Pete Reed and Michael Phelps: Lung Capacity Legends

Pulmonary Function Tests, or PFTs, are a suite of measurements crucial for diagnosing and managing respiratory disease. These tests examine lung volumes, capacities, flow rates, and the gas exchange ability of the lungs. PFTs are used by general internal medicine practitioners, respiratory therapists, and occupational health organizations to detect problems in how the lungs function. They are essential in distinguishing between obstructive and restrictive lung diseases, pinpointing specific patterns in test results that guide diagnosis and ongoing monitoring. For patients with asthma, chronic obstructive pulmonary disease, or suspected restrictive lung conditions such as pulmonary fibrosis, PFTs provide quantitative data to track disease progression or response to therapy. They are also deployed to assess the impact of hazardous environmental or occupational exposures, making them a core element of respiratory surveillance programs in at-risk industries.

Lung volumes are distinct measurements describing the amount of air in the lungs at different phases of the breathing cycle. Total Lung Capacity, or TLC, represents the absolute maximum volume of air the lungs can accommodate after a maximal inhalation. TLC is the sum of the Vital Capacity (VC)—the volume of air that can be forcibly exhaled after maximum inhalation—and the Residual Volume (RV), which is the air that remains in the lungs after the most forceful exhalation. Tidal Volume, or TV, is the volume of air that moves in or out with each normal, restful breath. In an average adult male, TLC is around 6 liters, while TV is about 0.5 liters. These numbers serve as benchmarks when interpreting PFT results, allowing clinicians to compare patient values against established norms.

Spirometry is the most widely used PFT and provides measurements of the amount and speed of air a person can inhale and exhale. In a typical test, the patient takes a deep breath and exhales as forcefully and completely as possible into the device. Spirometry yields several indices, such as Forced Vital Capacity (FVC), the maximum volume of air expelled after a full inhalation, and Forced Expiratory Volume in one second (FEV1), the amount of air expelled in the first second of a forced exhalation. The ratio of FEV1 to FVC is a key metric for diagnosing obstructive lung diseases, as a reduced ratio indicates airflow limitation characteristic of conditions like COPD or asthma. For instance, in healthy adults, the FEV1/FVC ratio typically ranges from 0.8 to 1.0; a significantly lower value suggests obstruction.

Advanced PFT techniques supplement spirometry and provide further insight into lung physiology. The helium dilution method uses a closed, rebreathing circuit to estimate residual lung volume, assuming the patient begins with no helium in their lungs. Nitrogen washout is another indirect approach, in which the patient inhales pure oxygen and the exhaled nitrogen is measured to determine lung volume. Body plethysmography, sometimes called the "body box," applies Boyle’s law to measure changes in volume and pressure within a sealed chamber, producing highly accurate lung volume data. Measurement of maximal respiratory pressures—maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP)—can indicate respiratory muscle strength and help monitor patients with neuromuscular diseases. Diffusing capacity tests evaluate how effectively gases such as carbon monoxide pass from the lungs into the bloodstream, an important factor in diseases affecting the alveolar-capillary membrane.

Impulse oscillometry (IOS) is a non-invasive method introduced in 1956 by Dubois and colleagues, which has gained traction for evaluating lung function without requiring forced breathing maneuvers. IOS uses sound waves applied during normal breathing to assess both large and small airway resistance. Studies have shown that IOS is particularly useful in patients who cannot perform effort-dependent exhalation, such as children or older adults. IOS can detect small airway dysfunction even when spirometry is normal, making it valuable for early diagnosis of asthma and COPD. In occupational health, IOS is increasingly used for surveillance, allowing monitoring of workers exposed to respiratory hazards before symptoms develop or spirometry abnormalities appear.

PFTs reveal characteristic patterns in obstructive versus restrictive lung diseases. In obstructive conditions such as asthma and COPD, airway narrowing impedes airflow, leading to a reduced FEV1/FVC ratio—sometimes as low as 0.6 in asthma or even down to 0.45 in advanced emphysema. However, lung volumes may remain normal or be increased due to air trapping. In restrictive diseases, such as pulmonary fibrosis or chest wall disorders, lung volumes including TLC and VC are decreased, but the FEV1/FVC ratio is often preserved or even elevated, generally within the range of 0.8 to 1.0. These distinct patterns allow clinicians to rapidly differentiate between the two categories and tailor further diagnostic workup and management.

Occupational lung diseases represent a broad spectrum of disorders caused by exposure to hazardous materials in the workplace. Examples include asbestosis, silicosis, coal worker’s pneumoconiosis, and occupational asthma. Workers in mining, construction, manufacturing, and industries involving asbestos, silica, or coal dust are at heightened risk. In the United States, occupational exposure accounts for around 15% of COPD cases. Asbestos exposure can lead to not only asbestosis but also mesothelioma and non-mesothelioma lung cancers. Silicosis, caused by inhaling fine particles of crystalline silica, can develop after more than ten years of low-level exposure or after short-term, high-intensity exposure. Regular PFTs in these populations can detect declines in lung function before irreversible damage occurs, enabling preventive action or withdrawal from exposure.

Impulse oscillometry has emerged as a powerful screening tool in occupational settings because it does not require the patient to perform maximal effort maneuvers. This is particularly useful for early detection of subtle airway changes in workers exposed to toxins, as IOS is sensitive to small airway dysfunction. In cases involving nanoparticles, flock, or other inhaled irritants, IOS can identify changes in airway mechanics that would otherwise be missed by conventional spirometry, supporting timely intervention.

Despite their value, PFTs have limitations and contraindications. Patients who have experienced a recent myocardial infarction, stroke, or suffered from an aneurysm are typically advised against undergoing PFTs, as the stress of forced breathing maneuvers may provoke complications. Neuromuscular disorders can also pose challenges, as weak respiratory muscles may prevent accurate test performance and interpretation. Risks associated with PFTs include temporary dizziness, shortness of breath, coughing, or, very rarely, pneumothorax. Technicians and clinicians must weigh the diagnostic benefit against these potential hazards, especially in vulnerable populations.

The clinical interpretation of PFTs is guided by published standards from organizations such as the American Thoracic Society and the European Respiratory Society, which specify protocols for test performance and reproducibility. For example, to consider a forced vital capacity measurement valid, three reproducible efforts are required, with FVC and FEV1 within 150 milliliters of each other. In practice, however, adherence to these guidelines varies, and deviations can result in false-positive or false-negative results. Standardization is especially critical when using PFTs in occupational health surveillance, where small changes in lung function may signal early disease.

The diffusing capacity of the lung for carbon monoxide (DLCO) is another advanced test, measuring the ability of the lungs to transfer gas from air to the blood. This is particularly useful in differentiating between restrictive lung diseases caused by interstitial fibrosis, which reduces DLCO, and those due to chest wall restriction, which may leave DLCO unaffected.

In healthy adults, the average respiratory rate is 12 to 20 breaths per minute, and the volume of a single tidal breath is about 0.5 liters. These reference values are used to interpret deviations that may signal pathology. Air pollution, even at low levels, can reduce FEV1 and FVC in asthmatics and in healthy adults, highlighting the sensitivity of PFTs to environmental exposures beyond the workplace.

The largest recorded lung capacity is reported at 11.68 liters in Pete Reed, a British rower and three-time Olympic gold medalist, while US swimmer Michael Phelps is also said to have a lung capacity of around 12 liters. These values far exceed the average TLC of 6 liters in adult men and illustrate the broad physiological range possible in elite athletes.