Pulmonary volumes and capacities
To identify many pulmonary disorders the amount of air a person inspires and expires and the rate of ventilation are measured by a method called spirometry. The movement of air in and out of the lungs is recorded resulting in a spirogram (see figure below).
The tidal volume is the volume of air inspired or expired with each normal breath. The inspiratory reserve volume is the extra volume of air that can be inspired above the tidal volume. The expiratory reserve volume is the extra volume of air that can be expired by forceful expiration after the end of a normal tidal expiration. The residual volume is the volume of air remaining in the lungs after the most powerful expiration.
The inspiratory capacity is the sum of the tidal volume and the inspiratory reserve volume. It resembles the amount of air a person can breathe beginning at the normal expiratory level and distending the lungs to the maximal amount. The functional residual capacity equals the expiratory reserve volume plus the residual volume. This is the amount of air that remains in the lungs at the end of normal expiration. The vital capacity equals the inspiratory reserve volume plus the tidal volume plus the expiratory reserve volume. This is the amount of air a person can expel from the lungs after first filling the lungs to their maximum extent and then expiring to the maximum extent. The total lung capacity is the maximum volume to which the lungs can be expanded with the greatest possible inspiratory effort. It is the sum of the vital capacity and the residual volume.
In pulmonary function studies a number of abbreviations and symbols have become standardized. Some frequently used ones are listed in the table below:
V T |
tidal volume |
PO 2 |
partial pressure of O 2 |
FRC |
functional residual capacity |
PaO 2 |
PO 2 in arterial blood |
ERV |
expiratory reserve volume |
PAO 2 |
PO 2 in alveolar gas |
RV |
residual volume |
PCO 2 |
partial pressure of CO 2 |
IC |
inspiratory volume |
PaCO 2 |
PCO 2 in arterial blood |
IRV |
inspiratory reserve volume |
PACO 2 |
PCO 2 in alveolar gas |
TLC |
total lung capacity |
RQ |
respiratory exchange ratio |
FEV 1 |
forced expiratory volume in 1s |
SO 2 |
% saturation of blood with O 2 |
V'O 2 |
amount of consumed O 2 |
SaO 2 |
SO 2 in arterial blood |
V'CO 2 |
amount of produced CO 2 |
DLO 2 |
diffusing capacity of the lung for O 2 |
Raw |
airway resistance |
DLCO |
diffusing capacity of the lung for CO |