Lung Volumes and Capacities

Lung Volumes

• Primary lung volumes:

• RV - residual volume
• ERV - expiratory reserve volume
• TV - tidal volume
• IRV - inspiratory reserve volume

• Secondary derived capacities

• TLC - total lung capacity
• VC - vital capacity
• IC - inspiratory capacity
• FRC - functional residual capacity

Volume refers to one of the 4 primary, non-overlapping subdivisions of TLC, each capacity includes two or more of the primary lung volumes

• Total ventilation is 7500ml/min
• Alveolar ventilation is 5250ml/min
• Pulmonary blood flow is 5000ml/min
• Alveolar ventilation and pulmonary blood flow are roughly equal

• Neither FRC nor RV can be measured with a spirometer alone, so must be measured by gas dilution or plethysmograph:
• Gas dilution technique:

• A patient is connected to a spirometer with a known concentration of helium (virtually insoluble in blood), and the patient breathes until helium concentrations in spirometer and lungs equalise
• C x V then used to determine total volume of the system

• Body plethysmograph:

• Patient sits in plethysmograph and is asked to inhale against a pressure sensor. Pressure in lungs drops and volume expands.
• Boyle's law: pressure x volume is constant at a constant temperature - PV=K
• P₁V₁ = P₂(V₁ - ΔV)
• And V can be obtained and used in P₃V₂ = P₄(V₂ + ΔV)
• V₂ is the FRC.

• Helium dilution only measures communicating gas/ventilated lung volume, whereas the body plethysmograph method measures all gas in lung including trapped volume

Functional Residual Capacity:

• Definition: The volume of gas left in the lungs at the end of normal tidal expiration
• The lung volume in which gas exchange takes place
• Small fluctuations of alveolar and arterial gas tensions occur with each tidal breath as fresh gas mixes with alveolar air
• Acts as a buffer:

• Maintains constant A and a gas tensions with each breath
• Prevents rapid changes in alveolar gas with changes in ventilation/inspired gas

• Increased with increasing height, being male, emphysema, asthma, increasing PEEP
• Decreased with decreased diaphragmatic muscle tone (in anaesthesia), lying down, increased abdominal contents