Lung Volumes
- Tidal volume (VT) is the volume inspired or expired during a normal breath.
- Inspiratory reserve volume is the additional volume that can be inspired above tidal volume.
- Inspiratory reserve volume is recruited during exercise.
- Expiratory reserve volume is the additional volume that can be expired after a normal tidal expiration.
- Residual volume is the air remaining in the lungs after maximal expiration.
- Residual volume cannot be measured by simple spirometry.
- Spirometry measures air that moves into or out of the lungs, so it cannot directly measure air that remains after maximal expiration.
- Anatomic dead space is the volume of conducting airways.
- Anatomic dead space is approximately 150 mL.
- Physiologic dead space is the volume that does not participate in gas exchange.
- Physiologic dead space is approximately equal to anatomic dead space in healthy lungs.
- Ventilation-perfusion defects increase physiologic dead space.
- Conducting airways move air but do not exchange gas, while gas-exchanging regions must receive both ventilation and blood flow.
- Bohr relationship calculates physiologic dead space.
- Bohr relationship equation is `VD/VT = (PaCO₂ - PECO₂)/PaCO₂`.
- VD in the Bohr relationship means dead-space volume.
- VT in the Bohr relationship means tidal volume.
- PaCO₂ means arterial or alveolar partial pressure of carbon dioxide (CO₂).
- PECO₂ means expired-air partial pressure of carbon dioxide (CO₂).
- Dead-space fraction represents dilution of alveolar carbon dioxide (CO₂) by dead-space air.
- Dead-space air contains little carbon dioxide (CO₂), so adding more dead-space air lowers mixed expired carbon dioxide (CO₂) compared with alveolar carbon dioxide (CO₂).
- Minute ventilation is tidal volume multiplied by respiratory rate.
- Minute ventilation equation is `VE = VT x breaths/min`.
- VE means minute ventilation.
- Alveolar ventilation is the volume of fresh air that reaches gas-exchanging surfaces each minute.
- Alveolar ventilation equation is `VA = (VT - VD) x breaths/min`.
- Minute ventilation counts all air moved per minute, whereas alveolar ventilation subtracts dead-space air and better reflects fresh air reaching exchange surfaces.
- VA means alveolar ventilation.
- Dead-space example with tidal volume 0.5 L, rate 15 breaths/min, arterial PCO₂ 40 mm Hg, and expired-air PCO₂ 36 mm Hg gives a dead-space fraction of 0.1.
- Alveolar ventilation example with tidal volume 0.5 L, dead-space fraction 0.1, and rate 15 breaths/min gives approximately 6.75 L/min.
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