Ventilatory defects can be broadly classified into obstructive, restrictive, mixed obstructive/restrictive defects and non-specific ventilatory patterns (Table 2.1) (2, 3). You will note that spirometry parameters alone are used to define results that are within normal limits or obstructive defects. Total lung capacity (TLC) measured by static lung volumes, however, is required for identifying restriction, mixed obstructive/restrictive defects and non-specific ventilatory patterns in addition to spirometry parameters. This is because (F)VC can be reduced in spirometry due to airflow limitation or reduced lung size (Figure 2.3) and, without measuring TLC, the cause of the reduced (F)VC cannot be determined (4).

^a TLC is measured via static lung volumes.

Obstructive ventilatory defects are primarily related to pathologies that result in airway narrowing (Table 2.2). Restrictive ventilatory defects (reduced lung volume) may result from pathologies intrinsic or extrinsic to the lungs (Table 2.2). It is also possible that individuals may present with concomitant obstruction and restriction, resulting in a mixed obstructive/ restrictive pattern. The non-specific pattern is described in more detail in Chapter 3, but is, as its name suggests, an abnormal pattern that does not fit the other defined patterns of abnormality.

^a Note: Static lung volumes are required to confirm restriction.

The steps in interpretation using spirometry alone are shown in the flow diagram in Figure 2.4. The final level of the diagram gives an example of a written technical interpretation.

Spirometry before and after bronchodilator (BD)

• 
  

  Baseline spirometry (also referred to as control or pre-bronchodilator (pre-BD) spirometry) is generally used as the basis for interpretation of spirometry.

  
  

  Determining normality using post-bronchodilator spirometry should only be performed if no baseline data are recorded and this should be clearly stated. For example, There is an obstructive ventilatory defect on post-bronchodilator spirometry or Post-bronchodilator spirometry shows an obstructive ventilatory defect.

  
  
• After classifying the pattern of the baseline spirometry result, the bronchodilator response can be assessed.

Assessing bronchodilator response

• A significant response to inhaled bronchodilator is defined as an increase ≥12% and ≥200 mL in either FEV₁ or FVC between the baseline and post-bronchodilator results (2).
  
  
  
  • When there is a significant response to inhaled bronchodilator:
    
    
    
    • If spirometry returns to within normal limits (FEV₁/(F)VC, FEV₁ and (F)VC within the normal range), then there is complete reversibility of airflow limitation.
      
    • If obstruction remains apparent after inhaled bronchodilator, then there is incomplete reversibility of airflow limitation.
  
  
• Also consider responses that are insignificant by definition, but return spirometry to within normal limits and whether or not this is clinically important information.

Analysing the shape of the flow–volume curve

The shape of the flow–volume curve can provide information regarding the type of ventilatory defect (Figure 2.5). Be careful using the flow–volume curve alone for identifying obstruction. The shape of the expiratory limb changes with age, and in an older population it may have a concave appearance similar to that seen in obstruction. Use the values in addition to the shape to identify the pattern seen. Also see Special Cases of Spirometry Interpretation Upper Airway Obstruction following for identification of upper airway obstruction using flow–volume curves.

Variable or fixed upper airway obstruction is most easily identified by the shape of the flow–volume curve (also known as flow–volume loop) (Figure 2.6). Although not specific, parameters such as the FIF₅₀/FEF₅₀ have been suggested to be helpful (2).

• Variable intrathoracic upper airway obstruction is characterised by flattening of the expiratory limb of the flow–volume curve (Figure 2.6b,c). FIF₅₀/FEF₅₀ > 1 (2).
  
• Variable extrathoracic upper airway obstruction is characterised by a flattening of the inspiratory limb of the flow–volume curve (Figure 2.6f). FIF₅₀/FEF₅₀ < 1 (2).
  
• Fixed upper airway obstruction results in flattening of both the inspiratory and expiratory limbs (Figure 2.6d,e). FIF₅₀/FEF₅₀ ∼ 1 (2).

Note:

1. FEV₁, FVC and FEV₁/FVC values may not be affected in all cases of intrathoracic airway obstruction (see Case 12). This highlights the importance of reviewing the flow–volume curves as part of the interpretation strategy.
   
2. In order to detect extrathoracic upper airway obstruction, a high-quality inspiratory portion of the flow–volume curve is essential. The inspiratory manoeuvre is entirely effort dependent and false-positive findings of extrathoracic upper airway obstruction can occur as a result of suboptimal effort.

Hyper-reactive airways

In a small number of individuals, the act of performing forced expiratory manoeuvres may result in progressive airways obstruction (see Case 11). The typical pattern seen with hyper-reactive airways presents as follows:

• The first effort is often within normal limits.
  
• The FEV₁ falls with subsequent consecutive efforts (as opposed to variable FEV₁ where changes may occur in both directions).
  
• Often the repeatability criteria are not met.
  
• Following bronchodilator, the FEV₁ may or may not return to the best baseline level, but generally post-bronchodilator FEV₁ is repeatable.

The test operator plays an important role in identifying and noting the pattern in the technical comments. It is also important that the lowest FEV₁ obtained is documented in the technical comments for reporting. The reporter should look at the raw data to confirm the pattern.