Spirometry is a common test that measures how much air a person can inhale and exhale, and how quickly they can exhale. It helps healthcare professionals diagnose respiratory conditions and monitor lung health over time. People might undergo spirometry for routine check-ups, to diagnose breathing difficulties, or to track existing lung conditions.
Key Spirometry Measurements
The spirometry test captures several important measurements that reflect different aspects of lung performance. Forced Vital Capacity (FVC) is one such measurement, representing the total volume of air a person can forcibly exhale after taking a maximal breath. This value indicates the overall capacity of the lungs.
Forced Expiratory Volume in 1 Second (FEV1) quantifies the amount of air exhaled during the first second of the FVC maneuver. It reflects the speed at which air can be expelled, providing insight into potential airflow limitations.
The FEV1/FVC ratio compares FEV1 to FVC, often expressed as a percentage. This ratio indicates the proportion of total lung capacity exhaled in the initial second of forced exhalation, serving as a key indicator to differentiate lung function patterns.
Understanding Numerical Results
Spirometry results are presented numerically and compared against “predicted values.” These statistical averages are derived from healthy individuals with similar age, sex, height, and ethnicity. Your actual results are then expressed as a percentage of these predicted values.
For most measurements, a result is considered normal if it is above 80% of the predicted value. The FEV1/FVC ratio typically falls between 70% and 85% for healthy adults aged 20-60, though some guidelines suggest above 70% as normal, noting it can decrease with age.
These percentages are general guidelines. Individual results vary, and a healthcare professional interprets them within a person’s overall health history and symptoms. A definitive diagnosis always requires professional medical evaluation.
Interpreting the Spirometry Graph
Beyond numerical values, spirometry tests also generate visual graphs that offer additional insights into lung function. The Flow-Volume Loop plots airflow rate against lung volume during exhalation and inhalation. A normal loop has a characteristic shape: rapid airflow increase during exhalation, reaching a peak, then gradually declining. The inspiratory phase typically shows a smooth, symmetrical curve. Deviations provide clues about airflow limitations.
The Volume-Time Curve illustrates the volume of air exhaled over time. A normal curve shows a rapid increase in exhaled volume during the first second, followed by a plateau as exhalation nears completion. This rapid rise and plateau indicate efficient, complete exhalation. The total exhalation duration, typically at least six seconds for a complete test, is also assessed.
Common Interpretive Patterns
Spirometry results, both numerical and graphical, often fall into specific patterns that suggest different types of lung conditions. An obstructive pattern indicates difficulty exhaling air due to narrowed airways. Numerically, it shows a reduced FEV1 and a significantly reduced FEV1/FVC ratio (typically below 0.70). While FVC may be normal or slightly reduced, the disproportionate FEV1 reduction is key.
On a flow-volume loop, an obstructive pattern often appears “scooped-out” or concave on the expiratory limb, reflecting airflow limitation. The volume-time curve will show a slower rise to maximum volume and prolonged exhalation. Conditions like asthma and chronic obstructive pulmonary disease (COPD) commonly present with this pattern.
Conversely, a restrictive pattern suggests lungs cannot fully expand, leading to reduced lung volumes. Both FEV1 and FVC are reduced, often proportionately, resulting in a normal or even increased FEV1/FVC ratio (typically 0.70 or higher).
The flow-volume loop appears smaller overall but retains a relatively normal triangular shape, indicating reduced lung capacity rather than airflow obstruction. The volume-time curve will show a smaller total exhaled volume, often reaching its plateau more quickly. Conditions like pulmonary fibrosis or chest wall abnormalities are associated with this pattern. A medical professional uses these patterns, alongside other clinical information, for diagnosis.