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Lung Volumes & Capacities : Foundations of Pulmonary Function Testing -Physiology USMLE Step 1
What Is Pulmonary Function Testing?
How do you objectively assess how healthy a patient’s lungs are?
Pulmonary function testing (PFT) is a noninvasive way to help determine overall lung health based on values for volumes, air flow, alveolar diffusion, and underlying inflammation. These very common tests can be vital for deciding which medication is best for your patient or checking whether an occupational hazard is affecting lung health.
What Are Lung Volumes and Capacities?
These include tidal volume (TV), inspiratory reserve volume (IRV), expiratory reserve volume (ERV), and residual volume (RV):
Tidal volume is the normal amount of air-about half a liter-that you breathe during quiet breathing, just the simple in (tidal inspiration) and out (tidal expiration) breathing of everyday life.
Inspiratory reserve volume is the additional air that you can inhale or inspire above the tidal inspiration-the extra air you’d inhale before diving into water or walking past a dumpster.
Expiratory reserve volume is the additional air that you can exhale or expire beyond the tidal expiration-the extra exhale of blowing out all 20-something trick candles on a birthday cake with only one breath.
Residual volume is the air that, no matter how hard you bear down, cannot be forced out, which helps prevent the collapse of the alveoli.
Most lung volumes are determined by spirometry (a quantitative breathing test). However, residual volume cannot be measured using spirometry and requires additional testing.
Lung capacities are determined by adding two or more volumes. They include inspiratory capacity, functional residual capacity, vital capacity, and total lung capacity:
Inspiratory capacity (IC) is all the air that can be inhaled; it is calculated by adding together the tidal volume and the inspiratory reserve volume.
Functional residual capacity (FRC) is the air that is not expired after normal expiration; it is the sum of the expiratory reserve volume and the residual volume.
Vital capacity (VC) is all the air that can be expired after maximal inspiration; it is the sum of tidal volume, inspiratory reserve volume, and expiratory reserve volume.
Total lung capacity (TLC) is the maximum amount of air that can be contained by the lungs; it is the sum of all lung volumes.
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