Pulmonary function studies provide information about the volume, pattern, and rates of airflow involved in respiratory function. These studies may also include tests involving the diffusing capabilities of the lungs (i.e., volume of gases diffusing across a membrane). A complete pulmonary function study includes the determination of all lung volumes, spirometry, diffusing capacity, maximum voluntary ventilation, flow-volume loop (see figure f00001), and maximum expiratory and inspiratory pressures. Other studies include small airway volumes.
Pulmonary function studies are classified according to lung volumes and capacities, rates of flow, and gas exchange. The exception is the diffusion test, which records the movement of a gas during inspiration and expiration. Lung volumes and capacities constitute the amount of air inhaled or exhaled from the lungs; this value is compared to normal reference values specific for the patient’s age, height, and gender. The following are volumes and capacities measured by spirometry that do not require timed testing.
Total amount of air inhaled and exhaled with one breath.
Residual volume (RV)
Amount of air remaining in the lungs after a maximum expiration effort (not measured by spirometry, but can be calculated from the functional residual capacity [FRC] minus the expiratory reserve volume [ERV]); this indirect type of measurement can be done by body plethysmography (see monograph titled “Plethysmography”)
Inspiratory reserve volume
Maximum amount of air inhaled after normal inspirations
Expiratory reserve volume
Maximum amount of air exhaled after a resting expiration (can be calculated by the vital capacity [VC] minus the inspiratory capacity [IC])
Maximum amount of air exhaled after a maximum inspiration (can be calculated by adding the IC and the ERV)
Total lung capacity
Total amount of air that the lungs can hold after maximal inspiration (can be calculated by adding the VC and the residual volume [RV])
Maximum amount of air inspired after normal expiration (can be calculated by adding the inspiratory RV and tidal volume)
Functional residual capacity
Volume of air that remains in the lungs after normal expiration (can be calculated by adding the RV and ERV)
The volumes, capacities, and rates of flow measured by spirometry that do require timed testing include the following:
Forced vital capacity in 1 sec
Maximum amount of air that can be forcefully exhaled after a full inspiration
Forced expiratory volume
Amount of air exhaled in the first second (can also be determined at 2 or 3 sec) of forced vital capacity (FVC, which is the amount of air exhaled in seconds, expressed as a percentage)
Maximal midexpiratory flow
Also known as forced expiratory flow rate (FEF25–75), or the maximal rate of airflow during a forced expiration
Forced inspiratory flow rate
Volume inspired from the RV at a point of measurement (can be expressed as a percentage to identify the corresponding volume pressure and inspired volume)
Peak inspiratory flow rate
Maximum airflow during a forced maximal inspiration
Peak expiratory flow rate
Maximum airflow expired during FVC
Flows and volumes recorded during forced expiratory volume and forced inspiratory VC procedures (see figure f00002)
Maximal inspiratory-expiratory pressures
Measures the strength of the respiratory muscles in neuromuscular disorders
Maximal voluntary ventilation
Maximal volume of air inspired and expired in 1 min (may be done for shorter periods and multiplied to equal 1 min)
Other studies for gas-exchange capacity, small airway abnormalities, and allergic responses in hyperactive airway disorders can be performed during the conventional pulmonary function study. These include the following:
Diffusing capacity of the lungs
Rate of transfer of carbon monoxide through the alveolar and capillary membrane in 1 min
Measures the closure of small airways in the lower alveoli by monitoring volume and percentage of alveolar nitrogen after inhalation of 100% oxygen
Flow-volume loop test followed by inhalation of a mixture of helium and oxygen to determine small airway disease
Measures thoracic gas volume and airway resistance
Quantifies airway response after inhalation of methacholine
Arterial blood gases
Measure oxygen, pH, and carbon dioxide in arterial blood
Values are expressed in units of mL, %, L, L/sec, and L/min, depending on the test performed.
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