Medicine
Spirometry Interpretation Strategies in Restrictive Lung Patterns
Quick fact
In restrictive lung disease, the FEV1/FVC ratio is often normal or even increased (e.g., 80%), because both FEV1 and FVC are reduced proportionally—a key clue that differentiates it from obstruction, where the ratio is decreased.
Why this is interesting
You've just watched a patient blow into a spirometer, and the numbers look 'low.' But is it a lung that can't empty, or a lung that can't fill? The answer isn't in the numbers alone—it's in the ratio.
Read the full explanation
Understanding Spirometry Interpretation Strategies in Restrictive Lung Patterns
Think of the lungs as a balloon. In restrictive lung disease, the balloon is stiff and hard to inflate—like trying to blow up a frozen balloon. The lung tissue (or the chest wall, or the muscles) resists expansion, so the total amount of air you can breathe in (FVC) is reduced. Because the airways are not blocked, the air that is there can still be expelled quickly, so the FEV1 is also reduced but proportionally. That's why the ratio (FEV1/FVC) stays normal or even rises. The spirometry shows a 'small but quick' breath. The strategy is to first look at FVC: if it's low, then check the ratio: if the ratio is normal or high, think restrictive.
A deeper explanation
The mechanism behind the restrictive pattern is reduced lung compliance—the lung's ability to stretch. This can stem from intrinsic lung disease (like pulmonary fibrosis, where scar tissue stiffens the lungs), extrapulmonary causes (like chest wall deformities or obesity), or neuromuscular weakness (like ALS). Spirometry alone can't confirm restriction; it just suggests it. The gold standard is measuring Total Lung Capacity (TLC) via plethysmography or gas dilution. If TLC is below the lower limit of normal, the restriction is confirmed. The interpretation strategy is therefore a two-step algorithm: first, identify the low FVC with a normal/high ratio; second, confirm with TLC measurement. This prevents misdiagnosis—because a low FVC could also occur in severe obstruction with air trapping.