see also:

• respiratory medicine
• Aust. asthma Foundation QRG to spirometry for GPs
• Aust. Asthma Foundation's buying guide for spirometers for GPs
  • eg. amongst the least expensive are the very light, compact PC-based spirometers such as:
    • MIR MiniSpir 2 at $AU999 excl. gst but includes 60 free turbine mouthpieces

• spirometry provides an assessment of lung function by measuring:
  • FEV₁ = Forced Expiratory volume in 1 sec
  • FVC = Forced vital capacity
  • lung volume vs time curve
  • lung flow vs volume curve

• assess shape of flow-volume curve to fit with a clinical pattern:
  • poor quality
    • free from coughs, leaks, or a slow start
    • ensure at least three repeatable measurements are captured within an acceptable range
    • volume-time curve should extend longer than six seconds, and there are no signs of early termination or cutoff
    • test session is finished when the difference between the two largest FVC measurements and between the two largest FEV1 measurements is within 0.2 L
  • normal
  • obstructive - unable to blow out quickly
    • asthma
    • COPD
  • restrictive - small lung capacity
    • pulmonary fibrosis
    • interstitial lung disease
    • pleural or chest wall disease
    • weak inspiratory muscles
    • rib deformity
    • obesity
  • mixed pattern
    • eg. cystic fibrosis
• FEV₁/FVC
  • is this above lower limit of normal (LLN) which is usually ~0.7?
    • is the FVC below LLN?
      • possible restriction
    • is the FVC above LLN?
      • normal spirometry
  • is this below lower limit of normal (LLN)?
    • this suggests an obstructive issue
    • severity grading of obstructive disease as determined by % of predicted FEV₁:
      • > 70% = mild
      • 60-69% = moderate
      • 50-59% = moderately severe
      • 35-49% = severe
      • < 35% = very severe
    • is FVC < LLN?
      • could be a mixed pattern
    • mid-expiratory flow rate (FEF_25–75%) is the average forced expiratory flow rate over the middle 50 percent of the FVC
      • in the correct clinical situation, a reduction in FEF_25–75% of less than 60% of that predicted and an FEV1/FVC ratio in the low to normal range may confirm airway obstruction
    • assess response to bronchodilator:
      • positive response indicated by:
        • FEV₁ increasing by > 12%
        • FVC increasing by > 12% and, in adults, by more than 200mL

• predicted values are based on age, sex, height, and ethnicity
• PEFR:
  • Young Adults (20–40 years)
    • Males: ~500 to 600 L/min (8.3 to 10.0 L/sec)
    • Females: ~380 to 480 L/min (6.3 to 8.0 L/sec)
  • Middle-Aged Adults (40–60 years)
    • Males: ~450 to 550 L/min (7.5 to 9.1 L/sec)
    • Females: ~340 to 440 L/min (5.7 to 7.3 L/sec)
  • Older Adults (60+ years)
    • Males: ~350 to 450 L/min (5.8 to 7.5 L/sec)
    • Females: ~250 to 350 L/min (4.2 to 5.8 L/sec)
• FEV₁/FVC
  • within 5% of the predicted ratio (usually 0.7)
  • Young and Middle-Aged Adults (20–50 years):
    • Normal ratio generally falls between 75% and 85%.
    • Peak lung function occurs in early adulthood, after which values remain relatively stable before a very gradual decline begins
  • Older Adults (50–70+ years):
    • Normal ratio naturally drifts downward, often resting between 70% and 80%.
    • For individuals over age 65, a ratio down to around 65% to 70% can still fall within expected limits depending on personal height, sex, and baseline health
• FVC usually 80-120% of predicted (values below 80% suggest increasing severity of restriction as it decreases)
  • Ages 20–30 years:
    • Males: 4.0 to 5.5 liters
    • Females: 3.25 to 4.5 liters
  • Ages 40–50 years:
    • Males: 3.5 to 4.5 liters
    • Females: Lower than the 20-30 age group bracket, mirroring a slow natural decline
  • Ages 60+ years:
    • Males: 3.0 to 4.0 liters
    • Females: Continues a gradual age-related reduction
• FEV₁ 80-120% of predicted
  • 20–30 years: Normal values fall between 80% and 100% (sometimes up to 120% in peak young adults).
  • 30–40 years: Normal values fall between 70% and 100% of the predicted value.
  • 40–60 years: Normal values fall between 75% and 100% of the predicted value, though a slight natural decline can start after age 50.
  • 60–70 years: Normal values fall between 70% and 90% of the predicted value.
  • Over 70 years: Normal values fall between 60% and 90% of the predicted value
  • absolute volumes:
    • Ages 20–30: FEV1 varies between 3.5 and 4.5 liters.
    • Ages 40–50: FEV1 varies between 3.0 and 4.0 liters.
    • Note: Healthy non-smokers naturally experience an FEV1 decline of about 30 mL each year as they age
• FEF_25–75%
  • it is dependent on FVC making it highly variable, so by age 50, the normal reference range for FEF_25–75% can statistically span from 40% up to 160% of the predicted value
  • if FEF_25–75% < 60% it may provided add confirmation of obstructive airways disease
• TLC (Total lung capacity) 80-120% of predicted
• FRC (Functional residual capacity) 75-120% of predicted
• RV (Residual volume) 75-120% of predicted