Spirometry

Describe the pressure and flow-volume relationships of the lung, chest wall and the total respiratory system

Describe the measurement and interpretation of pulmonary function tests, including diffusion capacity.

Pulmonary function tests are performed with a spirometer, which measures either volume or flow (integrated for time) to quantify lung function.

Basic spirometry can be used to quantify:

• Lung volumes and capacities
  All except residual volume (and therefore FRC and TLC).
• Dynamic measurements
  • FEV₁
    Volume of air forcibly exhaled in one second.
  • FVC
    Forced vital capacity.
  • PEFR
    Peak expiratory flow rate.
  • Flow-volume loop

Additional testing can be performed to measure:

• Residual volume
  FRC and TLC can therefore be calculated.
• Diffusion capacity

Basic Spirometry

Basic spirometry includes:

• Forced spirometry
  Patient forcibly exhales a vital capacity breath, producing a exponential (wash-in) curve. This calculates:
  • PEFR from the gradient at time 0 (assuming maximal effort)
  • FEV₁ is the volume expired in 1s
    Normal is > 80% of predicted.
  • FVC is the total volume exhaled.
  • The FEV₁/FVC ratio
    Normal is > 0.7.
  • These values also quantify disease severity:
    • In obstructive airways disease:
      • FEV₁ <80% predicted
      • FEV₁/FVC ratio
    • Restrictive disease:
      • FEV₁ <80% predicted
      • FVC
      • FEV/FVC ratio >0.7
        The ratio is normal as the FEV₁ and FVC fall proportionally.

• Volume-Time Graph (also known as a spirograph or spirogram)
  Quantifies static lung volumes by having a patient perform:
  • Normal tidal breathing
  • Vital capacity breath
  • Vital capacity exhalation

Flow-Volume Loops

• Normal
  • Peak expiratory flow of ~8L.s^-1
    Initial flow is highest as the increased lung volume increases the calibre of lung airways, reducing airways resistance.
    • This is called the effort dependent part of the curve
  • Flow tails off later in expiration
    • Lungs collapse, and airway calibre falls
    • Small airways are compressed
      Any increase in expiratory pressure will increase airway resistance proportionally.
      • This is called dynamic airways compression, and results in a uniform flow rate that is independent of expiratory effort
        This is therefore labeled the effort independent part of the curve.

• Obstructive lung disease
  • RV and TLC are increased due to gas trapping
  • Peak flow is limited
  • Effort-independent portion becomes concave

• Restrictive lung disease
  • TLC is reduced, but residual volume is unchanged
  • Peak flow may be reduced (as seen here)
    However, this reduction is proportional to the decrease in volume, such that the FEV₁:FVC ratio is normal. If peak flow is preserved, the FEV₁:FVC ratio will be increased.
  • Effort independent part is linear

• Fixed upper airway obstruction
  Describes an upper airway obstruction that does not change calibre during the respiratory cycle.
  • Peak inspiratory and expiratory flow rates are limited by the stenosis

• Variable extrathoracic obstruction
  Variable as the obstruction changes during the respiratory cycle:
  • During (negative pressure) inspiration the lesion is pulled into trachea, reducing inspiratory flow
  • During expiration the lesion is pushed out of the trachea
    The way to remember this is an extrathoracic obstruction impedes inspiration
  • The reverse effect occurs in positive pressure ventilation

• Variable intrathoracic obstruction
  The opposite to extrathoracic obstruction.
  • During inspiration the airway calibre increases and inspiratory flow is unimpeded
  • During expiration the airway calibre falls and expiratory flow is reduced

References

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