If the FEV1 is decreased but the FEV1/FVC ratio is normal, what condition does the patient have?

When the FEV1 (Forced Expiratory Volume in the first second) is decreased but the FEV1/FVC (Forced Vital Capacity) ratio remains normal, this pattern is indicative of a restrictive lung disease. In restrictive diseases, although the FEV1 may be low, the proportion of FEV1 to FVC stays within the normal range. This is because both FEV1 and FVC decrease proportionally due to the restriction, which can be caused by factors such as lung parenchyma diseases, pleural diseases, or neuromuscular issues.

In contrast, obstructive diseases typically present with a decreased FEV1 and a reduced FEV1/FVC ratio, reflecting that the obstruction limits airflow more during expiration than inspiration. Normal lung function would not show a decrease in FEV1, and chronic bronchitis, which is an obstructive disease, would also show a decreased FEV1/FVC ratio due to its obstructive nature. Therefore, the scenario of a decreased FEV1 with a normal FEV1/FVC ratio fits the profile of restrictive lung disease.