M04.07.009 Pulmonary Function Testing

Pulmonary function tests (PFTs) are essential in assessing respiratory health. They measure lung volumes, capacities, and airflow rates to differentiate between obstructive and restrictive pulmonary diseases.

Vital Capacity

Definition
Vital capacity (VC) is the maximum volume of air that an individual can exhale in a single breath. The forced vital capacity (FVC) is the most useful assessment of VC and is performed by expiring as quickly and forcefully as possible.

Key Measurements

• FVC (Forced Vital Capacity): The total volume of air exhaled during the FVC maneuver.
• FEV1 (Forced Expiratory Volume in 1 Second): The volume of air expelled in the first second of the FVC maneuver.

The FEV1/FVC ratio is a critical parameter; a ratio of 0.8 (or 80%) or greater is considered normal.

Points to Remember

• FVC is influenced by factors like age, gender, and body size, and is expressed as a percent of the predicted value.
• A decrease in the FEV1/FVC ratio indicates obstructive disease, while a normal or increased ratio suggests restrictive disease.

Physiology of Pulmonary Function Testing

During PFTs, various physiological parameters are evaluated:

• End of Inspiration: At total lung capacity (TLC), the intrapleural pressure (IPP) is very negative.
• Forced Expiration: A forced expiration generates positive IPP, facilitating airflow out of the lungs.
• Dynamic Compression: During forced expiration, airway resistance increases due to airway collapse, especially in smaller airways, leading to residual volume (RV) that cannot be expired.

Tables

Table 1: Comparison of Obstructive vs. Restrictive Patterns

Flow–Volume Loops

Flow-volume loops visually represent the relationship between airflow (liters/sec) and lung volume during breathing maneuvers.

• Normal Loop: Shows a typical pattern of airflow and volume.
• Obstructive Disease Loop: Starts and ends at higher lung volumes with lower expiratory flow. The downslope of expiration appears scalloped, indicating increased airway resistance.
• Restrictive Disease Loop: Begins and ends at lower lung volumes. While peak flow is reduced, flow at specific lung volumes may be greater than normal.

Figures

1. Flow-Volume Loop for Obstructive Disease:
   • The expiratory flow is lower than normal with a scalloped appearance.
2. Flow-Volume Loop for Restrictive Disease:
   • Shows decreased lung volumes but relatively preserved flow rates at specific lung volumes.

Conclusion

PFTs are vital tools for diagnosing and differentiating between obstructive and restrictive pulmonary diseases. Understanding the dynamics of lung volumes, capacities, and flow rates can significantly enhance the management and treatment of patients with respiratory conditions.

Bibliography

1. West, J.B. (2012). Respiratory Physiology: The Essentials. 10th ed. Lippincott Williams & Wilkins.
2. Crapo, R.O., et al. (1981). “Lung Volume Measurements.” American Review of Respiratory Disease, 123(2), 178-184. doi:10.1164/arrd.1981.123.2.178
3. Ruppel, G. (2014). Pulmonary Function Testing. 3rd ed. Jones & Bartlett Learning.
4. Koulouris, N.G., & Roussos, C. (2001). “Respiratory Muscle Physiology.” Physiology of the Respiratory System. Academic Press.