Volumes and Capacities

Explain the measurement of lung volumes and capacities, and factors that influence them

State the normal values of lung volumes and capacities

Define closing capacity and its clinical significance and measurement

The lung has four volumes and four (main) capacities:

• A volume is measured directly
• A capacity is a sum of volumes

Volumes

• Tidal volume (V_T)
  Volume of air during normal, quiet breathing.
  • Normal is 7ml.kg^-1, or 500ml

• Inspiratory reserve volume (IRV)
  Volume of air that can be inspired above tidal volume.
  • Normal is 45ml.kg^-1, or 2500ml

• Expiratory reserve volume (ERV)
  Volume of air that can be expired following tidal expiration.
  • Normal is 15ml.kg^-1, or 1500ml

• Residual volume (RV)
  Volume of air in the lungs following a maximal expiration.
  • Normal is 15-20ml.kg^-1, or 1500ml

Capacities

• Functional Residual Capacity (FRC)
  FRC = RV + ERV.
  • Normal is 30ml.kg^-1 or 3000ml
  • FRC decreases 20% when supine, and a further 20% under general anaesthesia

• Vital Capacity (VC)
  VC = ERV + V_T + IRV.
  • Normal is 4500ml

• Inspiratory Capacity (IC)
  IC = V_T + IRV.
  • Normal is 3000ml

• Total Lung Capacity (TLC)
  TLC = RV + ERV + V_T + IRV.
  • Normal is 6000ml

Functional Residual Capacity

The FRC has many important physiological functions:

• Gas exchange
  The FRC allows blood in the pulmonary circulation to become oxygenated throughout the respiratory cycle (if there was no FRC, then at expiration the lungs would be empty and no oxygenation would occur).

• Oxygen Reserve
  FRC is the only clinically modifiable oxygen store in the body, and allows continual oxygenation of blood during apneic periods.

• Minimise Work of Breathing
  Work of breathing is a function of lung resistance and compliance.
  • The lung sits on the steepest part of the compliance occurs at FRC
    Compliance is optimised as:
    • Alveoli are open and minimally distended
      • Below FRC, some alveoli collapse and the volume of lung available to receive the tidal volume decreases
        Re-expansion of collapsed alveoli requires more work than expanding open alveoli.
      • Above FRC, some alveoli will become overdistended and their compliance will fall
  • Airway resistance decreases as airway radius increases as lung volume increases

• Minimise RV Afterload
  PVR is minimal at FRC.
  • Above FRC, compression of intra-alveolar vessels occurs and PVR increases
  • Below FRC, extra-alveolar vessels collapse and PVR increases

• Maintain lung volume above closing capacity
  If closing capacity (see below) exceeds FRC, then shunt will occur.

Factors affecting FRC:

• FRC is reduced by:
  • Supine positioning
    Falls by ~20%.
  • Anaesthesia
    Falls by ~20%.
  • Raised intra-abdominal pressure
  • Impaired lung and chest wall compliance
• FRC is increased by:
  • PEEP
    • Extrinsic
    • Intrinsic (gas trapping)
      • PEEP
  • Emphysema
  • Acute asthma
  • Age
    May increase slightly.

Measurement of Lung Volumes and Capacities

• ERV, V_T, and IRV can all be measured directly using spirometry
  • A spirometer is a flow meter
    • The patient exhales as fast as possible through the flow meter
    • A flow-time curve is produced
    • This curve can be integrated to find volume
• Any capacity which is a sum of these (IC, VC) can therefore be calculated

• RV cannot be measured by spirometry, as it can't be exhaled
  • Therefore FRC and TLC cannot be calculated
• RV can be measured using:
  • Gas dilution
  • Body plethysmography

Gas Dilution

• Gas dilution relies on two principles:
  • Conservation of Mass
  • Helium has poor solubility and will not diffuse into circulation

• Limitations of gas dilution:
  • Only gas communicating gas can be measured - will underestimate FRC in gas-trapping

• Method:
  • Patient takes several breaths from a gas mixture containing a known concentration of helium (giving time for equilibration)
  • The concentration of expired helium is then measured
    From the law of conservation of mass:
    $C_1V_1 = C_2V_2$
    • $V_2$ is equal to the volume of the gas mixture the patient was breathing from ($V_1$) and the patients FRC
    • Therefore:
      $C_1V_1 = C_2(V_1 + FRC)$
      $FRC = V_1{C_1 - C_2 \over C_2}$

Body Plethysmography

• Body plethysmography relies on:
  • Boyle's law
    Pressure and volume are inversely proportional at a constant temperature, i.e. ($P \times V = k$).

• Method:
  • Patient is placed in a closed box, with a mouthpiece that exits the box
  • The patient inhales against a closed mouthpiece:
    • When the patient inhales, the volume of gas in the box decreases (the patient takes up more space) and therefore the pressure increases
    • The change in volume of the box is given by:
      $P_1V_1 = P_2V_2$, where:
      • $V_2$ is the change in box volume, or $V_1 - \Delta V$
      • Therefore:
        $P_1V_1 = P_2(V_1 - \Delta V)$
        As $\Delta V$ is the only unknown value, it can be calculated.
    • The change in volume of the lung must be the same as the volume of the box ($\Delta V$)
      • In the case of the lung, the initial volume ($V_1$) is FRC
      • Therefore:
        $P_1 \times FRC = P_2(FRC + \Delta V)$
        $FRC = {P_2 \Delta V \over P_3 - P_4}$

Closing Capacity

• Closing capacity is volume at which small airways begin to close
  Closing capacity is the sum of residual volume and closing volume.
  • Because dependent lung is compressed by gravity, dependent (typically basal) airways are of smaller calibre than non-dependent (typically apical) airways
  • During expiration, these airways are compressed first
    Alveoli connected to these airways are isolated, and V/Q scatter or shunt occurs.
  • If closing capacity exceeds FRC, then airway closure occurs during normal tidal breathing
    This occurs when:
    • FRC is decreased
    • CC is increased
      • Increases with age
        • CC exceeds FRC in the supine patient at 44
        • CC exceeds FRC in the erect patient at 66
  • This is clinically relevant during preoxygenation, as it will limit the denitrogenation that can occur

Measurement of Closing Capacity

Closing capacity is measured using Fowler's Method, and is covered under Dead Space.

References

1. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.