Pulmonary Function Tests (& Peak Flow)

Pulmonary Function Tests (PFTs) tell us how well the lung is working, and also help determine what kind of disease might be present.  We order them for patients with chronic lung symptoms, mainly shortness of breath, and also maybe for cough. PFTs are performed with special equipment in a pulmonary laboratory.

Standard PFTs include three components:

  • Spirometry: how much air gets breathed out & how fast (also called “flow rates”)
  • Volumes:  how much air fills the lung, and how much gets trapped inside
  • Diffusing Capacity (DLCO): how much oxygen crosses from lung into the blood

The spirometry should be repeated after an inhaled bronchodilator treatment (standard asthma medication), to see if anything changes.  If numbers improve, it helps with diagnosis.  However, even if the numbers stay the same, prescribed inhalers may still help the patient.

A Peak Flow Meter is a quick cheap version of Spirometry above, measuring how much air is exhaled by a strong forced breath. These meters are widely available to clinicians and patients alike. The Peak Flow should be measured while standing, for 3 tries (resting a moment in between); only the highest value counts (not an average). You can only compare values on the same meter, not between different ones, because they’re too simple to be standardized.

In terms of diagnostics, PFTs help distinguish Obstructive from Restrictive Lung Disease.  Obstructive Lung Disease involves the airway passages (bronchi); mainly Asthma and COPD.  Restrictive includes a variety of conditions that affect lung alveoli and tissue around the airway, such as Hypersensitivity Pneumonitis and Interstitial Lung Disease (see Diagram — Anatomy of the Lungs), and also conditions that stop normal lungs from obtaining enough air, such as neurologic conditions causing muscle weakness, skeleton abnormalities, and extreme obesity.

The DLCO  =  “Diffusing capacity of the Lung for CO (carbon monoxide)”.  Don’t be scared: a tiniest harmless amount of CO is used for the test.  This helps determine how much oxygen is able to get into the bloodstream from the air we breathe.  Normal is >80% of predicted (expected value based on person’s height & weight), bad = <70%, real bad = <40%, <20% heads for lung transplant.  The DLCO is used to assess disability.  

In patients with COPD, the DLCO can determine how much of their disease is chronic bronchitis (for which medications help), and how much is emphysema (nothing helps).ย  Low DLCO here suggests emphysema.ย  For patients with PFTs showing Restrictive Lung Disease, a low DLCO suggests lung disease, while a normal level suggests a condition outside the lung.ย  A low DLCO with normal PFTs (not obstructive nor restrictive) implies problems with blood flow from heart to lungs, like Pulmonary Hypertension, chronic Pulmonary Embolism, and Heart Failure.

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