What are the Forced Vital Capacity (FVC), Forced Expiratory Volume in one second (FEV1), and FEV1/FVC ratio measured by spirometry, and how are they helpful in assessing respiratory function?

Spirometry Parameters and Their Role in Respiratory Function Assessment

Spirometry parameters FVC, FEV1, and FEV1/FVC ratio are essential measurements that provide critical information about airway obstruction, lung restriction, and overall respiratory health, serving as the cornerstone for diagnosing and classifying pulmonary disorders. 1

Key Spirometry Parameters Explained

Forced Vital Capacity (FVC)

• The volume of air delivered during an expiration made as forcefully and completely as possible
• Expressed in liters
• Normal values are typically above 80% of predicted values or above the lower limit of normal (LLN)
• Reduced FVC may suggest restrictive lung disease, but cannot definitively diagnose it without additional lung volume measurements 2, 1

Forced Expiratory Volume in 1 Second (FEV1)

• The maximal volume of air exhaled in the first second of a forced expiration
• Expressed in liters
• Normal values are typically above 80% of predicted values or above the LLN
• Reduced FEV1 can indicate airflow obstruction, especially when the FEV1/FVC ratio is also reduced 2, 1

FEV1/FVC Ratio

• The proportion of the vital capacity exhaled in the first second
• Expressed as a percentage or decimal
• The most sensitive measure for detecting early airway obstruction
• A low ratio (below the 5th percentile of predicted or LLN) indicates airflow obstruction 2, 1

Clinical Utility in Respiratory Assessment

Diagnosing Obstructive Patterns

• Obstruction is defined as: FEV1/VC below the 5th percentile of predicted value 2
• The European Respiratory Society recommends using the 5th percentile as the lower limit of normal rather than a fixed value of 0.70, as this prevents overestimation of ventilatory defects in older individuals 2
• Severity of obstruction is graded based on FEV1 percentage of predicted:
  • Mild: FEV1 ≥80% predicted
  • Moderate: 50% ≤ FEV1 < 80% predicted
  • Severe: 30% ≤ FEV1 < 50% predicted
  • Very severe: FEV1 < 30% predicted 1

Diagnosing Restrictive Patterns

• Restriction is defined as: Total Lung Capacity (TLC) below the 5th percentile of predicted 2
• A reduced FVC with normal or increased FEV1/FVC ratio may suggest restriction, but:
  • Spirometry alone has a relatively low positive predictive value (41-58%) for confirming restriction 3
  • A normal FVC has an excellent negative predictive value (93.9-95.7%) for excluding restriction 3, 4
  • Definitive diagnosis of restriction requires measurement of TLC using techniques like body plethysmography 2, 1

Identifying Mixed Patterns

• Mixed ventilatory defect: Both FEV1/VC and TLC below their 5th percentiles of predicted values
• Cannot be accurately diagnosed with spirometry alone; requires lung volume measurements 2

Clinical Pitfalls and Caveats

1. Overreliance on spirometry for restriction diagnosis:

   • A reduced FVC alone is insufficient to diagnose restriction; only 41% of patients with low FVC have confirmed restriction on lung volume testing 3
   • Always measure TLC when restriction is suspected clinically but FVC is normal 2

2. Using fixed cutoffs instead of LLN:

   • Using a fixed FEV1/FVC ratio of 0.70 can overestimate obstruction in older individuals 2
   • The 5th percentile of predicted (LLN) is preferred for more accurate diagnosis 2, 1

3. Interpreting reduced FVC and FEV1:

   • A concomitant decrease in both parameters is most commonly caused by poor effort during testing
   • May also reflect severe airflow obstruction with air trapping
   • Confirmation of the pattern requires lung volume measurements 2

4. Quality control importance:

   • Poor testing technique can significantly impact results
   • Acceptable maneuvers require proper effort, with the two largest values of FEV1 and FVC within 0.150 L of each other 1

Additional Considerations

• Using the largest available VC (whether obtained on inspiration, slow expiration, or forced expiration) is preferable to using only FVC for calculating the FEV1/VC ratio 2
• FEV6 (volume exhaled in the first 6 seconds) can be a reliable surrogate for FVC, and FEV1/FEV6 can be used as an alternative to FEV1/FVC 1, 5
• Response to bronchodilator (increase in FEV1 or FVC ≥12% and ≥200 mL) helps characterize the nature of airway disease 1

Spirometry provides essential information about respiratory function but should be interpreted in the context of clinical presentation and, when necessary, supplemented with additional pulmonary function tests for comprehensive assessment.