A pulmonary function test (PFT) is a common diagnostic tool used to assess how well the lungs are working. PFTs measure the mechanics of breathing and the efficiency of gas exchange, providing a comprehensive look at respiratory health. While primarily designed to diagnose and monitor lung diseases, PFT results can sometimes suggest the presence of an underlying heart problem. This relationship between PFT results and potential cardiac issues is indirect, yet significant, offering physicians an initial clue that warrants further investigation.
Understanding Pulmonary Function Tests
Pulmonary function tests are a non-invasive suite of breathing maneuvers that quantify the volume, speed, and efficiency of air moving in and out of the lungs. The most common part is spirometry, which measures the amount of air a person can exhale and how quickly they can do it. Spirometry yields two key values: the Forced Vital Capacity (FVC), the total volume of air exhaled, and the Forced Expiratory Volume in 1 second (\(\text{FEV}_1\)). Another component is the Diffusion Capacity (\(\text{DL}_{\text{CO}}\)), which assesses the efficiency of gas transfer across the alveolar-capillary membrane into the bloodstream. Full PFTs also measure static lung volumes, such as Total Lung Capacity (TLC), to determine if breathing difficulty is caused by problems with airflow, lung size, or gas uptake.
Primary Role of PFTs in Respiratory Diagnosis
The intended use of PFTs is to diagnose and differentiate between various types of lung disorders. Results are analyzed for two broad patterns of dysfunction: obstructive and restrictive. An obstructive pattern, characterized by a low \(\text{FEV}_1\)/FVC ratio, indicates difficulty exhaling air due to narrowed airways, common in conditions like COPD and asthma. A restrictive pattern is defined by a reduction in lung volumes (low FVC and TLC), suggesting the lungs cannot fully expand. This occurs in diseases like pulmonary fibrosis, where lung tissue is stiff or scarred.
The Indirect Link: How Cardiac Issues Affect Lung Mechanics
While PFTs do not directly measure heart function, an abnormal result can signal a secondary problem caused by cardiac failure, most often Congestive Heart Failure (CHF). In CHF, the heart’s inefficiency causes blood to back up, increasing pressure in the lung vessels and leading to pulmonary edema. This fluid makes the lungs stiff and less compliant, restricting their ability to fully expand. This restriction presents on a PFT as a restrictive pattern (decreased FVC). Additionally, the fluid thickens the barrier between air sacs and blood vessels, impairing oxygen transfer, which is reflected in a reduced Diffusion Capacity (\(\text{DL}_{\text{CO}}\)).
PFT Limitations and Definitive Cardiac Diagnosis
An abnormal PFT pattern suggesting a restrictive defect with reduced diffusion capacity strongly indicates a cardiopulmonary issue, but it cannot pinpoint the underlying cause of the heart problem. The test reveals the effect of a disease on the lungs, not the source of the pathology, such as coronary artery disease or valve failure. Therefore, the PFT serves only as a screening tool, highlighting a potential problem that requires specialized cardiac evaluation. To reach a definitive diagnosis, a physician must order specific cardiac tests. These include an Echocardiogram (Echo) to visualize the heart’s structure and pumping efficiency, an Electrocardiogram (ECG) to assess electrical activity, and stress tests to evaluate function under exertion.