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Interpreting lung function tests

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By Lee Guion, MA, RRT, FAARC

Neurorespiratory Clinical Specialist

After each exhalation is measured by the spirometer, your results will be compared to “normal” or “predicted” values and expressed as a percentage of predicted (FVC %, FEV1 %). These reference values come from a large national database of healthy people the same age, gender, and height as you. Weight does not figure into the reference values, although a large abdomen may reduce the amount of air you can breathe in.

Spirometry tests should be repeated during each ALS clinic visit. Your physician will compare your initial test results with subsequent ones and note the trend.

Here’s what the numbers mean.

FVC:

  • A forced vital capacity (FVC) of 80% to 100% of predicted for your gender, age, and height is classified as normal. However, it’s important to determine your individual baseline FVC.
  • Actual values (lung volumes) are expressed in liters.
  • An FVC above normal has little clinical significance and is considered normal.
  • An FVC below normal warrants further investigation. Your respiratory therapist and physician will ask if you have clinical symptoms of reduced lung volume during the day or at night. For example, are you more tired with activities, have trouble catching your breath, have trouble sleeping through the night, or do you wake up feeling tired after a full night’s sleep?

FEV1:

  • An FEV1 of 80% or higher of predictive is considered normal.
  • Actual values (lung volumes) are expressed in liters per second.
  • A forced expiratory volume in the first second (FEV1) that meets or exceeds normal values indicates normal lung function. However, it needs to be compared to your FVC results.
  • An FEV1 that is below normal compared to FVC warrants further investigation and the results should be evaluated in the context of your medical history. Your respiratory therapist and physician may ask if you have a history of smoking cigarettes or have been diagnosed with asthma, emphysema, or bronchitis. They may ask if you have worked with or had hobbies that involved dust or fumes that are irritating to the lungs.

The FEV1/FVC ratio puts both numbers in context:

  • Abnormal spirometry results (FVC and FEV1 below 80% of predicted) are generally classified as either an obstructive or a restrictive process.
  • In ALS, we typically see lung restriction. Restriction is due to stiffening lungs and loss of elasticity. As the diaphragm weakens, your inhaled breath (lung volume) becomes smaller. This causes the amount of air that can exhaled quickly (volume in the first second of exhalation) to become smaller as well. In ALS, FVC and FEV1 are reduced together, so a FEV1/FVC ratio of at least 70% is considered normal.
  • This is the classic definition of restriction as measured by spirometry. (Reduced FVC and FEV1 values with normal FEV1/FVC ratio.) Restriction is further classified as mild, moderate, or severe based on how stiff the lungs have become.

Here is a sample of spirometry results for a 72-year-old woman who is 5 feet 2 inches tall.

Test                             Results            *Predicted         % Predicted

FVC  (L)                     2.15                    2.95                      73

FEV1 (L)                     1.74                    2.22                      78

FEV1/FVC (%)               81                      76                     110

* Best of three acceptable, reproducible results (three good blows)

Interpretation: Mild restriction (reduced FVC and FEV1 with a normal FEV1/FVC ratio)

Unlike obstructive lung disease, restricted lungs do not gain much symptomatic improvement from aerosol medications. The clinical focus is on reducing energy expenditure, pacing yourself, and maximizing your lung’s capabilities. Treatment with bi-level positive pressure breathing is highly recommended. These topics will be covered in detail in the next series of postings.