirometry are compared with theoretical values that reference values relative to the height, age, and sex of the subject in whom lung volumes are measured.
Spirogram showing static lung volumes.
(Reproduced from Morgan & Mikhail’s Clinical Anesthesiology, 5th ed. McGraw-Hill. Figure 23-5.)
STATIC LUNG VOLUMES AND CAPACITIES
The “static lung volumes” are individual volumes that cannot be further divided (Table 135-1):
TABLE 135-1Lung Volumes and Capacities ||Download (.pdf) TABLE 135-1 Lung Volumes and Capacities
|Measurement ||Definition ||Average Adult Values (mL) |
|Tidal volume (Vt) ||Each normal breath ||500 |
|Inspiratory reserve volume (IRV) ||Maximal additional volume that can be inspired above Vt ||3000 |
|Expiratory reserve volume (ERV) ||Maximal volume that can be expired below Vt ||1100 |
|Residual volume (RV) ||Volume remaining after maximal exhalation ||1200 |
|Total lung capacity (TLC) ||RV + ERV + Vt + IRV ||5800 |
|Functional residual capacity (FRC) ||RV + ERV ||2300 |
Tidal volume—The amount of air that is mobilized with each unforced breath (300-500 mL). To find out how much air arrives to the alveoli (and therefore participates in the gas exchange), one must calculate the alveolar volume, subtracting the anatomical dead space from the tidal volume. The anatomical dead space is given by the initial portion of the airways (from the mouth to the terminal bronchioles). Anatomical dead space does not participate in the exchange of O2 and CO2 between air and blood, but has only one function to bring the air to the alveoli. The dead space volume is on average 150 cc, and it can be calculated approximately by multiplying the weight in kilograms by 2.
Inspiratory reserve volume—The maximum amount of air that, after normal inspiration, may still be forcibly introduced in the lungs.
Expiratory reserve volume—The maximum amount of air that, after a normal expiration, can still be expelled with a forced exhalation.
Residual volume—The air that remains in the lungs after a forced exhalation. This volume cannot be measured directly and is calculated using various methods: plethysmography, helium mixing, nitrogen washout. Increased residual volume is a sign of lung hyperinflation due to bronchoconstriction or pulmonary emphysema. It is also very important in forensic medicine, because the absence of this residual air is an indication of death by suffocation.
The lung capacities are sums of volumes: