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.
Last updated 2021-08-23