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Spirometry

Basic spirometry procedure

  • Wash hands; Introduce self; Patient’s name and DOB; Explain procedure and obtain consent
  • Obtain patient details: sex, age, height, ethnic origin (to work out predicted values)
  • Check that their current condition is stable
  • Attach a clean mouthpiece to the spirometer
  • Ensure patient is seated upright and apply nose clip
  • Measure FEV1 and FVC
    • Ask them to take a deep breath in to full capacity
    • Place their lips around the mouthpiece forming a tight seal
    • Breathe out as hard and fast as possible through their mouth
    • Encourage them to keep exhaling until their lungs feel empty
    • Repeat 2 more times
  • Measure VC
    • Ask them to take a deep breath in to full capacity
    • Place their lips around the mouthpiece forming a tight seal
    • Breathe out steadily at a comfortable pace
    • Encourage them to keep exhaling until their lungs feel empty
    • Repeat 2 more times
  • Calculate % of predicted values:

Calculate % of predicted values

% predicted = (best patient reading / predicted value) x 100

  • Calculate the FEV1/FVC ratio:

Calculate the FEV1/FVC ratio:

FEV1/FVC ratio = Best FEVreading / Best FVC reading

  • Thank the patient and document results

Background physiology

Spirometry graph measurements

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Obstructive vs. Restrictive disorders

  • Obstructive diseases such as asthma and COPD result in obstructed airways. This creates airway resistance to expiratory flow, so the patient struggles to get air out quickly and has a decreased FEV1. A smaller FEV1 also results in a smaller FEV1/FVC ratio. 
    • NB: an obstructive pattern due to asthma will show reversibility following administration of a bronchodilator such as salbutamol.
  • Restrictive diseases such as pulmonary fibrosis/interstitial lung disease, obesity, neuromuscular and chest/spine deformitiesrestrict lung expansion, reducing the amount of air the lungs can hold (the vital capacity). This means the patient has a lower FVC. As there is decreased compliance and elasticity, it is also harder for the lungs to force air out quickly, resulting in a decreased FEV1. As both the FEVand the FVC have decreased, the FEV1/FVC ratio remains near normal.

Basic value interpretation

ObstructiveRestrictive
FEV1(<80%)(<80%)
FVCN(>80%)(<80%)
FEV1/FVC ratio(<0.7)N or ↑(0.7-0.8)  (>0.8)  

1. What is the FEV1? (<80% predicted = lung disease) 

  • FEV1 volume expelled in the first second of forced expiration (calculated as % of predicted value)
  • Physiology: FEV1 is reduced in obstructive disorders because there is airway resistance to expiratory flow. It is also reduced in restrictive disorders because there is decreased compliance and elasticity, so the lungs cannot force air out quickly.
  • Results
    • FEV1 <80% predicted = lung disease
    • FEV1 >80% predicted = normal = no lung disease (breathlessness due to another cause, e.g. PE, vasculitis)
  • Other points: FEV1 is also used to grade severity of COPD into mild (≥80%), moderate (50-80%), severe (30-50%), or very severe (<30%). Asthma is not usually diagnosed with spirometry (as tests are often normal when the patient is asymptomatic). However, reversibility of >12% of FEV1 with a bronchodilator may suggest the diagnosis. Asthma is usually diagnosed clinically or with serial peak flow demonstrating >20% diurnal variation.

2. What is the FVC? (<80% predicted = restrictive lung disease)

  • FVC = total volume expelled without time limit from maximal inspiration to forced maximal expiration (calculated as % of predicted value)
  • Physiology: FVC is reduced in restrictive disorders because there is reduced lung expansion, so the volume the lungs can hold is smaller. In obstructive disorders, there is airway resistance to expiratory flow, but a normal volume of air in the lungs, so the FVC is normal.
  • Results: FVC <80% predicted = restrictive disorder (FVC is normal in obstructive disorders)
  • Other points: VC = non-forced total volume expelled without time limit from maximal inspiration to maximal expiration (calculated as % of predicted value). The value will be similar to the FVC, which is more commonly used. 

3. What is the FEV1/FVC ratio?

  • FEV1/FVC ratio = proportional volume breathed out in first second compared to the whole breath (normally 0.7-0.8)
  • Physiology: This is calculated by dividing the FEV1 value by the FVC value. It is low in obstructive disorders because the FEV1 is low and the FVC is normal (as above). It is normal or high in restrictive disorders because the FEV1 is low and the FVC is proportionally as low or lower.
  • Results
    • FEV1/FVC ratio <0.7 = obstructive
    • FEV1/FVC ratio 0.7-0.8 = normal or restrictive
    • FEV1/FVC ratio >0.8 = restrictive (if FVC more affected than FEV1)

Advanced value interpretation

Lung volume (TLC and residual volume)

  • TLC = total volume of air inside the lungs, including the VC (maximal volume that can be breathed in and out) and the residual volume (the volume left inside the airways after maximal expiration)
  • Results
    • TLC is low in restrictive disorders (as described above) 
    • TLC may be high in emphysema because reduced elasticity causes hyperinflation and so results in a high RV. In this situation, the proportion of TLC comprised by the residual volume (the ‘RV%TLC’) will also increase.

Diffusion (TLCO and KCO)

  • TLCO = total diffusing capacity of lung. To measure the TLCO, the patient inspires a fixed amount of carbon monoxide in a single breath. The quantity expired again is then calculated to determine how much has diffused into the blood. 
  • KCO = TLCO/alveolar volume = diffusing capacity of lung per unit volume (i.e. as above but corrected for lung volume)
  • Results: a low TLCO may be due to:
    • Pulmonary vascular bed abnormalities (e.g. pulmonary embolism, pulmonary hypertension)
      • KCO is also low (because diffusion per unit alveolar volume is also affected)
    • Alveolar destruction (e.g. interstitial lung disease, emphysema)
      • KCO is also low (because diffusion per unit alveolar volume is also affected)
    • Reduced alveolar volume (e.g. pneumonectomy)
      • KCO is normal (because KCO corrects for alveolar volume)
    • Incomplete alveolar expansion (i.e. restrictive disorders)
      • KCO is normal (because KCO corrects for alveolar volume)
  • Other points: in the rare circumstance that both are raised, this suggests intra-alveolar haemorrhage (e.g. Granulomatosis with polyangiitis or Goodpasture syndrome)

Graph interpretation

Volume-time graph

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The graph plots the total volume of air expired by time, from full inspiration until full expiration.

  • Normal – rapid increase in volume of air expired initially, then curve forms a plateau
  • Obstructive – prolonged increase (because air cannot be expired as quickly due to airway resistance) but ends at the same point because the FVC is normal
  • Restrictive – rapid increase as normal, but curve forms a plateau much sooner (because total air volume in lungs is smaller)

Flow-volume curve 

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The graph plots the expiratory flow rate (in litres per second) versus the volume expired (in litres), from full inspiration until full expiration, when air flow stops.

  • Normal: rapid increase in flow rate, then gradual decrease until the end of expiration
  • Obstructive: decreased peak expiratory flow rate with steeper reduction in flow rate after it peaks creating a characteristic dip in the curve (worst in emphysema, due to small airway collapse)
  • Restrictive: curve is normal in shape but smaller due to proportionally reduced flow rates (total volume of lungs is restricted)
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